# Why won't a magnet slow down when falling through a copper coil?

1. Jul 3, 2014

### dEdt

As you can see, the magnet does not slow down (at least, not appreciably) when falling through the copper coiled around a PVC pipe, and I can't figure out why.

2. Jul 3, 2014

### UltrafastPED

Because the eddy currrents are too weak for the weight of the magnet.

Try sliding a very strong magnet along a thick piece of copper or aluminum. This will always show the effect.

3. Jul 3, 2014

### Simon Bridge

1. it does
... but as you notice, not by much.

The video is a good example of how not to do this demonstration.
You should have a go doing it yourself.

Off the top of my head...

Notice how the magnet is oriented as it falls through the copper pipe?
The poles are face-on to the walls. The eddy currents form circles about the poles ... the wires in the coil, being turned horizontally, won't let that happen.

Since the coils prevent (big) eddy-currents from forming (no sideways loops) so you only get resistance as the magnetic flux in the coil changes - that is when the magnet enters and leaves the coil. While the magnet is entirely within the coil, the magnetic flux inside the coil is not changing (much/enough) - so no opposing field.

Repeat the experiment with rows of coils down each side of the PVC tube - oriented so the open ends face to tube.

4. Jul 3, 2014

### Staff: Mentor

Or through a copper or aluminum pipe. That's how we do it in a classroom demonstration.

5. Jul 3, 2014

### Simon Bridge

... which is also how they did it in the video, where the demonstration has a pretty substantial copper coil too... presumably it is possible to have the same length and thickness of copper coil as copper pipe, but the pipe is substantially better at slowing the magnets than the coil.
This, I believe, is the essence of the question.

We could rephrase and ask how big the coil would have to be to have the same effect as a given copper pipe of given dimensions?

I'm saying that the insulation on the wire is preventing large eddy currents from forming - since that would involve currents passing through the insulation. Only currents that fit inside the width of the wire will form.

Presumably the effect would be bigger if the tube were small enough to keep the magnet oriented N-S up-down all the time.

6. Jul 3, 2014

### Simon Bridge

Last edited by a moderator: Sep 25, 2014