Two charged metallic spheres are joined by a very thin metal wire. If the radius of the larger sphere is four times that of the smaller sphere,the electric field near the larger sphere is

Shown in the figure are two point charges $+Q$ and $-Q$ inside the cavity of a spherical shell. The charges are kept near the surface of the cavity on opposite sides of the centre of the shell. If $\sigma_1$ is the surface charge density on the inner surface and $Q_1$ is the net charge on it,and $\sigma_2$ is the surface charge density on the outer surface and $Q_2$ is the net charge on it,then:

$A$ positive point charge $q$ is placed at a distance $2R$ from the surface of a metallic shell of radius $R$. The electric field at the centre of the shell due to the induced charge has a magnitude of:

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

$A$ charged spherical conductor of radius $R$ is connected momentarily to another uncharged spherical conductor of radius $r$ by means of a thin conducting wire. Then,the ratio of the surface charge density of the first to the second conductor is:

$A$ small conducting sphere is hung by an insulating thread between the plates of a parallel plate capacitor as shown in the figure. The net force on the sphere is