If in any part of an electrical circuit, some other form of energy is converted into electrical energy, an e.m.f. is said to exist at that part of the circuit. Whereas the ratio of work done in taking a test charge from one point to another in an electric field to that of the magnitude of the test charge is called the potential difference between those two points. Potential difference is said to be created at the point electrical energy is being converted into some other form of energy in a closed circuit. E.m.f. is the reason, while p.d. is its consequence. As expected, the magnitude of e.m.f. is always higher than the p.d. Both the terms describe energy transferred per unit chage, though, and is expressed in the same unit, Volts.