When a charge of $0.01 \ C$ is moved from point $A$ to point $B$ against an electric field,the work done is $15 \ J$. The potential difference $(V_B - V_A)$ is ....... $V$.

Along the $x$-axis,three charges $\frac{q}{2}, -q$ and $\frac{q}{2}$ are placed at $x=0, x=a$ and $x=2a$ respectively. The resultant electric potential at a point $P$ located at a distance $r$ from the charge $-q$ $(r > a)$ is ($\varepsilon_0$ is the permittivity of free space):

$A$ charge of total amount $Q$ is distributed over two concentric hollow spheres of radii $r$ and $R$ $(R > r)$ such that the surface charge densities on the two spheres are equal. The electric potential at the common centre is

If the electric potential of the inner sphere is $10\, V$ and that of the outer shell is $50\, V$,then the potential at the common centre is :- (in $, V$)

Four electric charges $+q, +q, -q$, and $-q$ are placed in order at the corners of a square of side $2r$. The electric potential at a point midway between the two negative charges is