The amount of work done in moving a unit charge from one point to another point is called potential difference between two points in an electric circuit.
For example :
If the work done in moving a charge of 2 coulomb (Q) is 20 Joules (W), then the potential difference across this circuit is the ratio of work done on moving this charge to the magnitude of thie charge.
Mathematically the potential difference (V) across this circuit is
All the other examples can be explained in the similar manner.