# Bar Magnet

## Main Question or Discussion Point

Hi All,

The classical theory of a bar magnet as I understand it is that there are many current loops at the atomic scale which have magnetic moments all pointing in the same direction. At the interior of the magnet, with one loop next to the other, currents go in opposite directions and their effects are canceled. However, at the surface of the magnet, small current loops are not canceled by adjacent loops which results in a current circulating around the outside of the magnet which causes the resulting magnetic field according to the Biot-Savart law.

If this description is true, which I think it is, how is this current measured? Also, is it possible to use this current and if so, what would the effect be? It seems that the bar magnet would gradually lose its magnetism.

Thank you,
Bob

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Meir Achuz
Homework Helper
Gold Member
That is the standard classical model, but it is not in fact the source of ferromagnetism.
Ferromagnetism arises from the magnetic moments of two electrons in the outer shell of iron. But these magnetic moments come from the spin of th electron, treated quantum mechanically by the Dirac equation. In this theory, the point electron has a magnetic moment, but there is no current loop. There still is a bound surface current at the surface of the magnet. It is a real current in the sense that it produces the strong solenoidal B field, but it cannot utilized in any other way because it is bound in the material. The bound current can be measured in the same way as most currents are measured, which is by the turning moment caused by the action of an external B field on it. For a compass, the turning moment is on the compass needle. For an ordinary current, the turning moment is caused by a B field in a galvanometer acting on a wire loop carrying the current.

That is the standard classical model, but it is not in fact the source of ferromagnetism.
Ferromagnetism arises from the magnetic moments of two electrons in the outer shell of iron. But these magnetic moments come from the spin of th electron, treated quantum mechanically by the Dirac equation. In this theory, the point electron has a magnetic moment, but there is no current loop. There still is a bound surface current at the surface of the magnet. It is a real current in the sense that it produces the strong solenoidal B field, but it cannot utilized in any other way because it is bound in the material. The bound current can be measured in the same way as most currents are measured, which is by the turning moment caused by the action of an external B field on it. For a compass, the turning moment is on the compass needle. For an ordinary current, the turning moment is caused by a B field in a galvanometer acting on a wire loop carrying the current.
Thanks Clem.

Nice explanation.