SAT Physics Subject Test

Chapter 11 Magnetic Forces and Fields


As we said at the beginning of this chapter, the source of magnetic fields are electric charges that move; they may spin, circulate, move through space, or flow down a wire. For example, consider a long, straight wire that carries a current I. The current generates a magnetic field in the surrounding space that’s proportional to I and inversely proportional to r, the distance from the wire.

The magnetic field “lines” are actually circles whose centers are on the wire. The direction of these circles is determined by a variation of the right-hand rule. Imagine grabbing the wire in your right hand with your thumb pointing in the direction of the current. Then the direction in which your fingers curl around the wire gives the direction of the magnetic field lines.

  7. The diagram below shows a proton moving with velocity v0, initially parallel to, and above, a long, straight wire. If the current in the wire is I as shown, what is the direction of the magnetic force on the proton?

Here’s How to Crack It

Using the second right-hand rule, we see the magnetic field B produced by the current points out of the page above the wire (where the proton is located). Using the first right-hand rule for a proton moving the right, feeling a magnetic field out of the page, we see that FB points downward, toward the wire.

  8. The diagram below shows a pair of long, straight, parallel wires, separated by a small distance, r. If equal currents are established in the wires, what is the magnetic field midway between the wires?

Here’s How to Crack It

Let B1 be the magnetic field due to Wire 1 and B2 be the magnetic field due to Wire 2. The total magnetic field at point P is just their vector sum, B1 + B2. Using the second right-hand rule, we know that B1 is directed counter-clockwise around Wire 1 and B2 is counter-clockwise around Wire 2. Therefore, at point P, B1 points upward and B2 points downward.

Since B is proportional to  and I and r are the same for both wires, B1 and B2 cancel to zero.