The magnetic force on the electron is:
where q is the electron charge, v the electron speed, B the magnetic field intensity and
is the angle between v and B. Since the electron is traveling perpendicular to the magnetic field,
, so we can write
This force acts as centripetal force of the motion,
, where m is the electron mass and r the radius of the circular orbit. So we can write:
and by re-arranging this equation, we find the radius of the circular orbit, r:
The correct answer is C) towards the center of the circle.
Although the object is moving at a constant speed it is constantly accelerating due to the constant change in direction as it describes the circular path. This causes a constant change in velocity as velocity is a vector quantity.
For the object to maintain the circular path there has to be centripetal force acting on the object and this centripetal force is directed towards the center of the circle.