Let us consider a conductor which has more negative charge (excess of electrons) at one end and more positive charge (comparatively less electrons). This separation exerts forces (Coulomb forces) on each other. This coulomb force is balanced by the internal forces of the conductor. This action causes energy to be stored. This stored energy is known as potential energy. 

Voltage is defined as ‘energy per unit charge’ or ‘work per unit charge’ required to move a positive charge from one end to the other end of the conductor. 

Voltage is expressed as 

        v=\frac{dw}{dq}, Volts (V)

The work done is represented by ‘w’ measured in joules and ‘q’ is the charge measured in coulombs. Hence one volt is equal to one joule per coulomb. The voltage can also be mentioned by  V. 

Example: Let an electric circuit requires 9.25 J to move 0.5 C from one point to another. Find the potential difference between these two points. 

        \bg_white \large \;v=\frac{dw}{dq}=\;\frac{9.2510-6\;J\;}{\;0.510-6\;C}=\;18.5\;V