$A$ charge $q$ is distributed uniformly on the surface of a sphere of radius $R$. It is covered by a concentric hollow conducting sphere of radius $2R$. What will be the charge on the outer surface of the hollow sphere if it is earthed?

Two charged spheres of radius $R_1$ and $R_2$ respectively are charged and joined by a wire. The ratio of the electric field at the surfaces of the spheres is . . . . . . .

$A$ solid conducting sphere has a cavity,as shown in the figure. $A$ charge $+q_1$ is situated away from the center. $A$ charge $+q_2$ is situated outside the sphere. Then the true statement is:

Two spherical conductors $A$ and $B$ of radii $1\, mm$ and $2\, mm$ are placed at a distance of $5\, cm$ from each other. They carry equal charges. If the spheres are connected by a conducting wire,they reach an equilibrium state. The ratio of the magnitudes of the electric fields at the surfaces of spheres $A$ and $B$ is ........

What is a Van de Graaff generator? Explain its principle.

The figure shows three concentric metallic spherical shells. The outermost shell has charge $q_2$,the innermost shell has charge $q_1$,and the middle shell is uncharged. The charge appearing on the inner surface of the outermost shell is