Help me explain how the magnet dropped down a copper tube demo conserves energy

  • #1

Main Question or Discussion Point

Help me explain how the "magnet dropped down a copper tube" demo conserves energy

I've tried explaining to this person that the potential energy you give a magnet when you do work against gravity to raise it up to the copper tube is all the energy it has to deal with, and once dropped down it, natural laws (Ampere's, Lenz's etc) governing the situation so happen to create a series of eddy currents, fields, and forces which oppose its descent. The magnet moves down the tube slower, heats up the pipe, and imparts some momentum to it as well. The heat cannot exceed the potential energy you give the magnet in the first place.

I tried thinking of the simplest analogy possible, and came up with: http://www.google.com/images?q=koosh ball&um=1&ie=UTF-8&source=og&sa=N&hl=en&tab=wi&biw=1280&bih=939" correctly sized and dropped down the tube would slow down, heat up, and impart some momentum to the tube, but we don't think it "creates energy" because...it's a koosh ball...and its tendrils are plainly visible.

He doesn't like this analogy because friction will eventually wear the koosh tendrils down, but "not so with a permanent magnet".

Now, I know exposing permanent magnets to opposing fields can (somewhat) disassociate electrons in its magnetic domains, but will performing the magnet drop/copper tube demo enough times analogously wear down the magnet? Or is this just not a good analogy in the first place?

If that's not a good analogy, most basically, there is a sum resistive force acting for the distance of a magnet's fall through a tube that doesn't exist with a normal ball.

How better do you explain to a layperson that it's not "energy out of nowhere"?

I'm trying to create the argument to show at the end "you can't get something from nothing; energy conservation works", so I don't want to use that as a supporting argument.

I know it sounds silly.

Anyone?

(Thank you whichever mod for placing this in a more appropriate forum)
 
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Answers and Replies

  • #2


Well you could explain to them that its not 'energy out of nowhere'. As you said, you're using energy to lift the magnet to the height at which you are dropping it from. That is the energy that is being dispersed in the copper that is slowing the magnets descent. Not sure if there is a good analogy out there to explain this. I think you explained it pretty well.
 
  • #3


@BillPrestonEsq:
(I thought so too! thanks)

You may be right about the analogy. I may have gotten myself in more trouble than necessary. Analogies are normally very useful...oh well.

So am I correct that an absurdly minute amount of energy may come from the demagnetization of the magnet while its exposed to opposing fields? Essentially harvesting a tiny tiny tiny tiny amount energy from the situation that ordered its structure in the first place?

I think it holds. And then it just becomes an order of magnitude problem; that the energy "extracted" from the magnet is peanuts (from a finite bag) compared to the work done against gravity.
 
  • #4
K^2
Science Advisor
2,469
28


The problem is that you can drop a superconducting magnet down the tube, one which cannot demagnetize, and you'll still get exactly the same result.

But the complaint is silly. It has nothing to do with the problem. Koosh ball analogy is qualitatively good. Actual drag forces are different, so numerically it will be different, but that's details.
 
  • #5


It's not that it's demagnetizing the magnet, the magnet is inducing a voltage in the conductor. When you induce a voltage in a conductor it creates it's own magnetic field which is opposing the magnetic field of the magnet, slowing it's descent through the copper tube
 
  • #6


@BillPrestonEsq:
Believe me, I get that part. My concern was if the analogy qualitatively (thank you K^2) holds, which it does, and his concern does miss the point.

However, I need to look up superconducting magnets now...
 
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  • #7


So am I correct that an absurdly minute amount of energy may come from the demagnetization of the magnet while its exposed to opposing fields? Essentially harvesting a tiny tiny tiny tiny amount energy from the situation that ordered its structure in the first place?
That's why I posted that, no big deal just wanted to clarify.
 
  • #8
1,679
3


You might point out that the generator at niagara falls does the same thing only 1) they wrap the pipe all the way out to your house and connect it to a light bulb and 2) they help the magnet fall by using water to weigh it down.

If that doesn't take the magic out of it I don't know what will.
 
  • #9
102
0


The energy is all coming from the potential energy of the magnet's height. The magnet inside the copper tube is not gaining as much kinetic energy because of the added losses. Maybe you could connect a voltmeter to the pipe to observe the change, but it might be too low to get a good measurement.
 
  • #10


If anyone cares to know, I have convinced the opposing side that energy is conserved with this argument, so I call it a success.

Thanks for all your input and reinforcement.
 

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