question_answer

An electron is launched with velocity \[\vec{v}\] in a uniform magnetic field\[\vec{B}\]. The angle \[\theta \] between \[\vec{v}\] and \[\vec{B}\] lies between 0 and\[\pi /2\]. Its velocity vector \[\vec{v}\] returns to its initial value in a time interval of

A) \[\frac{2\pi m}{eB}\]     

B) \[\frac{2\times 2\pi m}{eB}\]

C) \[\frac{\pi m}{eB}\]       

D) Depends upon angle between \[\vec{v}\] and \[\vec{B}\]