First slide

Force on a charged particle moving in a magnetic field

Question

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

Easy

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 .zy-plane

Solution

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

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