I have been baffled for some time now. Could anybody answer my question?
When I move a piece of iron away from a permanent magnet, I have to do work
on the piece of iron. Does this work enter the first law of thermodynamics
dU = DQ + DW and increase the internal energy of the iron, or does it
increase the external energy, the potential energy?
Thanks in advance: Rudy

I have been baffled for some time now. Could anybody answer my question?
When I move a piece of iron away from a permanent magnet, I have to do work
on the piece of iron. Does this work enter the first law of thermodynamics
dU = DQ + DW and increase the internal energy of the iron, or does it
increase the external energy, the potential energy?
Thanks in advance: Rudy

Hi Rudy

This is kind of a funny question. It's definitely first law, but it's a
little hard
to find where the energy goes.

In electrostatics, it's easy. You take two charged plate and double
their
separation. You have now the same field over twice the volume, so
there's
twice the field energy.

The magnetic case is strange because you have a geometry that looks
similar with a small gap between the iron and the magnet. But when you
double the separation, you doulbe the "magnetic resistance" of the
circuit,
and you get HALF the field strength over twice the volume. Since energy
goes as the square of the field strength, you have half the energy in
the
gap. (one quarter the field over twice the volume.)

The change in field energy does actually give you the correct value for
the work you do in moving the piece of iron...but the sign is wrong!

The explanation is not so easy. The electrostatic case is however of
some assistance. The analogy I made earlier of two plates with fixed
charge is wrong. The better analogy is two plates connected to a
battery
of fixed voltage. In this case, the work done to separate the plates is
the same, but you end up with half the charge on the voltage (because
you double the capacitance) and therefore half the energy (a quarter
the field energy over twice the volume) just like the magnetic case.
In this case the work done is in recharging the battery.

I think in the magnetic case the work done must be in dis-aligning the
internal poles of the iron. Paradoxically, it seems that a fully
magnetised
piece of iron must have less internal energy than a demagnetised piece,

Why doesn't a lump of iron then spontaneously magnetise? Because
of the extra cost of EXTERNAL magnetic energy. In fact, it does
spontaneously
magnetise on the grain level, so that there is no external field
energy. The alignment
of these magnetic domains must further decrease the internal energy.

That's the way I see it. I hope it makes sense. Do you remember me from
Pinawa, Rudy? I have a thread going in sci.physics.electromagnetis
about
quantum mechanics (The Case Against the Photon). You should check
it out.

Marty



Rudy von Massow wrote:
I have been baffled for some time now. Could anybody answer my question?
When I move a piece of iron away from a permanent magnet, I have to do
work
on the piece of iron. Does this work enter the first law of
thermodynamics
dU = DQ + DW and increase the internal energy of the iron, or does it
increase the external energy, the potential energy?
Thanks in advance: Rudy

When you move a piece of iron away from the magnet, you are not changing its
magnetic moment (which can be expressed in terms of the effect of an
equivalent current carrying coil) nor are you making any but a small change
in its internal flux. What you are mainly doing is changing the total
external field and the total energy stored in the field so considerations of
the total external field give decent results. Note that doubling the gap
doesn't necessarily halve the field strength in the gap. With a permanent or
an electromagnet the field strength is at a maximum when the gap is 0 and
for a range of gaps near 0, the field strength is essentially constant and
uniform because the gap reluctance is small compared to total reluctance

I think the case I analyzed is more to the point: as you do work by
pulling the iron away from the magnet, the energy stored in the field
changes. The problem is: the change is in the wrong direction! There
is LESS energy in the field after you do work on it.

But if you were to put a string on the iron and have it do work when it
gets pulled closer to the magnet, by the time it gets as close as it
was before you pulled it away, you'll have done about as much work as
it took to pull them apart.

So the field has more energy after you pull them apart. it's just set
up so at the beginning you get that energy out slower.

When you move a piece of iron away from the magnet, you are not changing
its
magnetic moment (which can be expressed in terms of the effect of an
equivalent current carrying coil) nor are you making any but a small
change
in its internal flux. What you are mainly doing is changing the total
external field and the total energy stored in the field so considerations
of
the total external field give decent results. Note that doubling the gap
doesn't necessarily halve the field strength in the gap. With a permanent
or
an electromagnet the field strength is at a maximum when the gap is 0 and
for a range of gaps near 0, the field strength is essentially constant
and
uniform because the gap reluctance is small compared to total reluctance

Hi Don

You are analyzing the case where the field in the gap is constant. I
think
this case is deceptive, and I'll tell you why. For me to physically
create
a system where the field stays constant as you move the magnet away,
I have to make the "reluctance" constant around the circuit: which I
can
do by taking a pair of U-shaped magnets and joining one set of poses
together. So I have a big rectangular loop with a gap in it. If I put a
piece
of iron in that gap, I can move it the short distance between the poles
and the field in the gap stays the same, because the reluctance around
the loop is the same. BUT....the iron moves effortlessly from one pole
to another. When there is no change in field energy, there really is no
force.
--------------

I think the case I analyzed is more to the point: as you do work by
pulling
the iron away from the magnet, the energy stored in the field changes.
The problem is: the change is in the wrong direction! There is LESS
energy in the field after you do work on it.
-----------

J Thomas wrote:
.

But if you were to put a string on the iron and have it do work when it
gets pulled closer to the magnet, by the time it gets as close as it
was before you pulled it away, you'll have done about as much work as
it took to pull them apart.

So the field has more energy after you pull them apart. it's just set
up so at the beginning you get that energy out slower.

No, that's my point. It's not like a spring or an elastic. And
it's not even like two electically charged bodies. With magnets,
there is actually less field energy when you pull them apart. I've done
the calculation. Simplest case: two parralel currents. Do they
attract or do they repel?

It doesn't seem to make sense but that's how it is. The energy
is somewhere else.

Start with two magnets that are 1 inch apart. You can get some work
done by harnessing the power they provide when they pull closer
together. Now start with the magnets 2 inches apart and harness the
power they release whenk they pull closer together first the one inch
to get to where you started last time, and then the second inch.

Which way do you get more work done? I think you get more work the
second way, where you get everything you got the first time, plus some
extra.

So why do you say the field has less power the second way?

I'm not saying you'r wrong, but if you're right I've misunderstood what
you're talking about.

Don, I think I can get to the gist of your argument by saying that
when you start with ZERO gap, and pull the iron away from the
magnet, you are creating a field where no field existed before, so
obviously the work done has gone into creating field energy.

I think the fallacy with your argument is that you are neglecting
the fact that there already WAS field energy...behind the iron, on
the back face. That's the point I was trying to illustrate with my
back-to-back U-shaped magnets. It's not obvious to me that
the TOTAL field energy (gap, external field, and internal field
in the iron) either decreases or increases in your example.

That's why I chose an example where I think it's clear that the
field energy decreases. Because even if your case DOES show
an increase in field energy (which I'm not prepared to accept based
on your arguments so far) I think you still have to explain the case
where the field energy clearly decreases.

That's why I cited the case of the two wires with parallel currents.
They attract each other and pull together...the field energy
obviously increases in this case, which is contrary to what you
would expect from your energy arguments.


marty
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