Question

A solid sphere of radius $R_{1}$ and volume charge density $\rho=\frac{\rho_{0}}{r}$ is enclosed by a hollow sphere of radius $R_{2}$ with negative surface charge density $\sigma$, such that the total charge in the system is zero, $\rho_{0}$ is a positive constant and $r$ is the distance from the centre of the sphere. The ratio $\frac{R_{2}}{R_{1}}$ is

• A. $\frac{\sigma}{\rho_{0}}$
• B. $\sqrt{2\sigma/ \rho_{0}}$
• C. $\sqrt{\rho_{0} /\left(2\sigma\right)}$
• D. $\frac{\sigma }{\rho _{0}}$

Answer 1

Answer: (C)

$\sqrt{\rho_{0} /\left(2\sigma\right)}$

Answer 2

For solid sphere of radius $R_{1}$ $q_{1}=\int\limits_{0}^{R_{1}} 4 \pi r^{2} d r \rho$ $=\int\limits_{0}^{R_{1}} 4 \pi r^{2} d r \frac{\rho_{0}}{r}$ $q_{1}=4 \pi \frac{R_{1}^{2}}{2} \rho_{0}$ $q_{2}=-4 \pi R_{2}^{2} \sigma$ $q_{1}+q_{2}=0$ $4 \pi \frac{R_{1}^{2} \,\rho_{0}}{2}-4 \pi R_{2}^{2} \sigma=0$ $\left(\frac{R_{1}}{R_{2}}\right)^{2}=\frac{2 \sigma}{\rho_{0}}$ $\frac{R_{2}}{R_{1}}=\sqrt{\frac{\rho_{0}}{2 \sigma}}$