The electric potential of the Earth is taken to be zero because the Earth is a good:

$A$ hollow metal sphere of radius $10 \ cm$ is charged such that the potential on its surface becomes $80 \ V$. The potential at the centre of the sphere is . . . . . . . (in $V$)

$A$ hollow metal sphere of radius $5\, cm$ is charged such that the potential on its surface is $10\, V$. The potential at the centre of the sphere is

Charges of $+ \frac{10}{3} \times 10^{-9} \, C$ are placed at each of the four corners of a square of side $8 \, cm$. The potential at the intersection of the diagonals is

Four equal charges $Q$ are placed at the four corners of a square of side $a$. The work done in moving a charge $-Q$ from its centre to infinity is

$A$ charge $+q$ is placed at the origin $O$ of $X-Y$ axes as shown in the figure. The work done in taking a charge $Q$ from $A$ to $B$ along the straight line $AB$ is