# A particle of mass m and charge q is thrown at a speed u against a uniform electric field E. The distance travelled by it before coming to rest

Q. A particle of mass m and charge q is thrown at a speed u against a uniform electric field E. The distance travelled by it before coming to rest

(a) (2mu2)/qE

(b) (mu2)/qE

(c) (mu2)/2qE

(d) (mu2)/2qE

Ans: (d)

Sol:

Retardation $\displaystyle a = \frac{q E}{m}$

$\displaystyle v^2 = u^2 – 2 a S$

$\displaystyle 0 = u^2 – 2 \frac{q E}{m} S$

$\displaystyle u^2 = 2 \frac{q E}{m} S$

$\displaystyle S = \frac{m u^2}{2 q E}$