Question

An electron is moving along positive $x-$axis. To get it moving on an anticlockwise circular path in $x-y$ plane, a magnetic field is applied

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

Answer 1

The correct option is B along positive $z-$axis

For the particle to move anti-clockwise, force should be in $\hat{j}$ direction.

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

$\Rightarrow \overrightarrow{B}=B\hat{k}$ since LHS is in $\hat{j}$ direction.