A charged particle of charge $q$ and mass $m$, gets deflected through an angle $\theta$ upon passing through a square region of side $a$, which contains a uniform magnetic field $B$ normal to its plane. Assuming that the particle entered the square at right angles to one side, what is the speed of the particle?
KVPY 2010, Diffcult
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(a)

In given condition, if $r$ is radius of curved path travelled by changed particle.

Then,

$\sin \theta \approx \tan \theta=\frac{a}{r}$

or $\quad r=a \operatorname{cosec} \theta \approx a \cot \theta \quad \dots(i)$

Also, $\quad r=\frac{m v}{B q} \quad \dots(ii)$

Combining Eqs. $(i)$ and $(ii)$, we get

$v=\frac{q B}{m} a \cot \theta$

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