Cracking the SAT Physics Subject Test

and you should at

least be able to eliminate

some of the choices.

A charge q in the wire feels two forces: an electric force, FE = qE, and a magnetic

force of magnitude

FB = qvB

If q is negative, FE is upward and FB is downward; if q is positive, FE is

downward and FB is upward. So, in both cases, the forces act in opposite

directions. Once the magnitude of FE equals the magnitude of FB, the charges in the

wire are in electromagnetic equilibrium. This occurs when qE = qvB—that is,
when E = vB.