Energy in a permanent magnet question

So how does a magnet pull up a nail? Work is done lifting things against gravity. Where did the energy come from?

act

The field in space changes shape! That stores the energy. Where there's a force there is always possible energy exchange. A force over some distance is an energy... always.

George H.

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C'mon, Nico, don't you remember electrodynamics? The energy density of the magnetic field is proportional to the volume integral of B**2. (It's the volume integral of B**2/(8*pi) in godsown, I mean Gaussian units, and in SI it's something with a mu-nought in it that I always have to look up.)

Cheers

Phil Hobbs

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I don't think that causes a loss in the 'magnetic field' energy. If I treat a system of magnet plus bits of iron... then the energy is 'stored' in moving the bits of iron away from the magnet. If you start with the iron away from a magent... then you start with some energy stored in the system.

George H.

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Look up magnetic cooling!

George H.

Tinfoil hat stuff, you have a fundamental misunderstanding of magnets.

Of course there is energy in a PM, just as there is energy in an inductor: stored energy proportional to I^2. Or do you also believe that an electromagnet stores no energy? Or believe that inductors store energy, but permanent magnets do not?

Typically we model a PM as having a large surface current which produces the field, since all the elementary loop currents of individual domains will cancel within the bulk of the material. Where did I last see this one, in Halliday and Resnik?

Let's look at the dual of your statement:

A magnet is very different than a brick. An inductor is very different than a brick. A capacitor is very different than a brick. Shall we even bring up "electrets" like PZT which need to be "poled" by applying high voltage while at high temperature? An electret is like a persistently-charged capacitor, while a PM is like a superconductor with a persistent surface current.

((((((((((((((((((((((( ( ( (o) ) ) ))))))))))))))))))))))) William J. Beaty Research Engineer beaty a chem washington edu UW Chem Dept, Bagley Hall RM74 billb a eskimo com Box 351700, Seattle, WA 98195-1700 ph206-762-3818

As a physical chemist friend of mine used to say,

"A physical chemist is someone who Talks physics to chemists, Chemistry to physicists,

and

Baseball to other physical chemists."

Cheers

Phil Hobbs

PS to all over-unity enthusiasts: Despite all wishful thinking and short-lived scientific grandstanding to the contrary, conservation of energy continues to be confirmed by experiment, massively, every day.

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Look up magnetic cooling!

George H.

Not true. If your picture were right, the tinfoil hats would be making money. The field contains energy, and the field does change when other magnetic objects move around.

The force on an object equals minus the gradient of the total energy of the system with respect to the object's displacement. You move the object, you change the energy. If the force is big enough to move the object by itself, it winds up in a position that gives lower energy to the whole system.

Cheers

Phil Hobbs

Incorrect argument. A magnetized permanent magnet (PM) is a system, it is magnet plus external field. The system changes greatly when something is stuck to it.

Here's the dual of your argument: since electrons don't change as they travel through a circuit, this proves that electric circuits cannot transfer any energy.

:)

Ah, but the fields outside the electrons do change greatly. It is wrong to look only at the electron particle itself while ignoring the fields. It's just as wrong to ignore the external b-field of a PM while looking only at the solid magnet itself.

((((((((((((((((((((((( ( ( (o) ) ) ))))))))))))))))))))))) William J. Beaty Research Engineer beaty a chem washington edu UW Chem Dept, Bagley Hall RM74 billb a eskimo com Box 351700, Seattle, WA 98195-1700 ph206-762-3818

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You just have to follow the energy. Magnetize a piece of stuff connected to the some temperature resoviour (a pool of liquid helium). Then put it in thermal contact with your sample, let it demagnetize and it takes energy away from the sample.

George H.

Yep; the nail reduces the field at a distance, thus reduces field energy.

Energy is stored in any binding; chemical binding energy, gravitational, magnetic, electric... it's a general principle among all types of attractive forces.

Magnets produce magnetic field; there's energy density associated with this field. If you introduce an object with permeability greater than air, the magnetic field will be sucked into that object, which creates lower overall energy configuration, which results in force pulling the object in.

Compare this to earth gravity---it doesn't decrease when an asteroid passes us by.

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Yes, Work done =3D force X displacement as we learned at school. Where there is a force there has to be energy behind it.

Hardy

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I still refuse to believe this

I can't figure out how it's done at present.

Hardy

Ah, but it does! The gravitational field between the objects is reduced. Not so noticable on Earth for a mere asteroid, but noticable for the asteroid. The tides exist because the Moon pulls on the oceans, or equivalently, because the Moon reduces gravity, allowing the oceans to float up towards space a little closer.

Both magnetic and gravitational fields are linear and obey superposition -- the analogous (and consistent) model for magnetism is, the permeable object is polarized by the influence of the magnet,and the resulting overall field is the sum of both magnets acting together.

Tim

That can happen with superconducting magnets, Normal magnets don't have sufficient energy density to vaporise metal (and eithout sudden evolution of gas or plasma ther can be no explosion)

If you dig a hole in the ground a lot of things (everything on the surface of the earth!) gains energy (PE) with respect to the bottom of the hole.

What happens when something falls in? All the objects on the surface of the earth still have the same PE wrt the bottom of the hole. The hole hasn't lost any energy but the object falling in has.

For the magnet to lift the nail it's magnetic PE must be greater than the gravitational PE needed to lift it. Basically the nail falls upwards into a into a magnetic "hole".

By "gravitational PE needed to lift it" I mean the mgh gained by the nail.