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I learned in my undergraduate physics class that atoms have magnetic fields produced by the orbit of electrons and the spin of electrons. I understand how an orbit can induce a magnetic field because a charge moving in a circle is the same as a loop of current.

What I do not understand is how a spinning ball of charge can produce a magnetic field. Can someone explain how spin works, preferably in a way I can understand?

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4 Answers

up vote 2 down vote accepted

An electron is not a spinning ball of charge and the intrinsic spin of particles cannot be understood in such terms. Not only is it difficult to make sense of what it means for a pointlike particle to spin, but also when treating the electron as a spinning ball of charge one finds a value of the ratio between the magnetic moment and the angular momentum that is a factor $2$ too small.

To understand why a rotating charged ball generates a magnetic field, note that every charge on the ball will move in a circle, so there is in fact a current, and that current will generate a magnetic field.

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I see. A point in space does not have a direction. That is why scientists assign some arbitrary vector like the dipole moment. –  Tad May 6 at 21:40
The dipole moment is not arbitrary, it is a measurable quantity. –  Robin Ekman May 6 at 21:44
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There is no classical analogue to visualise what spin is. We found out from experiments that particles have an intrinsic property which we named spin which produces a magnetic moment. You cannot visualise it since fundamental particles are zero-dimensional points in space so the term "spinning around its axis" makes no physical sense.

Its a strictly observational effect which fits well with our mathematical models and explains a great range of phenomena in nature.

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Another way to visualize electron spin is to consider the "Dirac Electron"

Dirac's one equation for a massive particle can be rewritten as two equations for two interacting massless particles, where the coupling constant of the interaction is the mass of the electron.

We can follow these rules and visualize a current loop capturing an orthogonal magnetic field. This way you can represent both electrons and anti-electrons as left or right handed particles. These particles can also exist as either up or down state relative to its environment.

enter image description here

These coupled spinors spin orthogonal to themselves. From the direction of spin, you can see there is no way to push the electron and anti-electron together without destroying them.

Also note, turning the inner (blue) loop 360 degrees, only turns the larger (red) loop 180 degrees leaving it upside down. You must turn another 360 degrees (720 total) before it is right side up.

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I really like the explanation on or about 29:30 of this older video. He causes the electrons to start spinning around the torus with the magnet, trapping the magnetic field lines orthogonal to the spinning electrons.

enter image description here


In his words "As the magnet is removed, the magnetic flux remains trapped within the tin ring. The trapped magnetic flux induces a current flow around the ring. The current flow causes the tin ring to act as a magnet. The magnetic property of the ring has been observed to persist for months at a time forcing the conclusion that there is no resistance to the current flow."

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Room temperature superconductors, they're the holy grail, are you quite certain? Does this answer the question? –  Duckisaduckisaduck May 7 at 14:58
This example is tin and is made superconducting in liquid helium. A spinning ball can have a magnetic field in the same way a spinning loop or torus does. The current loop is orthogonal to the induced magnetic field. For example, current loops inside the earth induce a magnetic field in and around earth with a north and a south pole. –  AnimatedPhysics May 7 at 18:39
sorry, I commented without having watched the clip. –  Duckisaduckisaduck May 7 at 22:52
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