Question

An $\alpha$-particle is travelling to its right with 1.5 kms${}^{-1}$. What uniform electric field be applied so that it starts moving with same speed to its left after 2.65 $\mu$s?

• A. 2.35 N/C toward left
• B. .235 NC${}^{-1}$ toward left
• C. 23.5 NC${}^{-1}$ toward left
• D. 2.35 NC${}^{-1}$ toward left

Answer 1

The correct option is C 23.5 NC${}^{-1}$ toward left

$v=u+at$
$-{1.5}_{km}\hat{i}={1.5}_{km}\hat{i}+a(2.65\times {10}^{-6})$ or
$a=\frac{-3\times {10}^6\times {10}^3}{2.65}\hat{i}$

Using $qE=ma$
$E=\frac{ma}{q}=-\frac{6.64\times {10}^{-27}\times 3\times {10}^6\times {10}^3}{2\times 1.6\times {10}^{-19}\times 2.65}$
$=-23.5\hat{i}N/C$
i.e. $23.5N/C$ towards left.