An electron moving along the +x-axis enters a magnetic field. If the electron experiences a magnetic deflection in the -y direction, what is the direction of the magnetic field in this region?
A) along the +z-axis
B) along the -z-axis
C) along the -x-axis
D) along the +y-axis
E) along the -y-axis

Question

Quizplus · Accepted Answer

The direction of the magnetic force is perpendicular to both the velocity of the electron and the direction of the magnetic field. 
In this case, since the electron is moving along the +x-axis and is deflected in the -y direction, the magnetic field must be pointing in the -z direction. This is because the magnetic force is directed towards the center of the circular path that the electron takes, which in this case is in the -y direction. The magnetic field must be perpendicular to the plane of the electron's motion, which is in the x-y plane, so it must be pointing in either the +z or -z direction. Since the electron is deflected in the -y direction, the magnetic field must be pointing in the opposite direction, which is the -z direction.

An Electron Moving Along the +X-Axis Enters a Magnetic Field