This is a quick and simple experiment to answer the question of magnetic field strength: Do four magnets have a stronger magnetic pull than one? You’ll find the answer quite surprising… which is: it depends. Here’s what you need to do to see for yourself:


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Materials:


  • stack of round magnets (7-9)
  • film canisters (or small containers like M&M containers)
  • clay or foam or tissue


 
Download Student Worksheet & Exercises


What’s going on here? When you bring the bottoms of the two film canisters together, you can feel the force of repulsion (if not, flip one of the stacks inside one of the the canisters). While you’ll definitely notice that the force of 4-on-4 magnets is larger than 1-on-1, you won’t be able to tell the difference between 4-on-1 or 1-on-4. Or, put more simply, you can’t tell which has one magnet and which has four. So what’s going on?


The more magnets you have, the more magnetic force they exert. The magnetic forces between two stacks of ten magnets magnets are equal and opposite. The magnetic force exerted by a stack of two magnets and five magnets is also equal and opposite, although somewhat less than the stack of ten.


It’s the same with gravity – the force of gravity between two masses (like the sun and the Earth) is also equal and opposite – or the earth would get pulled into the sun or go flying out of the solar system. The Earth exerts the same pull on the sun as the sun exerts on the Earth.


Say WHAT?!?

Did you expect four magnets to push with four times the force on the single magnet? That’s what common sense tells us. However, think of it this way: the 2nd, 3rd, and 4th magnets magnets are further away than the 1st magnet and so they each exert less and less of a push on the single magnet.


Exercises: 


  1. Why can’t you simply rub the needle back and forth with the magnet? Why do you have to stroke it in one direction?
  2. What other objects/materials can you use to make a compass?
  3. How do you know that the needle is magnetized?
  4. Why did we float the needle in water?

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