Question

A ball of mass $1\text{g}$ and charge ${10}^{-8}\text{C}$ moves from a point $A$ having electric potential $V_A=600\text{V}$ to a point $B,$ whose electric potential is zero. Velocity of the ball at point $B$ is $20{\text{cm s}}^{-1}$. Velocity of the ball at point $A$ is
(Neglect gravitation)

• A. $16.7{\text{ms}}^{-1}$
• B. $16.7{\text{cm s}}^{-1}$
• C. $2.8{\text{ms}}^{-1}$
• D. $2.8{\text{cm s}}^{-1}$

Answer 1

The correct option is B $16.7{\text{cm s}}^{-1}$

Work done by electric force for motion of charge from $A$ to $B$ :-
$(W{)}_{AB}=\Delta KE=q{V}_{AB}$
$\frac{1}{2}m{v}_B^2-\frac{1}{2}m{v}_A^2=q(V_A-V_B)$
$V_B=0,V_A=600\text{V}\frac{1}{2}m{v}_A^2=\frac{1}{2}m{v}_B^2-q{V}_A$
$v_A^2=v_B^2-\frac{2q{V}_A}{m}$
After solving
$v_A=16.7\times {10}^{-2}\text{m/sec}$
$v_A=16.7\text{cm/sec}$