Two conducting spheres of radii $9 \,cm$ and $1 \,cm$ are separated by a distance of $20 \,cm$ in free space. If the spheres are charged to the same potential of $10 \,V$ each, the force of repulsion between them is

Two point charges $-q$ and $+q$ are located at points $(0,0,-a)$ and $(0,0, a)$ respectively. The electric potential at a point $(0,0, z)$,where $z>a$ is

Three concentric spherical shells have radii $a, b$ and $c$ $(a < b < c)$ and have surface charge densities $\sigma, -\sigma$ and $\sigma$ respectively. If $V_A, V_B$ and $V_C$ denote the potentials of the three shells,then,for $c = a + b$,we have

Derive an expression for the electric potential at a distance $r$ from a positive point charge $Q$.

$A$ non-conducting ring of radius $0.5\,m$ carries a total charge of $1.11 \times 10^{-10}\,C$ distributed non-uniformly on its circumference,producing an electric field $\vec{E}$ everywhere in space. The value of the line integral $\int_{l = \infty }^{l = 0} { - \vec{E} \cdot d\vec{l} }$ (where $l = 0$ is the centre of the ring) in volts is:

$A$ charge of $20 \, C$ moves through a distance of $2 \, cm$. The work done is $2 \, J$. The potential difference between the two points is ........ $V$.