outside the wire; inside the wire, the field increases linearly from zero at the wire’s center.
A solenoid is set of wound wire loops. A current-carrying solenoid produces a magnetic field.
Ampére’s law can show that the magnetic field due to a solenoid is shaped like that of a bar magnet;
and the magnitude of the magnetic field inside the solenoid is approximately uniform, Bsolenoid = μ 0 nI
. (Here I is the current in the solenoid, and n is the number of coils per meter in the solenoid.)
- The magnetic field produced by a wire-wrapped torus (a “donut” with wire wrapped around it [see
Figure 20.13 ]) is zero everywhere outside the torus, but nonzero within the torus. The direction of
the field inside the torus is around the donut.
Figure 20.13 A wire-wrapped torus.Maxwell’s Equations
Okay, we’ll get this out of the way right now: You will not have to solve Maxwell’s equations on the AP
Physics exam . These four equations include integrals the likes of which you will not be able to solve
until well into college physics, if then. However, you can understand the basic point of each equation,
and, most importantly, understand the equations’ greatest consequence.
Accelerating charges produce oscillations of electric and magnetic fields. These oscillations propagate
as waves, with speedMaxwell obtained this wave speed as a mathematical result from the equations. He noticed that, when the
experimentally determined constants were plugged in, the speed of his “electromagnetic waves” was
identical to the speed of light.^5 Maxwell’s conclusion was that light must be an electromagnetic wave.
What are Maxwell’s equations? We’re not even going to write them out, for fear that you might throw