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) (2mu^{2})/qE

(b) (mu^{2})/qE

(c) (mu^{2})/2qE

(d) (mu^{2})/2qE

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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} $