Q.Explain the concept of velocity selector.

A velocity selector uses a uniform electric field $\vec E$ and a uniform magnetic field $\vec B$, set up mutually perpendicular in the same region, so that a charge $q$ moving with velocity $\vec v$ feels an electric force $q\vec E$ and a magnetic force $q(\vec v\times\vec B)$ acting in opposite senses. When these two forces exactly balance, $qE=qvB$, so only particles moving at the particular speed

$v = \frac{E}{B}$ …