A thunder cloud and the earth's surface may be regarded as a pair of charged parallel plates separated by a distance $h$ and the capacitance of the system is $C$. When a flash of mean current $i$ occurs for a time duration $t$, the electric field strength between the cloud and earth is reduced by

A combination of capacitors is set up as shown in the figure. The magnitude of the electric field, due to a point charge $Q$ (having a charge equal to the sum of the charges on the $4 \mu \mathrm{F}$ and $9 \mu \mathrm{F}$ capacitors), at a point distance $30 \mathrm{m}$ from it, would equal

Two identical capacitors have the same capacitance $C$. One of them is charged to potential $V_1$ and the other to $V_2$. The charged ends of the capacitors are connected together. When the positive ends also connected, the decrease in energy of the combined system is 

A capacitor of $2 \mathrm{\mu F}$ is charged as shown in the diagram. When the switch$S$  is turned to position $2$ the percentage of its stored energy dissipated is 

If $\sigma$ is the surface charge density of a charge for charged sphere of radius $R$ kept in a medium of dielectric constant $k$, then the electric intensity at a distance $r$ from its centre is ,where $r>R$ $\left({\epsilon }_{\circ }=permittivity of free space\right)$

A charge $Q$ is uniformly distributed over a long rod $A B$ of length $L$ as shown in the figure. The electric potential at the point $O$ lying at a distance $L$ from the end $A$ is

The region between two concentric spheres of radii $a$ and $b$, respectively (see figure), has volume charge density $\rho=\frac{A}{r},$ where $A$ is a constant and $r$ is the distance from the centre. At the centre of the spheres is a point charge $Q$. The value of $A$ such that the electric field in the region between the spheres will be constant, is:

Two capacitors $C_1$ and $C_2$ are charged to $120 \mathrm{V}$ and $200 \mathrm{V}$, respectively. It is found that by connecting them together, the potential on each one can be made zero. Then