Q. An electron having kinetic energy K is moving in a circular orbit of radius R perpendicular to a uniform magnetic induction. If kinetic energy is doubled and magnetic induction tripled, the radius will become

(a) $ \displaystyle \frac {2R}{3}$

(b) $ \displaystyle \frac {\sqrt2 R}{3}$

(c) $ \displaystyle \sqrt {\frac{2}{3}} R $

(d) $ \displaystyle \frac {2}{\sqrt3} R$

Ans: (b)