Question

A electron having a charge $e$ moves with a velocity $v$ in positive $x-$direction. A magnetic field exists in the region in positive $y-$ direction. The force on the electron acts in:
(perpendicular outward direction is taken as positive $z-$ axis).

• A. negative direction of $y-$ axis
• B. positive direction of $y-$ axis
• C. positive direction of $z-$ axis
• D. negative direction of $z-$ axis

Answer 1

The correct option is D negative direction of $z-$ axis



Magnetic force on a moving charge in magnetic field is given by

$\overrightarrow{F}=q(\overrightarrow{V}\times \overrightarrow{B})$

As per question,

$\overrightarrow{F}=-e(V\hat{i}\times B\hat{j})$

$\overrightarrow{F}=-eVB\hat{k}(\because \hat{i}\times \hat{j}=\hat{k})$

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

$(-\hat{k})$ means negative $z-$ axis.

Therefore, option $\left(d\right)$ is the correct answer.