A proton traveling due west in a region that contains only a magnetic field experiences a vertically upward force (away from the surface of the earth). What is the direction of the magnetic field?

Question

Quizplus · Accepted Answer

The direction of the magnetic force on a charged particle moving in a magnetic field is given by the right-hand rule. If the thumb of the right hand points in the direction of the velocity of the particle, and the fingers point in the direction of the magnetic field, then the palm gives the direction of the force. In this case, the proton is moving due west, so its velocity is to the left. The force is vertically upward, so it must be perpendicular to both the velocity and the magnetic field. The only way for this to happen is if the magnetic field is directed into the page, which means it is pointing due south.

A Proton Traveling Due West in a Region That Contains