Record Players

Wednesday December 31, 2014

Once upon a time, people used record players to hear music. Records were these big black discs that played on a machine. Spinning between 33 and 45 times per minute on a turntable, people used to listened to music just like this for nearly a century.


Edison, who had trouble hearing, used to bite down hard on the side of his wooden record player (called a phonograph) and “hear” the music as it vibrated his jaw.


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Here’s what you need:


  • an old turntable (do you have one in your garage?)
  • old record that can be scratched
  • tack
  • plastic container, like a clean yogurt or butter tub

If you have an old turntable and OLD record that can be scratched, here’s how to listen to the music without using regular speakers!



Yay! You’ve completed this set of lessons! Now it’s your turn to do physics problems on your own.


Download your Sound Problem Set here.

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Standing Waves

Sunday December 14, 2014

When most people think of waves, they imagine something like an ocean wave… it moves through the water and isn’t confined to one area. Some waves don’t travel at all – they are called standing waves.


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Picture two waves traveling in opposite directions. One is going to the right, the other going to the left. What happens when they combine? Using superposition, we can figure out what the interacting waves look like when they interfere with each other.



Did you notice how there are places along the string where the string is not moving at all? It’s at permanent rest… it’ doesn’t vibrate at all. Those places are called nodes. The places where the amplitude is greatest is called the antinodes.


Click here to go to next lesson on Nodes and Antinodes.

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Nodes and Antinodes

Sunday December 14, 2014

One common misconception is the idea that noes and antinodes are the same as the crest and trough of a wave. They’re not. A node is a place on the wave that is permanently at rest. An antinode is where the wave is at its maximum (it will travel through a large up and a large down displacement).
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You’ll find standing waves when you look at columns of vibrating air or strings vibrating at resonance. This happens because of two things going on simultaneously:  when the wave hits the barrier, instead of continuing on, it gets reflected back.


The reflected wave constructively interferes with the next incoming waves, and the overall effect is that you have places on the string that never move and places where it’s always at a maximum. This effect is what we call a standing wave.


Nodes and antinodes aren’t really part of the wave because a standing wave really isn’t a wave at all… it’s just a visual effect that looks like a wave that doesn’t move. You have to perfectly time the interference of two (or more) opposite-traveling waves to get this effect.


Click here to go to next lesson on Simple harmonic motion.

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Simple harmonic motion

Sunday December 14, 2014

Standing waves are basically two waves traveling in opposite directions that constructively interfere with each other so it looks like the whole system is moving in simple harmonic motion.
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Simple harmonic motion is when we used Hooke’s Law for the mass-spring system to figure out the displacement, period, or frequency of the system.



First harmonics have 2 nodes and 1 antinode. Second harmonics have 3 notes and 2 antinodes, third harmonics have 4 nodes and 3 antinodes, and so on.


Click here to go to next lesson on Build a Standing Wave Machine.

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Build a Standing Wave Machine

Sunday December 14, 2014

Standing wave machines are fun to make because they are easy to build and amazing to watch! Here’s a simple one you can make on your own using the materials from the previous lessons in electricity.


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Materials:


  • AA batteries (4, cheap “dollar-store” carbon-zinc kind work great)
  • AA battery case (4)
  • Alligator clip leads
  • DC, 3V motor (2)
  • Hot glue gun (and glue sticks)
  • Masking tape
  • Popsicle sticks
  • String
  • Scissors


How many wavelengths can you see in yours? Where are the nodes and antinodes? Change the tension in the strong to get different harmonics to show up! (Note – if you know how to change the motor speed using a potentiometer, you can do that also!)


Also note – if you’re finding that the string wads into a tight ball after only a few seconds, it means that one of your motors is going the wrong direction. Stop both motors and switch the wires in the back of ONE of the motors to reverse the polarity (plus and minus) so the motor spins in the opposite direction. You want the one motor to wind up the string and the other to un-wind it at the same time. Eventually, since the motors spin at slightly different speeds, the string will get wound up and you’ll need a new piece of string.


Click here to go to next lesson on Standing Wave Math.

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Standing Wave Math

Sunday December 14, 2014

It’s easy to calculate the frequency, period, wavelength and speed of waves at different harmonics.
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For a tight string, there’s a handy equation that we can use to relate the length of the string to the wavelength based on the harmonic number like this:


L = (n/2) λ


where L is the length of the string, and n is the harmonic number. Here’s how to use this equation to find everything we need about the wave:



You’ve finished this section! Now it’s your turn to do physics problems. Click here to download your homework problem set.

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Behavior of Waves

Sunday December 14, 2014

When a wave travels from one medium to another, like sound waves traveling in the air and then through a glass window pane, it crosses a boundary. Whether the wave continues to the new medium (and even how it goes through), or whether it bounces and reflects back, or a bit of both depends on the boundary.
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Imagine a jump rope attached to a door handle. The last particle of the rope is fixed tot he door handle, and doesn’t move at all. If you grab the free end of the rope and pull it taught, you have a nice, straight line. When you jerk the rope up, the pulse travels through the rope toward the door handle. Some of the energy carried by the pulse is reflected and comes back to you at the same speed and wavelength, but it’s upside down (called a reflected pulse) and not as large amplitude-wise. Some of the energy is also transmitted to the fixed end, causing the door handle to rattle and vibrate.



If you untie the rope from the door handle and instead tie it loosely to a pole (so it’s allowed to slide up and down easily) and repeat this experiment, you’ll find the pulse travels through the rope, turns around and reflects back right side up with the same speed and wavelength.


Now imagine untying the rope and instead attaching a rope denser, thicker rope to the end. The initial pulse travels toward the thicker rope, but two tings happen when it hits the thicker rope: first, a reflected pulse (same speed and wavelength, but inverted) returns back to you, but also some of the energy goes into the thicker rope so a smaller, slower wave (but with the same frequency as the original pulse) will travel along the thicker rope. Waves travel fastest in the least dense medium, so the reflected wave travels faster than the transmitted wave. Even though the waves travel at different speeds in different mediums, they are all vibrating with the same frequency.


What would you expect to happen if a sound wave traveled from the denser rope to a less dense rope? The initial pulse (also called the incident pulse) travels through the denser medium, and when it hits the lesser dense rope, it undergoes partial transmission and partial reflection like this: the reflected pulse  has the same speed and wavelength (and is right side up). Second, the wave transmitted to the less dense rope is right-side up, larger amplitude, and traveling faster than the reflected pulse.


The bottom line? Waves travel fastest in the least dense medium. Frequency doesn’t change when you cross a boundary. The wavelength is always greatest in the least dense rope. The amplitude is always greatest in the initial (incident) wave.


The reflected wave inverts when it moves from a less dense to a more dense rope due to Newton’s Laws of Motion. For the case when the rope is free to slide up and down on the pole, when the initial wave reaches the pole, the rope slides up and because of its inertia, it overshoots and exerts a reaction force on the string, and this reaction force sends a reflected wave back down the string (called a soft reflection). With the case of being fixed on a door handle, when the incident wave reaches the end, it exerts an upward force on the door handle, but Newton’s Third Law states that there’s an equal and opposite (reaction) force that the door handle exerts on the string, which generates an inverted pulse that travels back along the string (called a hard reflection).


Click here to go to next lesson on Wave Reflection.

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Wave Reflection

Sunday December 14, 2014

Straight waves are what happens when something  moves back and forth in a medium like water. These are interesting when they hit a diagonal plane barrier, because when the incident wave reaches the barrier, the waves always reflect at the same angle that they approached the barrier with (called the Law of Reflection).
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We’ll look at how waves reflect off curved surfaces(like a parabolic mirror) when we get to our section on light waves.


Click here to go to next lesson on Wave Refraction.

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Wave Refraction

Sunday December 14, 2014

Waves bend when they go from one medium to another when the speed changes. It’s a really important topic in light (not so much with sound), because it’s how lenses, eyes, cameras, and telescopes work. The bending of sound waves happens naturally in the air above the earth when it’s warmer than the surface of the earth. The sound waves that travel through the warmer air are faster and the ones that travel through cooler air are slower. When the sound waves go from warmer to cooler air (less dense to more dense air), they become bent back down toward the surface.
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So refraction can bend sound downward which in turn amplifies the sound by adding to the direct (original source) sound. If you’ve ever been near a cool lake, you’ve heard one of nature’s amplifiers!


But why does sound bend? You can imagine a toy car going from a wood floor to carpeting. One wheel hits the carpet first and slows down before the other, causing the toy to turn. The direction of the sound changes in addition to the speed. The slower speed must also shorten its wavelength since the frequency of the wave doesn’t change. Here’s the wave equation again from the previous section:



Note the first v is velocity, and the last ν is frequency in the equation.


Click here to go to next lesson on Diffraction.

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Diffraction

Sunday December 14, 2014

When waves pass around small (we’re talking small compared to the wavelength of the wave) objects, they diffract. People in the audience of a concert can hear really well if they are sitting right behind a pillar because the sound waves are large enough to bend around it (which is actually because of both diffraction and reflection effects). Diffraction helps sound bend around obstacles. You can sometimes hear conversations around corners because of diffraction.
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Waves also diffract if they spread out after moving through small (again, smaller than wavelength-size) openings, like light going through a slit cut with a razor.


The amount a wave diffracts (bends) depends on the wavelength. Lower frequencies bend around objects better than higher frequencies. If you’ve ever watched a lightning and thunder storm, you know that there’s a lot more sound (like a sharp crack) when the lightning is closer  (you hear both higher and lower frequencies) than when it’s more distant (mostly lower frequencies). Owls use low frequency sounds to transmit sounds further than the higher frequency bird twitters.


Click here to go to next lesson on Superposition.

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Superposition

Sunday December 14, 2014

Often two (or more) waves travel through the same spot. If you’ve ever listened to an orchestra, you’re hearing the sounds from many different instruments all playing at the same time. If more than two boats are on the lake, their wakes churn up the water together. Here’s how we handle this in physics…
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The principle of superposition states that when waves interfere with each other, the displacement at any location is the sum of the displacements of the waves at that location. Said another way, you simply add up the waves at the same spot to get the resulting amplitude.


Click here to go to next lesson on Interference.

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Interference

Sunday December 14, 2014

What if two waves of the same wavelength and amplitude travel in the same direction along a stretched string? What will the string look like? We know about the idea of superposition adding the waves together, but what does the string actually look like?
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How waves interact with each other depends on whether the waves are in phase or not. If they are in-step (in phase) with each other, then it’s easy to add up to double the displacement (constructive interference). If they are completely out of step, then they cancel each other out (destructive interference).



Click here to go to next lesson on Constructive Interference.

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Constructive Interference

Sunday December 14, 2014

When two waves have an increase in displacement when they  interact, it’s constructive interference.
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Click here to go to next lesson on Destructive Interference

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Destructive Interference

Sunday December 14, 2014

Destructive interference happens when two waves have opposite displacements. The pulses don’t destroy each other (as the name implies), but rather they cancel out the effect of each other when they interact with each other.
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They don’t have to cancel each other out completely to be destructive interference. They don’t even have to have the same amplitudes. (And actually, when two waves meet, they don’t even alter their path or alter the waveform itself after the interaction. They simply add or subtract when they interact, and then go on their merry way as they had been before the interaction when they’re done. It’s really quite amazing.)


Click here to go to next lesson on Which kind of interference is it?

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Vibrations

Sunday December 14, 2014

In previous lessons we’ve learned that energy is the ability to do work, and that work is moving something a distance against a force. The concept of energy is fairly easy to see as far as lifting things or pushing things go. We are exerting energy to lift a box against the force of gravity. We are exerting energy to pedal our bike up a hill. But how does this energy stuff relate to light, electricity, or sound? What’s moving against a force there?


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With energy, what’s happening is that outrageously tiny particles are moving back and forth outrageously tiny amounts, at outrageously high speeds.  With light, you’ve got little photons moving, with electricity little electrons. With sound, you’ve got molecules moving back and forth.


This back and forth motion is called vibration and these vibrations make waves. When one particle moves back and forth it does work on another particle, which does work on another particle and so on. As these particles do work on one another, they cause a wave to move from one place to another.


Energy moves by waves or, in other words, waves are energy-mobiles! Before we get in over our heads talking about waves however, we need to spend some time on this vibration thing.


If you imagine a swing at the park, it’s got a normal resting position (called an equilibrium position). That’s when it’s experiencing a balance of forces: the pull of gravity balances with the tension in the chain holding up the swing, and so forth. Everything adds up to zero, and stays at zero until a kid comes by.



When a kid starts pumping on the swing, the swing is no longer in equilibrium and the swing starts to move back and forth through an arc. This is a slow forced vibration that stops soon after the kid leaves the swing, and the kid needs to keep pumping back and forth to keep swinging at the same rate (or increase the swing rate). Eventually, the kid gets tired and stops pumping, and each repetition of the swing moving back and forth (vibration) is less and less (called damping). Eventually, the swing comes to a stop (equilibrium) until another energetic kid comes along.


Click here to go to next lesson on Damping.

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Which kind of interference is it?

Sunday December 14, 2014

Let’s practice figuring out which interference is being observed when waves interact…
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Click here to go to next lesson on Doppler Effect.

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Damping

Sunday December 14, 2014

Damping is when a spring, swing, or other vibrating object loses its energy over time. It means that without adding energy into the system, like pumping on a swing or hitting a drum head, the object will eventually come to its non-vibrating (equilibrium) position.


Imagine the kid on the swing again. Why does the kid move past the equilibrium point without stopping?
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It’s actually because of Newton’s law of inertia. The swing continues on its path as long as there are balanced forces acting on it. As the swing moves past its equilibrium point, a restoring force acts on it to move it back to its original equilibrium position.


The difference between a vibration (like a kid swinging) and translational motion (like a kid on roller skates) is that the kid on skates can be permanently displaced from his starting position. The kid on a swing doesn’t move away from its original equilibrium position for long – it stops, turns around, and comes back due to the restoring force. An object that vibrates actually wiggles and jiggles around a fixed position (the equilibrium position).



Other examples of vibrating objects include tuning forks (which are like inverted swings), piano strings, car suspension systems,  a weight on the end of a spring that bobs up and down, and drum heads. In each of these examples, when the object is disturbed (like hitting the tuning fork or drum head, or pulling the weight at the end of the spring), the object moves from its original resting position, stops and heads back toward its resting position and overshoots, stops and come back toward the resting position and the cycle repeats. The object moves along the same path over and over again, and if there was no friction or drag force (or other energy losses), the object would continue to move back and forth forever.



Did you notice that when you graph out the motion of the mass on a spring, it has a particular shape? The shape is described in math as a sinusoidal wave, or y = sin(x).  Did you notice how it is periodic and that you could also see the damping effect? The damping effect means that energy is being lost or dissipated. (Scientists wouldn’t say that it’s slowing down, because that indicates that the speed is decreasing, which isn’t the case when the mass stops and turns around to head back to the resting position.. it is actually speeding up!)


Click here to go to next lesson on Frequency.

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Frequency

Sunday December 14, 2014

The concept of frequency is very important to understanding energy. When it comes to electromagnetic waves it is frequency that determines whether the wave is radio, light, heat, microwave or more. It’s all the same type of energy, it’s the frequency that determines what that energy actually does. With sound energy the frequency determines the pitch of the sound.


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As we move forward with energy, it is quite important that you know that all waves come from some sort of vibrating particle somewhere. The reason you can pick up a signal on your radio is because somewhere, maybe miles away, there is a particle vibrating at some ridiculous speed, creating a wave that moves across distances to finally vibrate the particles inside your radio’s antenna. It’s important to realize, however, that the particle does not move over that distance. The particle that started the wave back at the radio station is still there. It did not move to your radio it just vibrated at the antenna and started the wave.


Frequency is a measure of how many times something moves back and forth. A swing, a pendulum, a leg of a walking person all have a frequency. All those things start at one place, move, and come back to the same position that they started. This moving and coming back is one vibration. The faster something vibrates, the more frequency that something has.


Frequency is measured in Hertz. One Hertz (or Hz for short) is one vibration in one second. The Hertz is named after Heinrich Rudolf Hertz (1857-1894) a German physicist and professor. Hertz proved that electricity can be transmitted in electromagnetic waves, which travel at the speed of light and which possess many other properties of light. His experiments with these electromagnetic waves led to the development of the wireless telegraph and the radio.



A Hertz is a relatively slow vibration so there are also kilohertz (KHz), megahertz (MHz), and gigahertz (GHz). A kilohertz is 1000 Hz, a megahertz is 1,000,000 (a million) Hz, and a gigahertz is 100,000,000 (one thousand million) Hz.


In this experiment you will be adjusting the length of string of a pendulum until you get a pendulum that has a frequency of .5 Hz, 1 Hz and 2 Hz. Remember, a Hz is one vibration (or in this case swing) per second. So .5 Hz would be half a swing per second (swing one way but not back to the start). 1 Hz would be one full swing per second. Lastly, 2 Hz would be two swings per second. A swing is the same as a vibration so the pendulum must move away from where you dropped it and then swing back to where it began for it to be one full swing/vibration.


You’ll need:


  • 3 Foot Long String
  • A Weight that can be tied to the end of the string
  • A Timer or Stopwatch
  • Masking Tape
  • A Table or Chair
  • A Partner is helpful


“Advanced students: Download your What is Frequency?”


1. Tie your weight (the official name of the weight on the end is bob. Personally I’ve always preferred the name Shirley, but Bob it is.) to the end of the 3 foot string. If you’ve done the gravity lesson in the Mechanics set of lessons you’ll remember that the weight of the bob doesn’t matter. Gravity accelerates all things equally, so your pendulum will swing at the same speed no matter what the weight of the bob.


2. Tape the string to a table or chair or door jam. Make sure it can swing freely at about 3 feet of length.


3. I would recommend starting with 1 Hz. It tends to be the easiest to find. Then try .5 Hz and then 2 Hz.


4. The easiest way I’ve found to do this is to start the pendulum swinging and at the same time start the timer. Count how many swings you get in ten seconds.


5. Now, adjust the string. Make it longer or shorter and try again. When you get 10 swings in 10 seconds you got it! That’s one swing per second. You should be able to get quite close to one swing per second which is 1 Hz.


6. Now try to get .5 Hz. In this case you will get 5 swings in ten seconds when you find it. (A little hint, the string is pretty long here.)


7. Now speed things up a bit and see if you can get 2 Hz. Be prepared to count quick. That’s 2 swings a second or 20 swings in 10 seconds! (Another little hint, the string is quite short for this one.)


Did you get all three different frequency pendulums? It takes a while but my classes found it rather fun. You’ve created three different frequencies. 2 Hz being the fastest frequency. That was pretty fast right? Can you imagine something going at 10 Hz? 100 Hz? 1,000,000 Hz? I told you things were moving at outrageous speeds!


Some examples of things that work at these frequencies are AM radio stations which broadcast at KHz, FM stations which broadcast at MHz, and microwaves which cook your food with GHz. If your radio is “crankin” tunes from radio station 750 AM, a part of your radio is vibrating at 750,000 times a second. If you’re “pumping wattage into your cottage” with WSCI at 94.2 on your radio dial, a part of your radio is vibrating at 94,200,000 times a second. If your radio happens to be green, then light is vibrating off your radio at 6 x 1014 Hz. That’s 6 with 14 zeros behind it or 600,000,000,000,000 vibrations in one second. That’s some serious vibes!


(By the way, if you can hear the sound coming out of your radio, your speakers are vibrating anywhere between 20 and 20,000 Hz. See how vibrations are important? They’re everywhere!) Let’s look more carefully at what those vibrations make, and that’s waves.


Click here to go to next lesson on Period.

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Doppler Effect

Sunday December 14, 2014

Imagine a police car on the side of the road with lights and sirens on full blast. You’re also parked and you hear the same frequency (say 1,000 Hertz) of the siren. However, if you’re driving at 75 mph toward the police car. you’re going to hear a higher frequency (1096 Hz), and if you’re driving away at 75 mph, you’re going to hear a lower frequency of 904 Hz. Why is that?
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It has to do with a motion-related frequency change called the Doppler Effect.  This effect was initially conceived by Johann Doppler in 1842 and later tested by Buys Ballot in Holland in 1845 with a locomotive drawing an open car with trumpeters. There’s Doppler Effect for not only sound but also light including microwaves, radio waves, and visible light. Police use the Doppler Effect on their radar guns to track your speed. Astronomers use it to find the motions of stars, galaxies, and quasars… it’s really an amazing tool in our scientific toolbox!


Click here to go to next lesson on Standing Waves.

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Period

Sunday December 14, 2014

The period is the time it takes for one full cycle to complete itself and is measured in seconds per cycle. The frequency is the number of cycles that are make in a period of time, and is measured in cycles per second.


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Click here to go to next lesson on The Frequency and Period Relationship.

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The Frequency and Period Relationship

Sunday December 14, 2014

The frequency is the number of cycles that are made in a given period of time, like 10 swings in 5 seconds, and is measured in cycles per second. The period is the inverse of the frequency, given by this equation: T = 1/ν.


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Click here to go to next lesson on Amplitude.

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Amplitude

Sunday December 14, 2014

Amplitude is how high or low the wave is from its original equilibrium position. (not vibrating). How high can you get the swing to go? How far does the car system spring travel over that bump? All these are the amplitude of the vibration.


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Let’s try a sample problem for amplitude:



Click here to go to next lesson on Pendulums.

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Pendulums

Sunday December 14, 2014

The restoring force slows down the object as it moves from its resting but speeds it up when it heads back to the resting position, and that’s what creates the vibration. We’re going to take a look at the forces in a pendulum from the point of view of Newton’s Laws.
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Click here to go to next lesson on Energy and Vibration.

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Energy and Vibration

Sunday December 14, 2014

There’s more than one way to solve physics problems… and by looking at the total mechanical energy of the system, you’ll be able to solve much more complicated pendulum problems with ease.


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Click here to go to next lesson on Pendulum Period.

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Pendulum Period

Sunday December 14, 2014

You’ll need a pendulum for this experiment. A pendulum is really nothing more than a weight at the end of something that can swing back and forth. The easiest way to make one is to get a string and tape it to the edge of a table. (The string should be long enough so that it swings fairly close to the ground.) Tie a weight to the bottom of your string and you’ve got a pendulum.
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Now, for YOUR part of the experiment we are going to change one of three different things, and only one thing changes at a time. First, we’ll change the length of string and measure the period. Then we’ll change the mass of the object, and then the angle that you start the pendulum from. With each trial you will be changing only ONE of those three things. (The rest of the variables will be constant.)


  1. Make an observation. Play with the pendulum a bit and see how it behaves.
  2. Make a hypothesis. How will the length of string effect the number of swings in 10 seconds? Will there be more, less, or no change in the number of swings as the string gets shorter.
  3. Set a timer for 10 seconds or get someone who has a watch with a second hand to tell you when 10 seconds are up.
  4. Now for the test. Pull the pendulum back as far as you’d like (the pendulum swings smoother if you don’t lift the weight higher than the top of the string).
  5. Start the timer and let go of the weight at the same time.
  6. Count the swings the pendulum makes in 10 seconds. This is your frequency in #cycles per 10 seconds.
  7. Write down what you found (collect the data as shown in the video).
  8. Do two more trials with the string at that same length.
  9. Now change the changing variable. In other words, shorten the string. I would recommend shortening it at least an inch.
  10. Redo steps 3 through 9, recording each time.
  11. Continue shortening the string and doing trials until you get at least five different lengths of string.
  12. Convert frequency to period by taking the inverse. Do this for each trial.

Now report your results. Take a look at your data and see if you find a trend. Do you get more swings as the string shortens, less swings, or does the length of the string matter? Something interesting to notice is that at a certain length you will get 10 swings in 10 seconds or a swing a second. This is why pendulums are used in grandfather clocks. They keep good time!


Click here to go to next lesson on Hooke’s Law.

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Hooke’s Law

Sunday December 14, 2014

How are pendulums like springs? They both vibrate, but how you model them on paper is a little different. Let’s take a look at how you handle springs and what their periodic nature looks like:
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Click here to go to next lesson on Nature of a Wave.

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Nature of a Wave

Sunday December 14, 2014

Since we can’t see soundwaves as they move through the air, we’re going to simulate one with rope and a friend. This will let you see how a vibration can create a wave. You’ll need at least 10 feet of rope (if you have 25 or 50 feet it’s more fun), a piece of tape (colored if you have it), a slinky, and a partner. Are you ready?


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1. Give one end of the rope to your partner.


2. Stretch the rope out so that it is a bit slack.


3. Now move your hand up and down. Feel free to do it several times in a row. Your partner should keep his or her hands as still as possible.


4. Watch the waves move from your hand to the other end of the rope.


5. Now let your partner create waves.


6. If you wish, you can try to time your vibrations and create waves with specific frequencies. A frequency of one Hertz is fairly easy to do (one rope shake per second). Can you create rope waves of higher frequencies? You may find that your arm gets tired pretty quickly!


Your hand is the vibrating particle. As your hand vibrated up and down, you moved the particles of the rope up and down. As those particles of rope vibrated, they vibrated the particles next to them. As they vibrated, they vibrated the particles next to them and so on and so forth. So the wave moved from your hand across the room. Did your hands move across the room. Nope, but the wave you created with your vibrating hand did.


This is the way energy travels. Why is the rope wave energy? Because the particles moved a distance against a force. Work was done on the particles. In fact, when you shook the rope, your energy from your body moved across the room with the wave and was transferred (moved to) your partner. Your partner’s hands could feel the energy you put into the rope in the first place. The work you did on the rope was transferred by the rope wave and did work on your partners hand. You have moved energy across the room!


Now… let’s add another element to this experiment…


Transverse Waves

1. Put a piece of (colored if possible) tape in about the middle of the rope.


2. Tie your rope to something or let your friend hold on to one end of it.


3. Now pull the rope so that it is a bit slack but not quite touching the floor.


4. Vibrate your arm. Move your arm up and down once and watch what happens.


5. Now, vibrate your arm a bunch of times (not too fast) and see the results. Notice the action of the tape in the middle of the rope.


transvWhat you’ve done is create a transverse wave. With a transverse wave, if the particle (in this case your hand) moves up and down, the wave will move to the left and/or right of the particle. The word perpendicular means that if one thing is up and down, the other thing is left and right. A transverse wave is a wave where the particle moves perpendicular to the medium. The medium is the material that’s in the wave. The medium in this case is the rope.


For example, in a water wave, the medium is the water. Your hand moved up and down, but the wave created by your hand moved across the room, not up. The wave moved perpendicular to the motion of your hand. Did you take a look at the tape? The tape represents a particle in the wave. Notice that it too, was going up and down. It was not moving along the wave. In any wave the particles vibrate, they do not move along the wave.


Longitudinal Waves

Now that you’ve seen a transverse wave, let’s take a look at a longitudinal wave. Here’s what you do:


1. Put a piece of tape on one slinky wire in the middle or so of the slinky.


2. Let your friend hold on to one end of the slinky or anchor the slinky to a chair or table.


3. Now stretch the slinky out, but not too far.


4. Quickly push the slinky toward your friend, or the table, and then pull it back to its original position. Did you see the wave?


5. Now do it again, back and forth several times and watch where the slinky is bunched up and where it’s spread out.


6. Notice the tape. What is it doing?


longitudinalHere you made a longitudinal wave. A longitudinal wave is where the particle moves parallel to the medium. In other words, your hand vibrated in the same direction (parallel to the direction) the wave was moving in. Your vibrating hand created a wave that was moving in the same direction as the hand was moving in. Did you take a look at the tape? The tape was moving back and forth in the same direction the wave was going.


Do you see the difference between a transverse wave and a longitudinal wave? In a transverse waves the particles vibrate in a different direction (perpendicular) to the wave. In a longitudinal wave the particles vibrate in the same direction (parallel) to the wave.


What’s the Difference between Amplitude and Wavelength?

Here’s an easy way to get a feel for amplitude:


1. Put a piece of tape in about the middle of the rope.


2. Tie your rope to something or let your friend hold on to one end of it.


3. Now pull the rope so that it is a bit slack but not quite touching the floor.


4. Your friend should hold their hands as still as possible.


5. Vibrate your hand but only move it up and down about a foot or so. Have your partner pay attention to how that feels when the wave hits him or her.


6. Now, vibrate your hand but now move it up and down 2 or 3 feet. How does that feel to your partner?


7. Have your partner do the vibrating now and see what you feel.


You created two different amplitude waves. The first wave had a smaller amplitude than the second wave. What you and your partner should have felt was more energy the second time. The wave should have hit your hand with more energy when the wave had more amplitude.


Here’s a great way to visualize wavelength:


1. Tie your rope to something or let your friend hold on to it.


2. Now pull the rope so that it is a bit slack but not quite touching the floor.


3. Your friend should hold their hands as still as possible.


4. Now begin vibrating your hand fairly slowly. In this case, it works better if you move your hand in a circle.


5. Try to make a wavelength with the rope. In other words it will look like you’re playing jump rope.


6. Now try a one and a half wavelengths.


7. Can you get two or more wavelengths? You’ve really got to get your hand moving to get it.


waves


In this image, the left wave is ONE wavelength, the middle is 1.5 wavelegnths, and the right is TWO wavelengths.  See the difference?


Did you notice how the frequency of your hand determined the wavelength of the rope? The faster your hand, moved the more wavelengths you could get.
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Click here to go to next lesson on Motion of Waves.

Motion of Waves

Sunday December 14, 2014

This is a recording of a recent live teleclass I did with thousands of kids from all over the world. I’ve included it here so you can participate and learn, too!


Sound is a form of energy, and is caused by something vibrating. So what is moving to make sound energy?


Molecules. Molecules are vibrating back and forth at fairly high rates of speed, creating waves. Energy moves from place to place by waves. Sound energy moves by longitudinal waves (the waves that are like a slinky). The molecules vibrate back and forth, crashing into the molecules next to them, causing them to vibrate, and so on and so forth. All sounds come from vibrations.


Materials:


  • 1 tongue-depressor size popsicle stick
  • Three 3″ x 1/4″ rubber bands
  • 2 index cards
  • 3 feet of string (or yarn)
  • scissors
  • tape or hot glue

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What’s Going On?

Do you remember where all waves come from? Vibrating particles. Waves come from vibrating particles and are made up of vibrating particles.


Here’s rule one when it comes to waves…. the waves move, the particles don’t. The wave moves from place to place. The wave carries the energy from place to place. The particles however, stay put. Here’s a couple of examples to keep in mind.


If you’ve ever seen a crowd of people do the ‘wave’ in the stands of a sporting event you may have noticed that the people only vibrated up and down. They did not move along the wave. The wave, however, moved through the stands.


Another example would be a duck floating on a wavy lake. The duck is moving up and down (vibrating) just like the water particles but he is not moving with the waves. The waves move but the particles don’t. When I talk to you, the vibrating air molecules that made the sound in my mouth do not travel across the room into your ears. (Which is especially handy if I’ve just eaten an onion sandwich!) The energy from my mouth is moved, by waves, across the room.


Questions to Ask

  • Does the shape of the index card matter?
  • What happens if you change the number of rubber bands?
  • What if you use a different thickness rubber band?
  • What happens if you make the string longer or shorter?
  • can you make a double by stacking two together?
  • Can you get a second or third harmonic by swinging it around faster?
  • Why do you need the index card at all?

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Click here to go to next lesson on Energy of a Wave.

Energy of a Wave

Sunday December 14, 2014

Since  the particles don’t travel with the wave, what does a wave carry? Waves transport energy, not particles (or matter).
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Click here to go to next lesson on Mechanical and Electromagnetic Waves.

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Mechanical and Electromagnetic Waves

Sunday December 14, 2014

Some waves need a medium to travel through while others do not. Mechanical waves need a medium for the wave to travel through to transport energy. Ocean waves, jump ropes, pendulums, sound, and waves in a stadium are all examples of mechanical waves.
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Electromagnetic  waves do not need a medium to travel through. Light from the sun reaches up 93 million miles away by traveling though the vacuum of space because it’s an electromagnetic wave. The electromagnetic wave travels through the vacuum of space at the speed of light (299,792,458 m/s). These types of waves are made by vibrating charged particles, and we’re going to look at this more in depth in our next section on Light.


Matter waves are the ones you get to learn about when you study quantum physics, as they describe the way that matter (like a beam of electrons) under certain conditions acts like a wave. That’s way out of our scope here, but I want you to at least be aware that they exist.


Click here to go to next lesson on Properties of a Wave.

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Properties of a Wave

Sunday December 14, 2014

The words particle and wave are two words you’ll see in nearly every area of physics, but they are actually very different from each other. A particle is a tiny concentration of something that can transmit energy, and a wave is a broad distribution of energy that fills the space it passes through. We’re going to look at particles in more depth later, and instead focus on understanding waves.
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A wave traveling along a stretched string can have different shapes, but every wave will have a frequency and a wavelength. Wavelength refers to the repeating wave shape, and frequency refers to the oscillating source that make the wave in the first place. Waves are defined by a math equation that we’ll get to a little later. First, let’s take a look at the different parts of a wave.



A wave number is the number of waves per unit length.


For longitudinal waves, a rarefaction is the spot where the the wave is traveling and is the most “stretched out” (minimum wave density). The compression is the spot where the wave is most squished together. You can see this easily if you play with a slinky… the coils that are most spread out are a rarefaction point.


Click here to go to next lesson on Period, Frequency, Amplitude and Wavelength.

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Period, Frequency, Amplitude and Wavelength

Sunday December 14, 2014

A wave can have many different shapes, but it has a very specific frequency ν and wavelength λ.
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Click here to go to next lesson on Humming Bird.

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Humming Bird

Sunday December 14, 2014

Humming birds are really fascinating, because they can beat their wings so fast! Here’s a quick way to calculate the frequency and period of a humming bird’s wings.
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Click here to go to next lesson on Swinging Kid.

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Swinging Kid

Sunday December 14, 2014

Kids love swings, and it’s amazingly simple to find the frequency and period of the swing. Here’s how…
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Click here to go to next lesson on Energy and Wave Amplitude.

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Energy and Wave Amplitude

Sunday December 14, 2014

How is the energy related to the wave frequency, amplitude and wavelength?
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Click here to go to next lesson on Wave Speed.

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Wave Speed

Sunday December 14, 2014

Sound is a type of energy, and energy moves by waves. So sound moves from one place to another by waves; longitudinal waves to be more specific. So, how fast do sound waves travel? Well, that’s a bit of a tricky question. The speed of the wave depends on what kind of stuff the wave is moving through. The more dense (thicker) the material, the faster sound can travel through it.
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Remember that waves move because the particles bounce off one another? Well, the farther the particles are from one another, the longer it takes one particle to bounce off another. Think about a row of dominoes. If you put them all close together and push one over they all fall down pretty quick. If you spread them out a bit, the row falls much more slowly. Sound waves move the same way.


Click here to go to next lesson on More on Wave Speed .

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More on Wave Speed

Sunday December 14, 2014

Sound moves faster in solid objects than it does in air because the molecules are very close together in a solid and very far apart in a gas. For example, sound travels at about 760 mph in air, 3300 mph in water, 11,400 mph in aluminum, and 27,000 mph in diamond!
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The temperature of the material also makes a difference. The colder the material, the faster the sound. This is why sound seems to be louder or clearer in the winter or at night. The air is a little cooler and since it’s cooler, the molecules are a little more tightly packed.


The speed of a wave is based on the basic distance over time relationship. If you watch the crest of a wave, the speed is how fast the crest is observed to move a distance.


Click here to go to next lesson on Echoes.

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Echoes

Sunday December 14, 2014

An echo is when a wave travels through one medium (like air) and then meets a different medium (like a cave wall). The sound wave bounces and reflects back to you.
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Click here to go to next lesson on Bird Waves.

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Bird Waves

Sunday December 14, 2014

Let’s do a couple of simpler sample problems, and then I’ll show you how to do problems that are more complex and involve higher level math. First, let’s take a look at the wings of a bird in flight…
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Click here to go to next lesson on Sitting on the Ocean.

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Sitting on the Ocean

Sunday December 14, 2014

Ocean waves travel on the surface of the water can be observed and measured. Let’s try one just before a storm…
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Click here to go to next lesson on Seasick Waves.

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Seasick Waves

Sunday December 14, 2014

Ever gotten sea sick? It’s usually because the motion of what your body detects is different from what your eyes see. Let’s take a look at how you can calculate the wave speed by watching two boats bobbing up and down (without getting sick).
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Click here to go to next lesson on Wave Speed on Tight Strings.

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Wave Speed on Tight Strings

Sunday December 14, 2014

Waves traveling on a tight string, like a climbing rope, are dependent on only two things: the tension of the rope and a physical property of the rope (like what it’s made out of, the diameter, etc.).
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Click here to go to next lesson on Wave Equations.

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Wave Equations

Sunday December 14, 2014

Now it’s time for a little more math because the physics problems are going to get a little harder.
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The wave equation for transverse and longitudinal waves moving in the +x direction looks like this:



where ym is the amplitude of the wave, κ is the angular wave number, ω is the angular frequency, and (κx-ωt) is the phase.


The wavelength L and the wave number k are found by the following equation: 



where K is the number of waves per meter.


The period (T) and the frequency (ν) are related to ω by the following equation:



The main wave speed equation is given by:



Note that the first letter v is for velocity, and the last letter in the equation is the greek letter “nu” (ν) for frequency.


The problems in the video  involve using the first equation that relates the distance and time to find the amplitude, wave number, and frequency of a wave. (This is a typical problem that you’ll see in college level physics.)


Click here to go to next lesson on Waves on a Stretched String.

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Waves on a Stretched String

Sunday December 14, 2014

If a wave can travel through mediums like air, water, strings, rocks, etc., then it makes sense that as the wave moves through these mediums, the tiny particles that make up the medium will also vibrate. In order for this to happen, the medium has to have a way for energy (both potential and kinetic) to be stored, so the medium has both inertia and elasticity.
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The wave equation for a stretched spring is:



where the first letter v is for velocity, μ is the linear density of the string, and the tension is τ.



You can’t send a wave along a straight string without stretching the string. The tension in the string that does the stretching is the elasticity of the string that stores the potential energy as the wave passes through. The amount of tension in the rope will affect the wave speed. The wave speed doesn’t change if you change the frequency, however it will travel faster through a tighter rope.


This means that the speed of the wave depends on the medium, and not the wave itself. For example, waves travel faster through solid rock than they do through air because the particles in the medium are much closer together and can transmit the wave faster.


Here’s the power part of the video above explained in more detail:




 


Click here to go to next lesson on Behavior of Waves

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Circuit Connections

Monday December 8, 2014

It’s easy to see how the current flows through a circuit that has only one component, like one LED connected to the battery. If it’s a 3 Volt battery, then there’s 3 Volts across the LED also.


But what if there are two or three LEDs? How does the voltage look across each one? What if the LEDs are different sizes? Does it matter how you hook them up, meaning does one way make the LED last monger or glow brighter? Let’s take a look at the difference between series and parallel connections.


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You can have resistors in series, batteries in parallel, motors in series and parallel, and hundreds more different combinations! It all depends on what you want to do and how you want to do it.


For example, a 6V lantern battery is actually made up of four D-cell batteries connected in series. Each D-cell is 1.5V, and when you add four of them up, you get 6V. The older models of these used to have 24 AA batteries which were connected in series and parallel to make 6V. We’re going to learn how to decide whether to use a series or parallel circuit, depending on what we’re building and what we need the circuit to do.


Which bring up another point… what’s the difference between a D-cell and a AAA battery? They both are 1.5 volts. Why use one over the other? And 9V batteries are smaller and lighter than a D-cell but have  6 times the voltage… why wouldn’t yo use 9V for everything?


It has to do with current (the rate of charge flowing through the circuit). A D-cell has much more current than a AAA battery, and you’d use D-cells for things like motors. AAA are perfect for LEDs and other low-current devices that don’t need as much amps to get them going. In your circuit, you want to choose the best option that will still work voltage and amperage-wise, is the least expensive, and lasts long enough so you’re not changing batteries every few minutes.


Click here to go to your next lesson on Series Circuits.

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Series Circuits

Monday December 8, 2014

Each electrical component is connected so that there’s only one option for the current to flow. There’s no branches or alternate routes for the electricity… it’s only got one way to move through the circuit. When you add more electrical components, like motors or LEDs to this circuit, the overall resistance in the circuit decreases since there’s only one path for the current.


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Let’s look at how charge flows through a series circuit:



Click here to go to your next lesson on Parallel Circuits.

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Parallel Circuits

Monday December 8, 2014

Each device, like a motor or LED, has its own branch lines in a parallel circuit, which means that the electricity has many different ways that it can travel along the circuit lines. When scientists and engineers draw electrical diagrams, they put one electric component on each branch, even if in reality there’s more than one when you actually build the thing.


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Unlike series circuits, when you add another branch, you also allow more pathways for the electricity to follow.



Click here to go to your next lesson on Calculating and Measuring Current.

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Calculating and Measuring Current

Monday December 8, 2014

Current and charge aren’t quite the same thing. Current is rate that charge goes through a circuit (just like acceleration is the rate of change in velocity… acceleration and velocity aren’t the same thing either, but they are related). Remember, charge doesn’t get used up by electrical components like LEDs or resistors.


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Click here to go to your next lesson on Ohm’s Law and Current.

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Ohm’s Law and Current

Monday December 8, 2014

One more thing to note about the difference between series and parallel circuits is that in a parallel circuit, the current in each branch can be different, but they all add up to be the same everywhere once you reduce the branches into a single branch. Just like the water analogy, when you connect the main hose into five different smaller hoses, the sum of all five is going to equal flow through the main hose.


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Click here to go to your next lesson on Ohm’s Law and Power.

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Series and Parallel Circuits in Everyday Life

Monday December 8, 2014

It’s easy to get sucked into doing math and equations all day without understanding how it applies to the real world. Let’s take a look at how to actually hook up series and parallel circuits in everyday life, and how they are different, and when to use each one…


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Click here to go to your next lesson on Breadboards.

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Breadboards

Monday December 8, 2014

Although you can’t see electricity, you can certainly detect its effects – a buzzer sounding, a light flashing, a motor turning… all of these happen because of electricity. Which is why electricity experiments are among the most frustrating. You can’t always tell where the problem is in a circuit that refuses to work right.


We’re going to outline the different electronic components (resistors, capacitors, diodes, transistors, etc) so you get a better feel for how to use them in a circuit. While we’re not going to spend time on why each of these parts work (which is a topic best reserved for college courses), we are going to tackle how to use them to get your circuit to do what you want. The steps to building several different electronics projects are outlined very carefully so you can really understand this incredible micro-world.


In this video, you’ll learn how to identify each electronic component.  You’ll also learn how to use a breadboard to quickly build circuits that can be easily changed.  Plus, you’ll learn how to make sure you don’t damage your components.


Before you use a breadboard, you need to know how the “holes” in it connect to each other.  Once you get this, they’re easy to use, but until you understand their secret, they can be totally confusing.  Be sure to pay attention to this part, and it will make things a lot easier.  Once you have this down, you’ll wire up a few simple circuits on the breadboard just to try out your new knowledge.


Which part is which? Click here to access a reference sheet so you can tell which resistor is which.


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Click here to go to your next lesson on Series Switches.

Series Switches

Monday December 8, 2014

Switches are a lot of fun, they allow us to control when a circuit is on and when it’s off. In this experiment we’ll see how two switches can be connected in series to control when a single device is on or off.


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Click here to go to your next lesson on Parallel Switches.

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Parallel Switches

Monday December 8, 2014

We’ve seen how switches can be used to turn stuff on and off, now let’s take a look at how a switch in different locations can turn the same thing on or off. A lot like a doorbell with a button at the front door and a button at the back door.


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Click here to go to your next lesson on Light Actuated Circuit.

Light Actuated Circuit

Monday December 8, 2014


In this circuit, we’re going to use a special kind of resistor, called a CdS photocell to detect light and dark.  When light is shined on the photocell, the LED will light up.  When it is dark, the LED goes out.  And with just a little light, the LED is dim.  Remember the explanation of how a transistor works?  We talked about having a small voltage (or current) control a larger one, kind of like turning the knob on a light switch dimmer in your house?  That’s what this circuit is doing.


The photocell is a kind of resistor that changes it’s resistance depending on how light or dark it is.  In this circuit, when it is light, the photocell delivers more current to the base lead of the transistor.   When this happens, the transistor allows more voltage to flow from the emitter lead to the collector lead, which in turn lights up the LED.  One resistor are simply used to reduce the amount of current that goes into the transistor (so it doesn’t get too much current)  if the photocell has a really low resistance because of how much light is on it.  The other one is called a pull-down resistor.  Think of it like a door closer spring for electricity.  A door closer closes the door when you let go of it (instead of leaving the door to sway in the wind).  A pull-down resistor makes sure that when the transistor is “off”, it will “spring” toward a connection to the negative side of the battery (a pull-up makes it spring toward the positive).


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If this is all confusing, don’t worry.  It will make more sense as you build more circuits and look at more schematic diagrams.


Click here for Unit 14 (Lesson 1) schematics.


This light-actuated relay circuit will remain actuated for a brief time after you remove the light.  You can substitute a 1.5-3V buzzer for the LED if you’d rather have a loud circuit.


Materials:


  • breadboard
  • AA battery case
  • 2 AA batteries
  • LED
  • CdS photocell
  • transistor (2N2222A)
  • 4.7k resistor
  • 1k resistor
  • jump wires
  • wire strippers

Here’s what you do:



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Click here to go to your next lesson on Light De-actuated Circuit.

Light De-actuated Circuit

Monday December 8, 2014

Make sure you’ve already made the Light Actuated Circuit before starting this project!


Photoresistors (also called CdS photocells) are made of a material that reacts with light, very similar to solar cells.  When light hits the material, it knocks a few electrons loose. When you hook up the cell to a circuit, the electrons now have a place to go, and electricity flows through your wires. You’ll notice your CdS cell works when you shine a light on it from either the front side or the back side. If you want to use a phototransistor, make a note as to the frequency of light it’s been tuned to – some will only work with IR light (like your remote control or sunlight).


In this circuit, the LED is actuated only when the photoresistor is dark. If you want a faster response to your light, you can substitute a phototransistor for the photoresistor (CdS cell) and adjust the value of the 1-kOhm resistor (change it lower or higher, or use a potentiometer) to control the sensitivity.


This is basically the same as the light-actuated circuit.  The difference is that the transistor is connected to control power to the LED in the opposite way as the light-actuated circuit.  So, as there is more light on the photocell (and the base lead gets more current), the voltage to the transistor is reduced.


Click here for Unit 14 (Lesson 1) schematics.


Notice what is the same in the circuit, and what is different.


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Materials:


  • breadboard
  • AA battery case
  • 2 AA batteries
  • LED
  • CdS photocell
  • transistor (2N2222A)
  • 4.7k resistor
  • 1k resistor
  • jump wires
  • wire strippers


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Click here to go to your next lesson on How to Read Schematics.

How to Read Schematics

Monday December 8, 2014

Do you remember the first time you tried to read a map? There were all those weird symbols and curving lines that you had to figure out before you could get anywhere. Electric circuits are kind of the same way… people use schematic diagrams to write down how their circuit is wired so others can build it, too.


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Now that you’ve built a couple of the breadboard circuits, it’s time to figure out how to read the symbols and build a circuit from a diagram. At first, it may seem a bit overwhelming with all the strange symbols and lines drawn all over the page, but don’t worry – after a couple of circuits, you’ll be cruising through these like a pro.


The first thing you need to do is download the schematics (if you haven’t already) and then watch this video:



Click here to go to your next lesson on Transistor Circuits.

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Transistor Circuits

Monday December 8, 2014


This Flashing Circuit used to be a real ‘wowser!’ with students before LEDs become commonplace (around 1995). You’re going to build a circuit that has a control knob that will allow you to set the flash speed of the LED. You can try different LEDs or mini-lamps to see what kind of an effect you get. Are you ready?


NPN and PNP transistors are similar in that when current is applied to the base, electricity flows through them.  But, they way they are used is different.  NPN transistors are often used to control whether a circuit is completed by connecting it to ground or not, where PNP control the positive current going into a device (or portion of a circuit).  NPN transistors are often used where larger currents need to be controlled, because it’s easier for a transistor to control the ground side of a circuit than the plus power side of it.


So, why does the LED flash?  Remember, a capacitor is like a storage tank for electricity.  You fill it up, then empty it out.  But, it takes time to fill up and empty.  This circuit uses the time it takes to fill and empty as a delay for turning the LED on and off.  How fast it fills up depends on the value of the resistor that is connected to it.  We’re using a variable resistor, so we can adjust how fast it fills up, and thus adjust the flash rate.


Click here for Unit 14 (Lesson 1) schematics.


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Materials:


  • breadboard
  • AA battery case
  • 2 AA batteries
  • LED
  • potentiometer
  • transistor (2N2222A and 2N4403)
  • 5.6k resistor
  • 1k resistor
  • 10k resistor
  • jump wires
  • wire strippers


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Click here to go to your next lesson on Audible Probes.

Audible Light Probe

Monday December 8, 2014

Resistors look like candy-striped hot dogs. Their job is to limit current to keep sensitive electronics from being overloaded. If you break open a resistor, you’ll find a pile of graphite. If you have a digital multimeter, draw a line on a sheet of paper with a graphite pencil, and place one probe near the end of the line. You can measure the change in resistance along the line with your other multimeter probe!


Make sure you’ve made the Light Flasher before starting this circuit. This circuit, the Audible Light Probe, is actually a very sensitive circuit that will emit all sorts of sounds reminiscent of junior high school boys locker rooms. The frequency from the speaker will change as the light intensity changes. One of the neat features about this circuit is that it will allow you to test different transistors (both PNP and NPN) to see (hear) the changes. You can also play with the capacitor and resistor values to change the range.


Click here for Unit 14 (Lesson 1) schematics.


TIP: If your tones won’t stop, or are too high to hear, try operating your circuit in a very dark room before adding the light. Can you get your speaker to *click*?


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Materials:


  • breadboard
  • AA battery case
  • 2 AA batteries
  • LED
  • CdS photocell
  • transistor (2N2222A and 2N4403)
  • 1k resistor
  • 0.47-?f electrolytic capacitor
  • 8-ohm speaker
  • jump wires
  • wire strippers


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Click here to go to your next lesson on Lie Detectors.

Lie Detectors

Monday December 8, 2014

Lie Detectors are electronic circuits that are able to measure your skin’s resistance. When you sweat (or if your skin is wet), the resistance is different than if it’s dry.


However since most people don’t sweat when they lie, this type of detector isn’t the most reliable type of detector around, but it’s one of the simplest to create. We’re going to build one from simple electronic components like resistors, capacitors, and transistors.


Our lie detector uses a speaker that changes pitch depending on the resistance of your skin – it’s much more entertaining than blinking an LED on or off. You can think of this circuit as more of a skin humidity indicator. Are you ready?


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Materials:


  • Breadboard (2″x3″, 400-hole)
  • Hookup wire (AWG 22g, solid), 6 feet
  • 4.7K-ohm resistor
  • 100K-ohm resistor
  • PN2222A or 2N3904 (NPN) transistor
  • 2N3906 or 2N4403 (PNP) transistor
  • 0.01 micro-f (mf) capacitor – Code on capacitor: 103
  • 8-ohm speaker
  • 9V battery snap and fresh 9V battery



Download your Current Electricity Problem Set here.

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Special Science Teleclass: Chemistry

Monday December 8, 2014

This is a recording of a recent live teleclass I did with thousands of kids from all over the world. I’ve included it here so you can participate and learn, too! (Click here if you’re looking for the more recent version that also includes Chemical Engineering.)


When you think of slime, do you imagine slugs, snails, and puppy kisses? Or does the science fiction film The Blob come to mind? Any way you picture it, slime is definitely slippery, slithery, and just plain icky — and a perfect forum for learning real science. But which ingredients work in making a truly slimy concoction, and why do they work? Let’s take a closer look…


Materials:


  • Sodium tetraborate (also called “Borax” – it’s a laundry whitener) – about 2 tablespoons
  • Clear glue or white glue (clear works better if you can find it) – about 1/2 cup
  • Yellow highlighter
  • Pliers or sharp razor (with adult help). (PREPARE: Use this to get the end off your highlighter before class starts so you can extract the ink-soaked felt inside. Leave the felt inside highlighter with the end loosely on (so it doesn’t dry out))
  • Resuable Instant Hand Warmer that contains sodium acetate (Brand Name: EZ Hand Warmer) – you’ll need two of these
  • Scissors
  • Glass half full of COLD water (PREPARE: put this in the fridge overnight)
  • Mixing bowl full of ice (PREPARE: leave in freezer)
  • Salt
  • Disposable aluminum pie place or foil-wrapped paper plate
  • Disposable cups for solutions (4-6)
  • Popsicle sticks for mixing (4-6)
  • Rubber gloves for your hands
  • Optional: If you want to see your experiments glow in the dark, you’ll need a fluorescent UV black light (about $10 from the pet store – look in cleaning supplies under “Urine-Off” for a fluorescent UV light). UV flashlights and UV LEDs will not work.

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Key Concepts

If you’ve ever mixed together cornstarch and water, you know that you can get it to be both a liquid and a solid at the same time. (If you haven’t you should definitely try it! Use a 2:1 ratio of cornstarch:water.) The long molecular chains (polymers) are all tangled up when you scrunch them together (and the thing feels solid), but the polymers are so slick that as soon as you release the tension, they slide free (and drips between your fingers like a liquid).


Scientists call this a non-Newtonian fluid. You can also fill an empty water bottle or a plastic test tube half-full with this stuff and cap it. Notice that when you shake it hard, the slime turns into a solid and doesn’t slosh around the tube. When you rotate the tube slowly, it acts like a liquid.


Long, spaghetti-like chains of molecules (called polymers) don’t clump together until you cross-link the molecule strands (polymers) together into something that looks more like a fishnet. This is how we’re going to make slime.


What’s Going On?

Imagine a plate of spaghetti. The noodles slide around and don’t clump together, just like the long chains of molecules (called polymers) that make up slime. They slide around without getting tangled up. The pasta by itself (fresh from the boiling water) doesn’t hold together until you put the sauce on. Slime works the same way. Long, spaghetti-like chains of molecules don’t clump together until you add the sauce – something to cross-link the molecule strands together.


The borax mixture holds the glue mixture together in a gloppy, gelatinous mass. In more scientific terms, the sodium tetraborate cross-links the long polymer chains in the glue to form the slime.


Why does the slime glow? Note that a black light emits high-energy UV light. You can’t see this part of the spectrum (just as you can’t see infrared light, found in the beam emitted from the remote control to the TV), which is why “black lights” were named that. Stuff glows because fluorescent objects absorb the UV light and then spit light back out almost instantaneously. Some of the energy gets lost during that process, which changes the wavelength of the light, which makes this light visible and causes the material to appear to glow.


Questions to Ask

  1. What happens when you freeze your slime? Is there a color change?
  2. How long does it take to thaw your slime in the microwave?
  3. Do you see the little bubbles in your slime?
  4. How many states of matter do you have in your slime now?
  5. Does this work with any laundry detergent, or just borax?
  6. What happens if you omit the water in the 50-50- glue-water mixture, and just use straight glue? (Hint – use the glow juice with the borax to keep the glowing feature.)
  7. Does your slime pick up newsprint from a newspaper?
  8. What other kinds of glue work well with this slime?

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Electrical Resistance

Monday December 1, 2014

As electrons move through a load in an electric circuit (devices like a LED, buzzer, motor, etc.), they experience resistance (even through the wire itself), which corresponds to a drop in energy . This drop in energy is referred to as a voltage drop.  Resistance hinders the flow of electrons, even in the water itself.  You can think of resistance as the friction between the water and the pipe along the inside of the pipe.


The pipe, just like the wire, has a certain diameter and length. The longer the wire, the more resistance the electron will encounter, just as with a long pipe of water. If you increase the pipe diameter, more water will flow through it. The thicker the wire, the more current flows through it. The amount of resistance the charge encounters also depends on what the wire is made out of. Certain materials are more electrically conductive than others, with silver, copper and gold being at the very low end of electrical resistance (which is why most wires are made from copper, which is the least expensive of the three).


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It’s easy to find the resistance of objects using a simple mathematical equation that relates the length of the wire, the cross-sectional area, and the resistivity of the material. Here’s an example of how to do it:



Click here to go to your next lesson on Resistance over Distance.

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Resistors

Monday December 1, 2014

Resistors are a very important part of electronics, in fact without them we wouldn’t even have electronics. Resistors help us control the flow of current through a circuit and protect components from being damaged. This video talks about what resistors are, how to read them, and how to use them.


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Click here to go to your next lesson on Potentiometers.

Potentiometers

Monday December 1, 2014

potSo now you know how to hook up a motor, and even wire it up to a switch so that it goes in forward and reverse. But what if you want to change speeds? This nifty electrical component will help you do just that.


Once you understand how to use this potentiometer in a circuit, you’ll be able to control the speed of your laser light show motors as well as the motors and lights on your robots. Ready?


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Here’s what you need:


  • 2 AA batteries
  • AA battery case
  • 3 alligator wires
  • potentiometer
  • 1.5-3V DC motor
  • LED (any you choose)


Download Student Worksheet & Exercises


Exercises


  1. How does a potentiometer work?
  2. Does the potentiometer work differently on the LED and the motor?
  3. Name three places you’ve used potentiometers in everyday life.
  4. How do you think you might wire up an LED, switch, and potentiometer?

Click here to go to your next lesson on Measuring Voltage and Current.

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Measuring Voltage and Current

Monday December 1, 2014

meterOne of the most useful tools a scientist can have! A digital multimeter can quickly help you discover where the trouble is in your electrical circuits and eliminate the hassle of guesswork. When you have the right tool for the job, it makes your work a lot easier (think of trying to hammer nails with your shoe).

We'll show you how to get the most out of this versatile tool that we're sure you're going to use all the way through college. This project is for advanced students.

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Download Student Worksheet & Exercises

Exercises

  1. If you measure 2.65 volts from your battery pack, do you need new batteries or will they work?
  2. How do you think you would measure the resistance of an LED?
  3. Reset your meter for a quick practical test: Remove the wires from your DMM and set the dial at OFF. Wave your hand wildly and show how you can use the meter (you can add probes and turn it on now) to test the voltage on your LED in a simple circuit doing the steps from the experiment.

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Click here to go to your next lesson on Ohm's Law.

 


Ohm’s Law

Monday December 1, 2014

One of the most important equations in current electricity is: V = IR. With one glance, you can see how current, voltage, and resistance are related to each other. If the current decreases, so does the voltage. Charge  mores when the resistance decreases.


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Click here to go to your next lesson on Applying Ohm’s Law.

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Power and Ohm’s Law

Monday December 1, 2014

There are two main equations for power in the field of current electricity:


P = I2 R


P = V2/R


where P = power (watts), V = voltage (volts), R = resistance (ohms), I = current (amps)


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Using these three equations (the two above and V = IR) will solve most of your problems in physics having to do with current electricity. However, there are a few things to keep in mind before you dive in too deeply…



Click here to go to your next lesson on Hair Dryers.

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Hair Dryer Problem

Monday December 1, 2014

I do have a LOT of hair on my head. Here’s a neat way to figure out how much current I use every morning I fire up the hair dryer:


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Click here to go to your next lesson on Toasters.

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Special Science Teleclass: Thermodynamics

Monday December 1, 2014

This is a recording of a recent live teleclass I did with thousands of kids from all over the world. I’ve included it here so you can participate and learn, too!


You’ll discover how to boil water at room temperature, heat up ice to freeze it, make a fire water balloon, and build a real working steam boat as you learn about heat energy. You’ll also learn about thermal energy, heat capacity, and the laws of thermodynamics.


Materials:


  • cup of ice water
  • cup of room temperature water
  • cup of hot water (not scalding or boiling!)
  • tea light candle and lighter (with adult help)
  • balloon (not inflated)
  • syringe (without the needle)
  • block of foam
  • copper tubing (¼” diameter and 12” long)
  • bathtub or sink
  • scissors or razor
  • fat marker (to be used to wrap things around, not for writing)

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Key Concepts

The terms hot, cold, warm etc. describe what physicists call thermal energy. Thermal energy is how much the molecules are moving inside an object. The faster molecules move, the more thermal energy that object has.


There are three different scales for measuring temperature. Fahrenheit, Celsius and Kelvin. (There’s also a fourth temperature scale for absolute Fahrenheit called Rankine.) Temperature is basically a speedometer for molecules. The faster they are wiggling and jiggling, the higher the temperature and the higher the thermal energy that object has. Your skin, mouth and tongue are antennas which can sense thermal energy.


There are four states of matter: Solid, liquid, gas and plasma. Solids have strong, stiff bonds between molecules that hold the molecules in place. Liquids have loose, stringy bonds between molecules that hold molecules together but allow them some flexibility. Gasses have no bonds between the molecules. Plasma is similar to gas but the molecules are very highly energized. Materials can change from one state to another depending on the temperature and the bonds. Changing from a solid to a liquid is called melting. Changing from a liquid to a gas is called boiling, evaporating, or vaporizing. Changing from a gas to a liquid is called condensation. Changing from a liquid to a solid is called freezing. All materials have given points at which they change from state to state. Melting point is the temperature at which a material changes from solid to liquid.  Boiling point is the temperature at which a material changes from liquid to gas. Condensation point is the temperature at which a material changes from gas to liquid. Freezing point is the temperature at which a material changes from liquid to gas.


What’s Going On?

Heat is the movement of thermal energy from one object to another. Heat can only flow from an object of a higher temperature to an object of a lower temperature (this is the First Law of Thermodynamics). Heat is movement of thermal energy from one object to another. When an object absorbs heat it does not necessarily change temperature.  Objects release heat as they freeze and condense. Objects absorb heat as they evaporate and melt. Freezing points, melting points, boiling points and condensation points are the “speed limits” of the phases. Once the molecules reach that speed they must change state.


Heat capacity is how much heat an object can absorb before its temperature increases. Specific heat is how much heat energy a mass of a material must absorb before it increases 1°C. Heat capacity is influenced by the specific heat of the material and/or the amount of the material. Each material has its own specific heat. The higher a material’s specific heat is, the more heat it must absorb before its temperature increases. A larger amount of something will have a higher heat capacity then a smaller amount of something. Water has a very high heat capacity.


Questions:

  1. True or False: Water is poor at absorbing heat energy.
  2. True or False: A molecule that heats up will move faster.
  3. True or False: A material will be less dense at lower temperatures.
  4. For gases, if we increase the temperature, what happens to the pressure and the volume?
  5. What is specific heat?
  6. What is heat?
  7. Does heat flow from cold to hot? Give an example.
  8. What do the our body sense, heat flow or temperature? Are they the same thing?
  9. How can we boil room temperature water without heating up the water?

Answers:


  1. False.
  2. True.
  3. False. (Usually.)
  4. If we increase the temperature, the pressure increases and the volume decreases. This is called the Ideal Gas Law (remember the ping pong balls from the teleclass?)
  5. Specific heat is how much heat energy a mass of a material must absorb before it increases 1°C.
  6. Heat is the movement of thermal energy from one object to another.
  7. No. Heat flows from hot to cold. (This is the First Law of Thermodynamics.) A hot cup of coffee left out on a cold morning will eventually cool to the surrounding air temperature.
  8. Heat flow. No they are not the same thing. Temperature is a measure of how much energy the molecules have.
  9. By increasing the pressure by decreasing the volume, we can force the bubbles out of the water and it will boil. Boiling is when the liquid water turns into a gas, NOT when the liquid water heats up. Boiling can happen at many different temperatures when you change the pressure.

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Electric Current

Monday November 24, 2014

Imagine you have two metal plates that are parallel to each other, and one is positively charged ad the other is negatively charged. The direction of the electric field created by these two charged places is from the positive toward the negative plate. (Imagine placing a positive test charge int he field… which way would it go? Away from the positive plate and toward the negative plate… so that’s the direction of the electric field.) Now imagine connecting the two plates with a metal wire. What do you think would happen?


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The positive charges on the positive plate would flow toward the negative plate, evening out the charge until they were both at the same charge, and then there is no electric potential difference between the two. It’s like connecting a hose between two cups filled with different levels of water. The water flows out of the cup with more water into the one with less until they both even out. When the charges even out, there not charge flow because there’s no electrical potential difference.


Click here to go to your next lesson on Electric Circuits!

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Electric Circuits

Monday November 24, 2014

An electrical circuit is like a NASCAR raceway.  The electrons (racecars) zip around the race loop (wire circuit) superfast to make stuff happen. Although you can’t see the electrons zipping around the circuit, you can see the effects: lighting up LEDs, sounding buzzers, clicking relays, etc.


There are many different electrical components that make the electrons react in different ways, such as resistors (limit current), capacitors (collect a charge), transistors (gate for electrons), relays (electricity itself activates a switch), diodes (one-way street for electrons), solenoids (electrical magnet), switches (stoplight for electrons), and more.  We’re going to use a combination diode-light-bulb (LED), buzzers, and motors in our circuits right now.


A CIRCUIT looks like a CIRCLE.  When you connect the batteries to the LED with wire and make a circle, the LED lights up.  If you break open the circle, electricity (current) doesn’t flow and the LED turns dark.


LED stands for “Light Emitting Diode”.  Diodes are one-way streets for electricity – they allow electrons to flow one way but not the other.


Remember when you scuffed along the carpet?  You gathered up an electric charge in your body.  That charge was static until you zapped someone else.  The movement of electric charge is called electric current, and is measured in amperes (A). When electric current passes through a material, it does it by electrical conduction. There are different kinds of conduction, such as metallic conduction, where electrons flow through a conductor (like metal) and electrolysis, where charged atoms (called ions) flow through liquids.


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Here’s what you need:


  • 2 AA batteries
  • AA battery case
  • 2 alligator wires
  • LEDs (any you choose is fine)


Download Student Worksheet & Exercises


Be alert for:


1. Batteries inserted into the case the wrong way!


2. LED in the wrong way (LEDs are picky about plus and minus – they are POLARIZED)


3. Is there a metal-to-metal connection?  (You’re not grabbing ahold of the plastic insulation, are you?)


4. Bad wires can cause headaches – if all else fails, then swap out your alligator clip lead wires for new ones.


Exercises


  1. What does LED stand for?
  2. Does it matter which way you wire an LED in a circuit?
  3. Does the longer wire on the LED connect to plus (red) or minus (black)?
  4. Do you need to hook up batteries to make a neon bulb light up?  Why or why not?
  5. What’s the difference between a light bulb and your LED?
  6. What is the difference between a bolt of lightning and the electricity in your circuit?
  7. What is the charge of an electron?

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Click here to go to your next lesson on Detecting Current!

Detecting Current

Monday November 24, 2014

Galvanometers are coils of wire connected to a battery. When current flows through the wire, it creates a magnetic field. Since the wire is bundled up, it multiplies this electromagnetic effect to create a simple electromagnet that you can detect with your compass.
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Here’s what you need to do:


Materials:


  • magnet wire
  • sand paper
  • scissors
  • compass
  • AA battery case
  • 2 AA batteries
  • 2 alligator clip wires


Download Student Worksheet & Exercises


1. Remove the insulation from about an inch of each end of the wire. (Use sandpaper if you’re using magnet wire.)


2. Wrap the wire at least 30-50 times around your fingers, making sure your coil is large enough to slide the compass through.


3. Connect one end of the wire to the battery case wire.


4. While looking at the compass, repeatedly tap the other end of the wire to the battery. You should see the compass react to the tapping.


5. Switch the wires from one terminal of the battery to the other. Now tap again. Do you see a difference in the way the compass moves?


You just made a simple galvanometer. “Oh boy, that’s great! Hey Bob, take a look! I just made a….a what?!?” I thought you might ask that question. A galvanometer is a device that is used to find and measure electric current. “But, it made a compass needle move…isn’t that a magnetic field, not electricity?” Ah, yes, but hold on a minute. What is electric current…moving electrons. What do moving electrons create…a magnetic field! By the galvanometer detecting a change in the magnetic field, it is actually measuring electrical current! So, now that you’ve made one let’s use it!


More experiments with your galvanometer

You will need:


  • Your handy galvanometer
  • The strongest magnet you own
  • Another 2 feet or more of wire
  • Toilet paper or paper towel tube

1. Take your new piece of wire and remove about an inch of insulation from both ends of the wire.


2. Wrap this wire tightly and carefully around the end of the paper towel tube. Do as many wraps as you can while still leaving about 4 inches of wire on both sides of the coil. You may want to put a piece of tape on the coil to keep it from unwinding. Pull the coil from the paper towel tube, keeping the coil tightly wrapped.


3. Hook up your new coil with your galvanometer. One wire of the coil should be connected to one wire of the galvanometer and the other wire should be connected to the other end of the galvanometer.


4. Now move your magnet in and out of the the coil. Can you see the compass move? Does a stronger or weaker magnet make the compass move more? Does it matter how fast you move the magnet in and out of the coil?


Taa Daa!!! Ladies and gentlemen you just made electricity!!!!! You also just recreated one of the most important scientific discoveries of all time. One story about this discovery, goes like this:


A science teacher doing a demonstration for his students (can you see why I like this story) noticed that as he moved a magnet, he caused one of his instruments to register the flow of electricity. He experimented a bit further with this and noticed that a moving magnetic field can actually create electrical current. Thus tying the magnetism and the electricity together. Before that, they were seen as two completely different phenomena!


Now we know, that you can’t have an electric field without a magnetic field. You also cannot have a moving magnetic field, without causing electricity in objects that electrons can move in (like wires). Moving electrons create a magnetic field and moving magnetic fields can create electric currents.


“So, if I just made electricity, can I power a light bulb by moving a magnet around?” Yes, if you moved that magnet back and forth fast enough you could power a light bulb. However, by fast enough, I mean like 1000 times a second or more! If you had a stronger magnet, or many more coils in your wire, then you could make a greater amount of electricity each time you moved the magnet through the wire.


Believe it or not, most of the electricity you use comes from moving magnets around coils of wire! Electrical power plants either spin HUGE coils of wire around very powerful magnets or they spin very powerful magnets around HUGE coils of wire. The electricity to power your computer, your lights, your air conditioning, your radio or whatever, comes from spinning magnets or wires!


“But what about all those nuclear and coal power plants I hear about all the time?” Good question. Do you know what that nuclear and coal stuff does? It gets really hot. When it gets really hot, it boils water. When it boils water, it makes steam and do you know what the steam does? It causes giant wheels to turn. Guess what’s on those giant wheels. That’s right, a huge coil of wire or very powerful magnets! Coal and nuclear energy basically do little more than boil water. With the exception of solar energy almost all electrical production comes from something huge spinning really fast!


Exercises


  1. Why didn’t the coil of wire work when it wasn’t hooked up to a battery? What does the battery do to the coil of wire?
  2. How does a moving magnet make electricity?
  3. What makes the compass needle deflect in the second coil?
  4. Does a stronger or weaker magnet make the compass move more?
  5. Does it matter how fast you move the magnet in and out of the coil?

Click here to go to your next lesson on Conductivity!

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Conductivity

Monday November 24, 2014

switch-zoomMake yourself a grab bag of fun things to test: copper pieces (nails or pipe pieces), zinc washers, pipe cleaners, Mylar, aluminum foil, pennies, nickels, keys, film canisters, paper clips, load stones (magnetic rock), other rocks, and just about anything else in the back of your desk drawer.


Certain materials conduct electricity better than others. Silver, for example, is one of the best electrical conductors on the planet, followed closely by copper and gold. Most scientists use gold contacts because, unlike silver and copper, gold does not tarnish (oxidize) as easily. Gold is a soft metal and wears away much more easily than others, but since most circuits are built for the short term (less than 50 years of use), the loss of material is unnoticeable.
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Modify your basic LED circuit into a Conductivity Circuit by removing one clip lead from the battery and inserting a third clip lead to the battery terminal. The two free ends are your new clips to put things in from the grab bag. Try zippers, metal buttons, barrettes, water from a fountain, the fountain itself, bike racks, locks, doorknobs, unpainted benches… you get the idea!


Here’s what you need:


  • 2 AA batteries
  • AA battery case
  • 3 alligator wires
  • LEDs (any you choose is fine)
  • paper clip
  • penny
  • other metal objects around your house (zippers, chairs, etc…)


Why does metal conduct electricity?

Why does metal, not plastic, conduct electricity? Imagine you have a garden hose with water flowing through it. The hose is like the metal wire, and the water is like the electric current. Trying to run electricity through plastic is like filling your hose with cement. It’s just the nature of the material.



Download Student Worksheet & Exercises


Exercises


  1.       Name six materials that are electrically conductive.
  2.        What kinds of materials are conductors and insulators?
  3.      Can you convert an insulator into a conductor? How?
  4.        Name four instances when insulators are a bad idea to have around.
  5.      When are insulators essential to have?

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Click here to go to your next lesson on requirements for a circuit!

Requirements for a Circuit

Monday November 24, 2014

You need two important things for an electric circuit: first you need a closed conductive path that goes from the positive to the negative terminal of the battery. When I teach this activity to kids, there’s always a couple that try to light up the LED just using the LED and wires (they forget about the battery completely!) You always need a power source in the circuit in order for charges to flow. The charges only flow through something that conducts electricity. Sometimes kids forget about the conductive part, and just try to touch the plastic coating on the wires to the LED and are frustrated when it doesn’t work right. It must be a closed conductive loop.


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The second thing is that there needs to be an electric potential difference across the two ends of the circuit. (You can think of the ends as the ends that are connected to the battery.) Because it takes energy to move the charge from a low to a high potential (remember how it takes energy to go up a flight of stairs?), there needs to be something that supplies the charges with the energy they need to go from a low to a high electric potential so then they can move through the circuit on their own. As the charge moves though the circuit, it loses energy until it reaches the negative terminal at zero energy. Then the battery gives that charge energy and it moves back up to the positive terminal at high energy and does it all over again. The battery pumps up the charges and creates an electric potential difference across the two ends of the circuit.


In your house, the electric outlet that you plug appliances and lamps into is the electric potential difference. The three holes in the outlet are hot, neutral, and ground.


The hot terminal is the smaller one on the upper right, and this has the highest potential. Neutral is the larger one on the upper left, and it is the return path for current (and should always be considered to be high potential. It’s unfortunately pretty common for hot and neutral wiring to be reversed, because the outlet still works if it is.) Ground is the roundish one at the bottom and is there for safety. Ground and neutral are connected together back at the box, and they give electricity the quickest and simplest way back to the low potential.  An ungrounded device (including extension cords) can be dangerous if there’s no path to ground except through you.


The electrical outlet in US homes supplies 110-120 volts of voltage and 15-20 amps of current. On the other hand, a AA battery has 1.5 volts and about 50 mA (that;s 0.050 amps), depending on the type of battery and what kind of load you are putting on it.


And just in case you’re considering poking something into the outlet, over 4,000 people every year are in the emergency room for injuries that have to do with sticking something other than a plug into an electrical outlet, about one third of them are kids under 18 years old. And those are just the ones you made it to the hospital… some were not as lucky and never made it at all. The current can kill you because you’re made up mostly of water, which makes it a really good conductor of electricity. Electricity is always looking for the easiest way to ground, and though you is one of those ways and the amount of current that passes through you is enough to stop your heart and damage your cells. Bottom line: the only thing you should put into an outlet is a plug.


Click here to go to your next lesson on the water analogy!

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Water Analogy 

Monday November 24, 2014

This is a recording of a recent live robotics teleclass I did with thousands of kids from all over the world. I’ve included it here so you can participate and learn, too!


We’ll cover topics in electricity, magnetism, electrical charges, robot construction, sensors and more by building several projects together. For now, just watch the video and if you already have the materials to build the projects, feel free to do it along with me. If not, don’t worry… we’ll get to these projects soon in the course.


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Materials:


  • AA battery case (Jameco 216081)
  • Two AA batteries for your battery case**
  • A couple of LEDs (You can use any under 3V, but if you need recommendations, try Jameco 2-lead LEDs or 3-lead LEDs)
  • Set of 10 alligator wires (Jameco 10444)
  • One or two 1.5-3V DC motors (Jameco 231925)
  • Index card
  • 6 brass fasteners
  • Two large paper clips
  • One foam block (2” cube or larger)
  • 1 wooden clothespin
  • 10 wooden skewers
  • Drill & bit the size of the motor shaft diameter
  • Hot glue gun, razor and adult assistant
  • OPTIONAL: Buzzer (Jameco 24872)

**Note about batteries: The cheap dollar-store kind labeled “Heavy Duty” are recommended. Do not use alkaline batteries like “Duracell” or “Energizer” for your experiments with us during this class. (We’ll explain during the class.)



Download the worksheet for this teleclass HERE.


Key Concepts

A robot not only moves but it can also interact with its environment and it does that by using sensors, like light detectors that can see light, you can have motion detectors that can sense movement, touch sensors, pressure sensors, infrared light sensors, proximity sensors, water detectors, spit sensors, detecting all different kinds of stuff!


Robots need electricity to make the motors move, the LEDs light up, the buzzers to sound, and more. When you move electrons around, that’s what creates the electricity. When you rub a balloon on your head for example, you’re picking off the electrons from the atoms in your hair and sticking them on the balloon. There’s a static charge on your head due to the extra electrons.


The electrons have a negative charge, and so just like the north and south poles of a magnet attract each other, the negative charge of the electron is attracted to positive charges. That’s why your batteries have plus and minus signs on them. Electricity is when the electric charge is moving around inside the wires in the circuit.


Current is measured in amps, and is a measure of how much charge is flowing through the circuit at a certain point. We use the letter I  to mean current when we’re doing calculations on paper, and it’s important to remember that current is like velocity in that it’s a rate.Just like velocity is the change in position per unit time, current is the amount of charge per unit time: I = Q/t. This means that one amphere (amp) is one coulomb per second.


Click here to go to your next lesson on Franklin’s Mistake!

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Franklin’s Mistake

Monday November 24, 2014

If this next part is too confusing, just skip over it. I did want to let you know (for those of you who have spotted it already) that there’s a big problem with the positive test charge model we’ve been using. Well, it’s kind of a problem, but not really a big one once you get used to the idea.


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When we determined which one is the high and low potential (the plus and minus on a battery), we assumed that the test charge was positive. The particles that move charge through the wires are actually negatively, not positively, charged. We’ve been dealing with just positive charges so far, and now we’re going to mix things up a bit since electrons are in fact, negative. This means that current actually goes from the negative terminal to the positive terminal because that charge is being carried by electrons. Note that charge carriers don’t have to be electrons (they can also be positively charged or both traveling simultaneously in opposite directions!)


It was actually one of Ben Franklin’s not-so-great moments when he arbitrarily assigned the direction of electric current the way we think about it today, which is going from the positive to the negative terminal. Electrons actually move in  the opposite direction! BUT in the real world, we all think about current flowing from plus to minus (even though in reality its the opposite direction). Just file it away in your mind in case it ever comes up (which it probably won’t) that you would need to know the actual direction the electrons are moving in a circuit (which most people don’t need to know or care about).


Click here to go to your next lesson on learning about charge carriers!

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Charge Carriers are Not Used Up

Monday November 24, 2014

In electric circuits, the charge carriers are electrons, which are already inside the wire itself. The battery doesn’t add extra electrons to the circuit to make it go… the electrons inside the wires are already there. The battery provides an electric potential difference that signals the electrons to start moving, and this signal travels at the speed of light (or close to it), and then the electrons start moving (quite a bit slower than the speed of light). This means that electrons don’t have to start at the battery and them go all the way to the light before the light bulb lights up, because the electrons inside the filament itself are the ones that start glowing when they get the signal to start moving.


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All electrons everywhere start moving as soon as they get that signal from the battery, like like water flowing in the hose that we saw in the teleclass video. The electrons themselves don’t get used up, but rather the energy carried by the electrons is transformed to other forms of energy (including heat, light, sound, motion, etc.). The charge itself doesn’t transform or get used up, just the energy itself. Kids use up their energy as they run, play, and jump , and need to refuel before they can go back in the afternoon to the playground. The kids don’t get used up, but rather the energy the kid is carrying is being transformed from chemical energy int the food to motion energy , and needs to be replenished when the kid runs out of energy.


Click here to go to your next lesson on Switches!

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Switches

Monday November 24, 2014

knifeswitchWhen you turn on a switch, it’s difficult to really see what’s going on… which is why we make our own from paperclips, brass fasteners, and index cards.


Kids can see the circuit on both sides of the card, so it makes sense why it works (especially after doing ‘Conductivity Testers’).


SPST stands for Single Pole, Single Throw, which means that the switch turns on only one circuit at a time. This is a great switch for one of the robots we’ll be making soon, as it only needs one motor to turn on and off.
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Think of this switch like a train track. When you throw the switches one way, the train (electrons) can race around the track at top speed. When you turn the switch to the OFF position, it’s like a bridge collapse for the train – there’s no way for the electrons to jump across from the brass fastener to the paper clip. When you switch it to the ON position (both sides), you’ve rebuilt the bridges for the train (electrons).


Troubleshooting: The two tabs on the back of the motor are the places to clip in the power from the battery pack. Since these motors spin quickly and the shaft is tiny, add a piece of tape to the shaft to see the spinning action more clearly.


Kids can make their own switches so they can trace the path the electricity takes with a finger. See what you think about this SPST:


Here’s what you need:


  • 2 AA batteries
  • AA battery case
  • 3 alligator wires
  • index card
  • 2 brass fasteners
  • paper clips
  • buzzer, motor, or LED


Download Student Worksheet & Exercises


Exercises


  1.  If you want to reverse the spin direction of a motor without using a switch, what can you do?
  2.  A simple switch can be made out of what kinds of materials?
  3.  How would you make your SPST switch an NC (normally closed) switch?
  4.  How did you have to connect your circuit in order for both the LED and motor to work at the  same time? Draw it here:
  5.  Draw a picture of your experiment that explains how the SPST switch works, and show how      electricity flows through your circuit:

Extra Credit (for students who have completed Part 3):


  1. Draw a picture of your experiment that explains how the DPDT switch works in your circuit and show how to wire up the circuit.

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 Click here to go to your next lesson on Electric Circuit Loads!