Question

Forces $\overrightarrow{F}$ experienced by a particle having charge q and velocity $\overrightarrow{v}$ in a magnetic field B is given by $\overrightarrow{F}$ = q $(\overrightarrow{v}\times \overrightarrow{B})$. What is the direction of force acting on electrons (negatively charged particles) falling vertically, at a place where the Earths magnetic field is horizontal pointing towards North ?

• A. East
• B. West
• C. Vertically up
• D. Vertically down

Answer 1

Answer: (B)

West

Let North be towards $+\hat{j}$ direction and East be towards $+\hat{i}$. Hence the electrons falls towards $-\hat{k}$.

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

Hence direction of force is given by $\hat{F}=-e(-\hat{k}\times \hat{j})$

$=-\hat{i}$

which is towards west.