As shown in the figure, a point charge $Q$ is placed at the centre of conducting spherical shell of inner radius a and outer radius $b$. The electric field due to charge $Q$ in three different regions I, II and III is given by: $( I : r < a , II : a < r < b , III : r > b )$
$E _{ I }=0, E _{ II }=0, E _{ III } \neq 0$
$E _{ I } \neq 0, E _{ II }=0, E _{ III } \neq 0$
$E _{ I } \neq 0, E _{ II }=0, E _{ III }=0$
$E _{ I }=0, E _{ II }=0, E _{ III }=0$
Assertion : A metallic shield in form of a hollow shell may be built to block an electric field.
Reason : In a hollow spherical shield, the electric field inside it is zero at every point.
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
Three concentric metallic spherical shells of radii $R, 2R, 3R$, are given charges $Q_1, Q_2, Q_3$, respectively. It is found that the surface charge densities on the outer surfaces of the shells are equal. Then, the ratio of the charges given to the shells, $Q_1 : Q_2 : Q_3$ is
A conducting sphere of radius $10\, cm$ is charged $10\,\mu \,C$. Another uncharged sphere of radius $20\, cm$ is allowed to touch it for some time. After that if the sphere are separated, then surface density of charges, on the spheres will be in the ratio of
A thin metallic spherical shell contains a charge $Q$ on it. A point charge $+q$ is placed at the centre of the shell and another charge $q'$ is placed outside it as shown in fig. All the three charges are positive. The force on the central charge due to the shell is :-