Question

A ball of mass m = 0.5 kg is suspended by a thread and a charge $q=0.1\mu C$ is supplied. When a ball with diameter 5cm and a like charge of same magnitude is brought close to the first ball, but below it, the tension decreases to 1/3 of its initial value. The distance between centres of the balls is:

• A. $0.12\times {10}^{-2}m$
• B. $0.51\times {10}^{-4}m$
• C. $0.2\times {10}^{-5}m$
• D. $0.52\times {10}^{-2}m$

Answer 1

Answer: (D)

$0.52\times {10}^{-2}m$

Initially, $T=mg$
$T=0.5\times 10$
$T=5N$
When another ball is through near,
$\frac{k{q}^2}{r^2}+T_1=mg$

Given: $T_1=\frac{T}{3}$

$\frac{9\times {10}^9\times (0.1\times {10}^{-6}{)}^2}{r^2}+\frac{5}{3}=5$

$\Rightarrow r=0.52\times {10}^{-2}m$