As shown in the figure,a point charge $Q$ is placed at the centre of a 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: $(I: r < a, II: a < r < b, III: r > b)$

$A$ metallic rod is placed in a uniform electric field. Select the correct option.

$A$ 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:

Consider a finite insulated,uncharged conductor placed near a finite positively charged conductor. The uncharged body must have a potential

Assertion : In a cavity within a conductor,the electric field is zero.
Reason : Charges in a conductor reside only at its surface.

Three identical uncharged metal spheres are at the vertices of an equilateral triangle. One at a time,a small sphere is connected by a conducting wire with a large metal sphere that is charged. The center of the large sphere is on the straight line perpendicular to the plane of the equilateral triangle and passing through its center (see figure). As a result,the first small sphere acquires charge $q_1$ and the second acquires charge $q_2$ $(q_2 < q_1)$. The charge that the third sphere $q_3$ will acquire is (Assume $l >> R$,$l >> r$,$d >> R$,$d >> r$):