A proton, moving east, enters a magnetic field of a certain strength. Because of this field the proton curves downward. What is the direction of the magnetic field?

Question

Quizplus · Accepted Answer

The magnetic force on a moving charge is given by the Lorentz force equation: F = qvB sin(theta), where q is the charge of the particle, v is its velocity, B is the magnetic field strength, and theta is the angle between v and B. In this case, the proton is moving east, so v is in the x-direction. The magnetic field is causing the proton to curve downward, so F is in the y-direction. Since q and v are both positive, the only way for F to be in the y-direction is if B is in the z-direction, which is towards the south.

A Proton, Moving East, Enters a Magnetic Field of a Certain