Question

A copper ball of density $8.6gc{m}^{-3},1cm$ in diameter is immersed in oil of density $0.8gc{m}^{-3}$. If the ball remains suspended in oil in a uniform electric field of intensity $36000N{C}^{-1}$ acting in upward direction, what is the charge on the ball?

• A. $1.1\mu C$
• B. $4.2\mu C$
• C. $2.4\mu C$
• D. $3.7\mu C$

Answer 1

Answer: (A)

Apparent weight of the ball $=mg-$ upthrust
$=\frac{4}{3}\pi r^3(\rho -\sigma )g=\frac{4}{3}\pi \times (0.5{)}^3\times (8.6-0.8)\times 980$
$=4,002.4$ dyne $=4.0024\times 10^{-2}N$
Let $q$ be the charge on the ball. As the ball remiand suspended,
$qE=$ apparent weightl of the ball
or $q=\frac{\text{ apparent weight }}{E}=\frac{4.0024\times 10^{-2}}{36,000}$
$=1.11\times 10^{-6}C$