Electromotive Force , E.M.F

When you measure the potential difference across (or between) the terminals of a battery you are measuring the ”electromotive force” (emf) of the battery.
This is how much potential energy the battery has to make charges move through the circuit. This driving potential energy is equal to the total potential energy drops in the circuit. This means that the voltage across the battery is equal to the sum of the voltages in the circuit.

E.m.f. = sum of potential difference across the whole circuit, V_total

Worked example :
What is the voltage across the unknown resistor in the circuit shown?

Solution :
V_battery = V_total

V_battery = V1 + V_resistor

3V = V1 + 1V

V1 = 2V