When a particle with charge q and mass m is introduced into a medium with a uniform field...

When a particle with charge q and mass m is introduced into a medium with a uniform field B such that the initial velocity of the particle u is perpendicular to B (Fig.). the magnetic force exerted on the particle causes it to move in a circle of radius a. By equating Fm to the centripetal force on the particle, determine a in terms of q, m, u and B.

FIGURE : Particle of charge q projected with velocity u into a medium with a uniform field B perpendicular to u.