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.

[Al Skierkiewicz]

The second right hand rule relates to the field generated by a current flowing through a wire. If you place your right hand, thumb out, so that your thumb points in the same direction that current is flowing, the magnetic flux will flow around the wire in the direction your fingers are curling towards your palm. This of course only works on DC current. In an AC circuit the current and therefore the flux are constantly changing direction and intensity. The concept is really easy when you look at your hand. Thumb current and fingers flux. What is really cool is to visualize a coil. Hundreds and hundreds of wires, all pointing in the same direction and all having the identical current flowing and each producing a field with flux lines pointing in the same direction. Each adds to the one next to it and the flux is proportional to the number of windings. It even grows when you wrap more than one layer of wire, the more the better. Throw in a magnetic metal like nickel steel, and it concentrates the field even more. There are tremendous fields produced in our motors.
What is really cool is that fields tend to merge when they get close together. (or warp each other when the poles are opposite) Two bar magnets with opposite poles (north to south) together make a new field where the flux flows from the opposite ends of the two magnets. Since you can't really have any loss in a steady field, the flux leaving one end of the structure must return to the other end of the structure. Now comes the really cool part, the flux density can change as it spreads out over the outside of the structure. It will be strongest (most concentrated) at the end or the bar, then spread out (less concentrated) as it bends around the outside and back to the other pole and then becomes more concentrated at the far end. It is what makes our motors work. The fields are concentrated by the magnet structure and the armature design to make the motor turn and then the field is turned around and spread out by the motor body and the inside of the armature and returned to the magnet structure. (you need to have that closed loop, remember)

[Keith Chester]

Thanks for the help guys... I'll let you know how I do after the test...