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:
where q is the electron charge, v the electron speed, B the magnetic field intensity and
This force acts as centripetal force of the motion,
and by re-arranging this equation, we find the radius of the circular orbit, r: