Question

There was a demonstration I did our Zoom class with a falling magnet in a simple copper tube. Explain, in detail, the physics behind why the magnet slows its descent through the tube (it does not touch the sides). In your discussion be sure to explain why the magnet doesn't come to a stop in the tube and why it does not speed up. Your response should include at least 3 paragraphs to show your mastery of the material.

Answer 1

In sum: the magnet induced a current in the copper pipe, which in turn produced a magnetic field. The direction of this current then opposed the change in the magnet’s field, resulting in the magnet being repelled and thus falling more slowly.

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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