# Magnet question

Discussion in 'Electronic Design' started by HardySpicer, Mar 16, 2009.

1. ### HardySpicerGuest

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

2. ### Sylvia ElseGuest

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.

3. ### HardySpicerGuest

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

4. ### Sylvia ElseGuest

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.

5. ### J.A. LegrisGuest

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.

6. ### HardySpicerGuest

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

7. ### BenjGuest

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!

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 ).

--
Regards,

Design Engineer J & K Micro Systems
Microcomputer solutions for industrial control

9. ### Sylvia ElseGuest

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

Sylvia.

10. ### Sylvia ElseGuest

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.

11. ### J.A. LegrisGuest

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

12. ### Guest

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.

13. ### Sylvia ElseGuest

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.

14. ### HardySpicerGuest

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

15. ### J.A. LegrisGuest

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:

http://en.wikipedia.org/wiki/Magnetic_refrigeration

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.

--
Regards,

Design Engineer J & K Micro Systems
Microcomputer solutions for industrial control

17. ### Sylvia ElseGuest

If there's a complete circuit around a loop that contains flux, and the
flux changes, you'd expect a current to flow. If there's resistance in
the circuit, then that will cause an energy loss.

It would depend on the orientation of the loop whether there's any flux
through it, and thus whether there's any change in flux.

Sylvia.

18. ### Sylvia ElseGuest

It occurred to me after posting that maybe the presence of the screw
increases the effective curie point. If I understand the mechanism of
the curie point, it's the point where the domains have sufficient energy
to move away from their aligned positions, thus destroying the magnetic
field. However, if the field is more intense, because of the presence of
the screw, then the domains would presumably require more energy. That
is, the the effective currie point temperature has increased.

At the point where the screw falls away because the field has
disappeared, the magnet would then be above the temperature it would
need to reach to destroy the field in the absence of the screw. It now
need to cool down somewhat before it can pick up the screw again. The
extra energy is irretrievably lost, thus defeating any attempt to make a
free energy machine this way.

This wouldn't manifest as a change in specific heat except about the
curie point temperature.

Sylvia.

19. ### BenjGuest

Harry, this statement is either total ignorance of electromagnetics or
a complete lie. You tell us which one it is!
The truth is that were it not for non-conservative fields electrical
engineering would be quite simple and boring (no, batteries, no
transformers, etc. Just balloons rubbed on cats...)
I just don't understand how it is that the person publicly displaying
the most blatant ignorance of subjects are the very ones who are
always suggesting that someone "take a course". Harry, it is YOU who
is in need of a course in electromagnetics. I'll even give you a FREE
lesson.

The following from the classic FRESHMAN physics text Halliday and
Resnick Page 757.

"Electric fields associated with stationary charges are conservative,
but those associated with changing magnetic fields are
nonconservative. See Section 8-1. Since electric potential can only be
defined for a conservative force, it is clear that it has no meaning
for electric fields produced by induction..."

Note that shoving iron (screws or other ferromagnetic materials) in
the flux path of magnets produces changes in the field's distribution
in space hence, "changing fields".

I hope you are pleased with yourself that you just demonstrated to the
world in a public forum that you do not understand this subject even
to the Freshman level. I urge you to consider a college education
starting with a freshman course in basic physics. And I'm talking
about a REAL physics course not the mathless "physics appreciation"
courses that "science" teachers take today.

Benj

20. ### J.A. LegrisGuest

Just to be perfectly clear, we cannot shield one end of a magnet. The
covered pole just gives a magnet with one end bigger than the other.
In other words, more screws will be attracted to the growing blob of
screws until it's large enough to enclose the other pole and all the
flux in between.