(a) The work done in bringing a point charge from infinity to any point in the electric field is defined as the electrostatic potential energy of the charge and is measured in joules (J).
The work done against the electric field in carrying a unit positive test charge from infinity to any point in electric field is defined as the electric potential at that point and is measured in volts (V).
(b) The electric potential difference generally called as voltage. The electric potential difference and the voltage are the same physical quantities and both are measured in the units of volts (V).
Electric potential difference is defined as electric potential energy difference per unit charge, and therefore does not depend on the amount of charge moved. Thus, the expression for the electric potential difference is,
Here, the electric potential energy difference and q is the charge of the particle.