Question

An electron moving with a speed $u$ along the positive $x-$axis at $y=0$ enters a region of uniform magnetic field which exists to the right of $y-$axis. The electron exits from the region after some time with the speed $v$ at coordinate $y,$ then :

• A. $v=u,y<0$
• B. $v=u,y>0$
• C. $v>u,y>0$
• D. $v>u,y<0$

Answer 1

Answer: (A)

$\overrightarrow{v}=v\hat{i}$
$\overrightarrow{B}=u(-\hat{k})$
${\overrightarrow{F}}_{mag}=-e(\overrightarrow{v}\times \overrightarrow{B})=euB(\hat{i}\times \hat{k})=-euB\hat{j}$