An arc of radius $r$ carries a charge. The linear density of charge is $\lambda$ and the arc subtends an angle $\frac{\pi }{3}$ at the centre. What is the electric potential at the centre?

Two identical thin rings each of radius $R$ meters are coaxially placed at a distance $R$ meters apart. If $Q_1$ coulomb and $Q_2$ coulomb are respectively the charges uniformly spread on the two rings, the work done in moving a charge $q$ from the centre of one ring to that of other is

If the electric potential of the inner shell is $10$ volt and that of the outer shell is $5$ volt, then the potential at the centre will be: (the shells are uniformly charged)