Find the condition under which the charged particles moving with different speeds in the presence of electric and magnetic field vectors can be used to select charged particles of a particular speed.

(1) The velocity $\overrightarrow{\mathrm{V}}$, of the charged particles, $\overrightarrow{\mathrm{E}}$ and the and $\overrightarrow{\mathrm{B}}$vectors, must be mutually perpendicular.
Also the forces on q, due to $\overrightarrow{\mathrm{E}}$ and $\overrightarrow{\mathrm{B}}$, must be oppositely directed.
(2) If magnetic force= electrostatic force
$\begin{array}{r}{\mathrm{F}}_{\mathrm{m}}={\mathrm{F}}_{\mathrm{e}}\\ \mathrm{qvB}=\mathrm{qE}\\ \mathrm{v}=\mathrm{EB}\end{array}$