If the potential at the centre of a uniformly charged hollow sphere of radius $R$ is $V$ then electric field at a distance $r$ from the centre of the sphere is $(r > R)$

Two charges $3 \times 10^{-8}\; C$ and $-2 \times 10^{-8}\; C$ are located $15 \;cm$ apart. At what point on the line joining the two charges is the electric potential zero? Take the potential at infinity to be zero.

A hollow metallic sphere of radius $R$ is given a charge $Q$. Then the potential at the centre is

A thin spherical shell is charged by some source. The potential difference between the two points $C$ and $P$ (in $V$) shown in the figure is:

(Take $\frac{1}{4 \pi \varepsilon_0}=9 \times 10^9$ $SI$ units)

Three concentric metal shells $A, B$ and $C$ of respective radii $a, b$ and $c (a < b < c)$ have surface charge densities $+\sigma,-\sigma$ and $+\sigma$ respectively. The potential of shell $B$ is