We define the amount of electrical potential energy that a unit charge has as:

The bigger the energy change betwen two points in the circuit the bigger the difference in electrical potential. We call this the potential difference between those two points.

The potential difference between two points in a circuit is the work done in moving unit charge (i.e. one coulomb) from one point to the other. The units for potential difference are Joules per coulomb, or volts. (1
volt = 1 Joule/coulomb).

Along the connecting wire from the cell to B there is no resistance and so no loss of
electrical energy or drop in potential.

In the resistors r and R energy is converted to heat
and so the potential drops from B through to E.

From E to the cell there is no loss of
electrical energy so the potential at E is the same as that at the negative terminal of the cell –
zero.