CLEAN POWER

Lenz’s Law Demonstration – Magnetic Physicists Needed?

When a magnet is dropped down a long coil no e.m.f is induced in the middle section as (to my understanding)although flux linkage is at the maximum there is no change in flux linkage so Faraday’s Law gives an e.m.f of 0. however in the classic demonstration of a magnet being dropped down a copper pipe there is clearly and e.m.f being induced all the way down the pipe to opppose the motion so that it reaches a terminal velocity and takes longer than expected to come out of the other end of the pipe. Can anyone explain the difference in the coil to the pipe that causes the effect in the pipe but not the coil. Thanks.

The difference is that the pipe allows for complete current loops around the circumference of the pipe, which will generate a magnetic field in opposition to the falling magnet. In a coil, with the ends disconnected, you won’t have this happening. You induce some emf in the coil, but it’s not a closed circuit, or at least not one that can generate a magnetic field opposing the falling magnet. You could consider small current loops induced within the wire, or around the circumference of the wire, depending on the magnet’s orientation, but these will be small and weak compared to a current loop all the way around a pipe. However, if you wire one end of the coil to the other, you should get some Lenz’s law braking of the magnet.

It’s a very clever idea for a demonstration. I had to go set it up and test it myself with some magnet wire wound around a glass tube and a glass tube sheathed in a copper pipe, to see the effect you described. Like I said, if you connect the coil to itself, you will see some effect, but it’s not nearly as strong as the copper tube. I think this is because the emf is induced in the coil as a whole, and not as a local loop as in the pipe, so you have a field of the same total strength, but it’s not as strong locally. I think I’m going to have to play around with this some more and maybe do some actual math to figure out the full details.

Johnny Carson and Jack Webb

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