The capacity of an isolated sphere of radius $9 \mathrm{cm}$ is $C$. When it is connected to an earthed concentric thin hollow sphere of radius $R$, the capacity becomes $10C$. Then the value of $R$ is

Assertion: A parallel plate capacitor is connected across battery through a key. A dielectric slab of constant $k$ is introduced between the plates. The energy which is stored becomes $k$ times.

Reason: The surface density of charge on the plate remains constant or unchanged.

Assertion: The lightning conductor at the top of the high building has sharp pointed ends.

Reason: The surface density of charge at sharp points is very high resulting in the setting up of electric wind.

Putting a dielectric substance between two plates of condenser: capacity, potential and potential energy, respectively 

Four closed surfaces $S_1, S_2, S_{3 }\text{and }{S}_4$ together with charges $+q,-q$ and $-3 q$ are shown. Through which one of the surfaces is the net flux zero?

An electric field $\overrightarrow{E}=(2\hat{\mathrm{i}}+\hat{\mathrm{j}}) \mathrm{N} {\mathrm{C}}^{-1}$ exists in space. The potential difference $\left(V_{\mathrm{P}}-V_{\mathrm{Q}}\right)$ between two points whose positions vectors ${\overrightarrow{r}}_P=\hat{\mathrm{i}}+2\hat{\mathrm{j}} \mathrm{m}$ and ${\overrightarrow{r}}_Q=2\hat{\mathrm{i}}+\hat{\mathrm{j}}+\hat{\mathrm{k}} \mathrm{m}$ is

Assertion: Two concentric charged spherical shells are given. The potential difference between the shells depends on charge of inner shell.

Reason: Potential due to charge of outer shell remains same at every point inside the sphere.