Magnet question

• posted

Ok so I have a permanent magnet which I fix say 1 inch above the table. I then slide a screw under it and it pulls it up. It has done mgh amount of potential energy. If I keep doing this will the magnet eventually de-magnetise? ie the energy in the field has all gone in doing this work,

Hardy

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No. The energy is not in the field alone - it's in the combined system of magnet and screw. You put the energy back into the system when you pull the screw away from the magnet.

Sylvia.

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Really? I imagined it may be but I cannot see why?

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When you pull the screw away, you have to apply a force, and move the screw and/or magnet through a distance. This means you are doing work on the combined system. The energy you put into the system cannot be destroyed. Instead, you get it back when you allow the screw to be pulled back towards the magnet.

Sylvia.

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Or, suppose you dip the magnet into a bucket of iron filings. Almost all of the magnetic flux ("lines of force") will be confined to the resulting fuzz ball, and the available energy will approach zero.

-- Joe

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No matter whether the screw is moving towards the magnet or away from the magnet, the force as a function of distance is the same. This is a "conservative force".

Now if you invented a way to "turn off" the permanent magnet force while the screw is moving away but have it "turn on" while it is moving towards the magnet, then you could use it to say turn a rotor. You can do this by heating objects above and below the Curie temperature but this is no free lunch: it's just a very inefficient way of turning heat energy into mechanical energy :-).

Tim.

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When you move the screw away though you generate an emf in the screw don't you? Ok so it isn't connected to a load. Let's suppose I shorted the screw to a resistor.

Hardy

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Actually if it's a metal screw it is it's own resistor! However by bringing this up you've opened a can of worms because inductive E fields are not conservative! :-)

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The EMF generated is not conserved, but that is nothing to do with the force connecting the magnet to the screw. That is purely magnetics, not electric ( at least in the macro sense ).

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Regards,

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So? It will take energy to remove the iron filings, and then the situation will be as it was before.

Sylvia.

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Depending on the orientation, it would then take a bit more energy to pull the screw away from the magnet, with the extra energy being dissipated as heat in the resistor. Not all of the energy would be recoverable when the screw is returned to the magnet. Indeed, more energy would be dissipated by the same mechanism as the screw returns to the magnet.

But this still wouldn't be depleting any energy notionally contained by the magnetic field. You could repeat the cycle endlessly, and the magnetic field would remain unchanged.

Sylvia.

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It will eventually become surrounded by a mass of screws which will shield the magnet from the outside world, by shunting the flux locally. So eventually it will be unable to lift any more screws. As seen from a distance, it will indeed seem to be demagnetized. Inside the screwball, near the magnet the flux density will be higher than it would have been in free air, because the reluctance of the stuff surrounding the magnet has gone down.

Conservation of energy: the magnet can't do any more work lifting screws than the energy it took to magnetize it.

John

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Of course. I just figured that iron filings would demonstrate confinement of the flux better than a handful of screws.

-- Joe

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Nope; you pull the screw away, restoring the (magnetic) balance.

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de quoted text -

Sorry Adrian, but all electromagnetic fields are conservative, including their results. Were it not so, electrical engineering would not work. It does.

I suspect that your confusion is that you do not fully grasp the nature of an electromagnetic field. Many of the quacks believe that they can sneak around the conservative properties of an electromagnetic field though various complex and seemingly sophisticated mechanism, but at the bottom line, none of these work. When you better understand that nature of electromagnetic fields (take a course) you'll clearly see why.

Harry C.

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Seems to imply that will take more energy to heat up a magnet that's holding a screw than one that isn't - even if the screw is thermally insulated and doesn't itself heat up.

Sylvia.

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What about the other pole of the magnet (if it is say a bar magnet and only one side is covered with screws). Is there energy in both poles lost or just one pole.

Hardy

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If that's the case then the specific heat of a ferromagnetic material should be dependent on the flux density passing through it, which means that if you reduce a magnetic field on a piece of iron (or remove a screw from a magnet) it should cool down a little. I don't think it's quite that simple, but here's a related subject:

-- Joe

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quoted text -

I think you should be talking to the OP, not to me.

I know a little about electromagmetism, I do have a degree in physics, after all. And I was referring to the fact that the EMF generated by moving a conductor in a magnetic field generates eddy currents, which end up as heat dissipated, not conserved within the magnetics system. Sure, this does not come from the energy in the field, but from the work done in the movement, but thats a subtle difference.

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Regards,