Acosmic ray electron moves at 7.45 â 106 m/s perpendicular to the earth's magnetic field at an altitude where field strength is 1.10 â 10â5 t. what is the radius (in m) of the circular path the electron follows?

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: