Question

A particle of mass 7.00 10-8 kg and charge 7.8-µc is traveling due east. it enters perpendicularly a magnetic field whose magnitude is 29.0 t. after entering the field, the particle completes one-half of a circle and exits the field traveling due west. how much time does the particle spend traveling in the magnetic field?

Answer 1

The radial force on the particle is given by the Lorentz force.  They don't give you the velocity so you have to solve for it in terms of the radius and get them to cancel.