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I hope this is an okay place to ask this. Not a lot of places for amateurs to seek guidance for failed experiments or rudimentary knowledge.

This was an experiment I just tried, and want to understand why it didn't work.

I understand that moving a magnet through a copper coil will generate a magnetic field.

And my very basic understanding of Lenz's law is that dropping a magnet through a copper pipe induces an electric current that generates a magnetic field that opposes the magnetic field of the magnet, thus slowing it down.

I had been researching if it was possible to capture this current from a pipe, but as far as I could find, it only works with a coil.

I feel a bit like a kid here, because I'm sure this demonstrates how little I know: As an experiment, I attached some copper wire to both ends of a copper pipe, connected them to an LED, taped the whole thing to a PVC pipe so I could ensure the current was contained and dropped the magnet through the pipe. The LED did not light up.

Is it possible to generate enough electricity by dropping a magnet through a pipe to light an LED? Or does it actually generate any electricity at all? If not, why?

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  • $\begingroup$ I actually tried this exact same thing a while ago and I couldn't get it to work. I also tried it with a rotating disc in a constant magnetic field (Griffiths has a very similar problem that I was trying to make into a lab experiment), but it didn't work. The field was $10 T$, and the disc stopped rotating almost instantaneously, but I couldn't detect any current between the axes and the edge. $\endgroup$
    – Philip
    Commented Jun 10, 2020 at 4:55
  • $\begingroup$ I am curious if you tried the exact same experiment except with a copper coil, did the led light up? $\endgroup$
    – anna v
    Commented Jun 10, 2020 at 6:24

3 Answers 3

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The current generated in the pipe by the falling magnet is induced where the magnetic field is changing, which is only close to the magnet. Try slitting the pipe lengthwise along one side. Attach wires to the edges of the slit about halfway up the pipe, and repeat your experiment. You might get enough current through the wires to deflect a micro-ammeter, as the magnet passes the points where you've attached the wires. Using an LED as a detector requires your experiment to generate enough voltage to exceed the LED's threshold, so probably won't work.

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  • $\begingroup$ Okay, so the pipe, being solid provides a constant induced magnetic field and thus there is no charge, right? So in theory, what if the pipe was ribbed? The difference between a pipe and a tightly wound coil is just the insulation separating segments of the atoms that the magnet is affecting. What is it about being a solid that causes the pipe not to work? $\endgroup$
    – rhamilton
    Commented Jun 11, 2020 at 6:33
  • $\begingroup$ Currents do flow in the intact pipe, but won't flow out the wire because the easiest path is just to go around the pipe. The longitudinal or spiral slits force the current to go through the wire. $\endgroup$
    – S. McGrew
    Commented Jun 11, 2020 at 13:05
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Firstly, the currents you are trying to use to light up your LED are called Eddy Currents

Now, you can try modifying your setup either as S. Mcgrew mentioned in his answer or as shown in the image below. You can wound wire around your copper rod and place it close to another coil that is connected to the LED.

enter image description here

But even after all these modifications, the LED might not light up since the induced potential wouldn't be large enough to exceed the threshold.

Other possible changes (based on theory) may include (Though, I am not sure if any of these will light up the LED):

  • Using a stronger (and longer) magnet.

  • Using Transformer's equation and changing the number of turns in both the coils accordingly (If you are using the design similar to the one in the image given below)

  • Throwing the magnet down the rod faster (again, not sure if this will work practically or not) since that will increase the rate of change of Magnetic Flux and hence increase the magnitude of the induced voltage.

All the Best!

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  • $\begingroup$ Slitting the pipe in a spiral to create a coil would work too. $\endgroup$
    – S. McGrew
    Commented Jun 10, 2020 at 2:48
  • $\begingroup$ Just to clarify the emphasis on Eddy currents. The link says "Eddy currents flow in closed loops within conductors, in planes perpendicular to the magnetic field." Is this implying the current is not flowing through the width of the pipe but around? What if the poles of the field where perpendicular to the length of the pipe instead of parallel? $\endgroup$
    – rhamilton
    Commented Jun 11, 2020 at 6:24
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Try using a bunch of thin magnets have enough to keep it going for a good minute as fast as possible for no separation.

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    Commented Dec 5 at 6:41

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