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Lenz's Law: Magnet Through a Copper Tube

based on 25 ratings
Author: Alex Jacobsen

It is 1834, and you have just heard of this marvelous new phenomenon called eddy currents. Some fellow named Lenz discovered them, and you’re curious if you can find out something special about them yourself. The friend who mentioned it to you said that some interesting things happened when magnets and copper pipes interacted, which is strange, because copper isn’t magnetic! It’s time to find out what’s going on.

Problem: How can a copper pipe interact with a magnet?

Materials

  • Copper pipe or fresh tube of aluminum foil
  • Aluminum cookie sheet
  • Aluminum or non-magnetic metal washer
  • String
  • Small neodymium magnet
  • Voltmeter (optional)

Procedure

  1. Touch your magnet to your various materials to confirm that they’re not magnetic.
  2. Take your magnet and drop it from about the height of your pipe onto something soft.
  3. Now, drop it through your pipe or tube of aluminum foil. What do you expect to happen? What actually happens?
  4. Place your magnet on the cookie sheet and tilt the sheet so that it slides off. Does it behave oddly? Why do you think that is?
  5. Tie your magnet to the string, and swing it in a low circle so that it passes over a metal washer placed on a smooth surface. What happens to the Washer? Can you think of how this might be related to the other behavior you observed earlier?

Extra: If you have a voltmeter, attach it to the objects you are moving your magnet next to. When does the voltage change? Is it different depending on where you clip it to the metal?

Extra: if you have two neodymium magnets, get a length of PVC pipe that’s as long as your copper pipe or roll of aluminum foil. Race the magnets by dropping them through their respective pipes at the same time. Which magnet do you think will win based on the behavior you observed earlier in the experiment?

Results

When you drop your magnet through a copper tube, it slows down. The magnet will also slide down the cookie sheet slowly, and nudge the metal washer in the direction the magnet is spinning. The voltage will spike when the magnet is moving next to the metal, but not when the magnet is sitting still.

Why?

Magnetic fields are the result of electric currents. Changing a magnetic field (moving a magnet) next to a non-magnetic metal will induce an electric field (a voltage difference) in the metal, which subsequently generates a magnetic field with an opposite orientation with respect to your magnet.

When your magnet moves next to a metal it creates these fields, but the fields act in a very specific way. They want to cancel out the magnetic field in the metal, because metals don’t like having electric or magnetic fields inside of them (that’s why electricity flows through metals easily—they’re trying to cancel out the difference in electric potential by moving electrons around!). This phenomenon is known as Lenz’s Law.

The magnetic field induced in the metal attracts the falling magnet, creating resistance. This resistance is what slowed down your magnet. As your magnet slows down, it stops generating as much current, which reduces the resistance acting on the magnet’s movement. Gravity speeds the magnet back up again until it reaches a happy medium speed. Basically, your magnet is creating a whirlpool of electrons around it as it falls through your pipe. Neat, huh?

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