Presentation: magnetism

Look at these tiny compasses. Just look at them. And look at what happens to them when a bar magnet is brought nearby. Just look at them move around. (Video link: "PH MD SC TUTE 70031A V0541 Nuclear Magnetic Resonance NMR Model.")

In this presentation we discuss the attraction and repulsion of bar magnets in terms of magnetic fields, in parallel with a previous presentation discussing the attraction and repulsion of electric charges in terms of electric fields.

First, a "direct" model of magnetic forces.

By convention we label the two magnets that exert forces on each other as "source" and "test" poles, where the source magnet (with a north N and a south S pole) is said to be exerting a force on the test magnet (with a north n and a south s pole).

Throughout this discussion, don't worry about the magnitude of the forces these magnets exert on each other, as will focus only the direction of these magnetic forces. This force is attractive if the ends of the source magnet and test magnet face each other with opposite poles, and repulsive if the ends of the source magnet and test magnet face each other with like poles.

Note the convention where the source bar magnet (held stationary) has square ends, while the test bar magnet, which would be free to turn about a fixed center, is drawn like a compass with pointy ends.

Second, a more sophisticated "indirect" or two-step model of magnetic forces.

Instead of a source magnet directly exerting a force on a test magnet, in this two-step model, the source magnet is said to create a magnetic B field everywhere around it, and it is this magnetic field that exerts a force on a test magnet.

In order to visualize the magnetic field created by a source magnet, let's imagine filling space with many test magnets, all of them small enough that the only significant force exerted on them is due to the source magnet, and not due to the test magnets on each other. The directions of all of these test magnets shows us how the "influence" of the source magnet at each and every location in space--it is this influence throughout space that is the magnetic field of the source magnet.

Instead of drawing tiny test magnets at each and every location in space to represent the magnetic field of a source magnet, we place a series of test magnets end-to-end, and trace this magnetic field line. We can then replace the line of test magnets with magnetic field lines everywhere, where the direction of these lines denotes the direction of the north ends of the test magnets.

Let's focus on the first step of this two-step model. The source magnet, with a north pole N and a south pole S creates a magnetic field everywhere around it. (Again, don't worry about the magnitude of this magnetic field). The direction of all magnetic fields make closed loops, each coming out of the N pole (which is the "source" of magnetic field lines), and going into the S pole (which is the "sink" of magnetic field lines).

Notice how these magnetic field lines form closed loops, coming out of the N pole of the bar magnet, and coming into the S pole of the bar magnet.

Now what? If there is another magnet anywhere in the presence of this magnetic field, this magnetic field will exert a force on the test bar magnet's north pole n and south pole s. (Again, don't worry about the magnitude of this magnetic force). The direction of the magnetic force on the n pole is along the direction of magnetic field lines, and the force on the s pole is directed against the direction of magnetic field lines. As a result, the test magnet will often twist and move around corresponding to the forces exerted on its n and s poles.

Here, from before, we show the magnetic field lines filling in all space surrounding a source magnet with N and S poles. This magnetic field will cause a magnetic force to be exerted along a field line on the test magnet's n pole, and a magnetic force to be exerted against the field line on the test magnet's s pole. If we hold the middle of these test magnets stationary, but allow them to twist around, they will all align themselves accordingly where n poles "obey" and s poles "disobey" the field lines. (In any case, as a check the direction of the force on any test magnet poles should be attractive or repulsive depending on whether it is the opposite or same end as the source magnet N and S poles.)

Note that all of these magnets have both a north pole and a south pole. While there is speculation on the existence of magnetic monopoles, we will only consider magnetic dipoles, such as these bar magnets.