# why does a magnet slow down in a copper tube

Ahh, Physics, you never cease to astound me. Today s awesome science demo is brought to you by Lenz s Law. was a German physicist that formulated a law of electromagnetic induction back in 1833. The most stripped down
of the law is that when a current is induced in a conductor, a magnetic field is generated that opposes the action that produces the current. Check out a 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. Neat. This is a really simple experiment to try out. All you need is a neodymium magnet and a copper tube.

Any size will work, just make sure that the neodymium magnet is smaller in diameter than the copper tube so that it can free-fall. You can actually just get a magnet you like (either from an online magnet store or our ) and take a trip to the hardware store. There are different sizes of copper pipes you can try out. What happens for different diameters? Can you try a copper pipe that has thicker walls? Does it speed or slow the magnet s descent? What do you think is happening? You probably noticed that if you try to stick the magnet to the copper pipe, it just falls Pto the floor. If the copper is non-magnetic, how could it possibly slow the magnet as it falls to the ground?

The explanation is really cool! Lets demonstrate what s going on with our trusty explorabot. First we re going to magnetize him. We can draw his magnetic field lines in green. Next lets take the explorabot and drop him into the copper pipe. You ll notice that as he falls, the magnetic field lines pass through the pipe at different locations. It turns out that when this changing magnetic field passes through the copper, it creates electricity (called eddy currents). This is calledP inductance because we areP inducing an electrical current in the copper. When these eddy currents flow through our copper pipe, they create a magnetic field of their own.

This magnetic field always opposes the field of our magnet according to Lenz s Law. Brainiacs can read more. So now we have two forces on our magnet! POne is from gravity (pointing down), but now we have another one from our eddy currents (pointing up). This slows our magnet down, making it seem like the magnet is floating. How do you think you can slow the magnet in a copper pipe even more? One way is to have a thicker pipe so that there is less resistance to the eddy currents. Another way is to cool the copper to lower its electrical resistance. See whatPhappens when you cool the copper using liquid nitrogen -185 C (-300 degrees F)!

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