If the
velocity v is perpendicular to the magnetic field   B, the  magnetic
force is  perpendicular to both v and B and acts
like a centripetal force It has a magnitude q v
B Equating the two expressions for centripetal
force,
m v 2/r = q v B, which gives
r =  m v / qB
(4 5)
for the radius of the circle described by the
charged particle