An empty thick conducting shell of inner radius $a$ and outer radius $b$ is shown in figure.If it is observed that the inner face of the shell carries a uniform charge density $-\sigma$ and the surface carries a uniform charge density $ '\sigma '$

If a point charge $q_A$ is placed at the center of the shell, then choose the correct statement $(s)$

A conducting sphere $A$ of radius $a$, with charge $Q$, is placed concentrically inside a conducting shell $B$ of radius $b$. $B$ is earthed. $C$ is the common centre of the $A$ and $B$. 

A solid conducting sphere of radius $a$ has a net positive charge $2Q$. A conducting spherical shell of inner radius $b$ and outer radius $c$ is concentric with the solid sphere and has a net charge $-Q$. The surface charge density on the inner and outer surfaces of the spherical shell will be

Four metal conductors having different shapes

$1.$  A sphere     $2.$  Cylindrical

$3.$  Pear            $4.$  Lightning conductor

are mounted on insulating stands and charged. The one which is best suited to retain the charges for a longer time is

Figure shows a solid conducting sphere of radius $1 m$, enclosed by a metallic shell of radius $3 \,m$ such that their centres coincide. If outer shell is given a charge of $6 \,\mu C$ and inner sphere is earthed, find magnitude charge on the surface of inner shell is ............. $\mu C$

‘Inside a conductor electrostatic field is zero’. Explain.