question_answer

An electron is travelling along the x-direction. It encounters a magnetic field in the y-direction. Its subsequent motion will be                                                                                [AIIMS 2003]

A)            Straight line along the x-direction

B)            A circle in the xz-plane

C)            A circle in the yz-plane

D)            A circle in the xy-plane

Correct Answer: B

Solution :

                   \[\overrightarrow{F}=-e(\overrightarrow{v}\times \overrightarrow{B})\]Þ \[\overrightarrow{F}=-e[v\hat{i}\times B\hat{j}]=evB[-\hat{k}]\]                    i.e. Force on electron is acting towards negative z-axis. Hence particle will move on a circle in xz-plane.