Question

An electron is travelling along the $x$-direction. It encounters a magnetic field in the $y$-direction. Its subsequant motion will be

• A. Straight line along the $x$-direction
• B. A circle in the $x$-$z$ plane
• C. A circle in the $y$-$z$ plane
• D. A circle in the $x$-$y$ plane

Answer 1

The correct option is B A circle in the $x$-$z$ plane

Given, $\overrightarrow{v}=v\hat{i}\overrightarrow{B}=B\hat{j}$


Magnetic force on the charge particle is,

$\overrightarrow{F}=q(\overrightarrow{v}\times \overrightarrow{B})=-e[v\hat{i}\times B\hat{j}]$

$\overrightarrow{F}=evB(-\hat{k})$


Since force is in $-z$ axis, It will force $e^{-}$ to move in circle in $x$-$z$ plane.

Hence, option $\left(b\right)$ is correct answer.

Why this question ? To know the quantities on which magnetic force experienced by charge moving in the magnetic field depends and its relation with them along with use of Right-hand thumb rule. Note: The plane of the circle will be perpendicular to the magnetic field.