Consider an initially neutral hollow conducting spherical shell with inner radius $r$ and outer radius $2 r$. A point charge $+Q$ is now placed inside the shell at a distance $r / 2$ from the centre. The shell is then grounded by connecting the outer surface to the earth. $P$ is an external point at a distance $2 r$ from the point charge $+Q$ on the line passing through the centre and the point charge $+Q$ as shown in the figure. The magnitude of the force on a test charge $+q$ placed at $P$ will be

A metallic spherical shell has an inner radius ${{\rm{R}}_1}$ and outer radius ${{\rm{R}}_2}$. A charge $\mathrm{Q}$ is placed at the centre of the spherical cavity. What will be surface charge density on $(i)$  the inner surface, and $(ii)$ the outer surface ?

Assertion : In a cavity within a conductor, the electric field is zero.

Reason : Charges in a conductor reside only at its surface

Two isolated metallic solid spheres of radii $R$ and $2 R$ are charged such that both have same charge density $\sigma$. The spheres are then connected by a thin conducting wire. If the new charge density of the bigger sphere is $\sigma^{\prime}$. The ratio $\frac{\sigma^{\prime}}{\sigma}$ is

Obtain the relation between electric field and electric potential.