Magnetic Flux

[ZZII 527]

Mmmm...magnetic flux...

Flux of anything is the amount of that thing that "flows" through an area. The most common analogy is to flowing water and a wire loop. If you put the wire loop perpendicular to the flow of water, the amount that passes through it in a given time interval is the area of the loop times the velocity of the water. Similarly, the magnetic flux is the area of the surface being considered (usually the inside of loop of wire or something) multiplied by the strength of the magnetic field. This only works if the surface is perpendicular to the field lines, though. Otherwise, you have to take into account the angle. (Just like a wire loop put parallel to the flow of water has no water passing through it.) So the magnitude of the magnetic flux is actually the area of the surface times the magnitude of the field times the cosine of the angle between the field and the normal direction (perpendicular) to the surface. This gets messier when the field isn't constant or the surface isn't flat and involves calculus. Not sure if this is covered in AP...probably not.

The RHR is used to define directions for some of the weird stuff that goes on in magnetic fields. The force on a moving charged particle, for example, is proportional to v X B, where B is the magnetic field and v is the velocity of the charged particle. The "X" represent a cross product, indicating that the result is also a vector. This vector is in a direction perpendicular to v and B defined by the right hand rule. By pointing your right hand in the direction of v and curling your fingers toward B (the short way around), your thumb will point in the direction of the resulting force.

Hope this helps.