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Recall and use the equation for electromotive force (e.m.f.) E = W / Q.
Electromotive force (e.m.f.) is the energy supplied by a source per unit charge that moves through it. It is essentially the driving force that causes electric charge to flow in a circuit.
The unit of e.m.f. is the volt (V), which is equivalent to Joules per Coulomb (J/C).
The relationship between the work done (W) in moving a charge (Q) and the electromotive force (E) is given by the following equation:
$$E = \frac{W}{Q}$$Where:
This equation states that the e.m.f. is equal to the work done per unit charge. A larger e.m.f. means more work is done on the charge for each Coulomb of charge that passes through the source.
Consider a battery that does 100 Joules of work to move 2 Coulombs of charge. What is the electromotive force of the battery?
Using the equation E = W / Q:
E = 100 J / 2 C = 50 V
Therefore, the electromotive force of the battery is 50 Volts.
Potential difference (p.d.) is the difference in electric potential between two points. It is the 'push' that drives electric current through a circuit. It is often considered to be the same as the electromotive force in a simple circuit.
The unit of potential difference is also the volt (V).
In an ideal circuit (without internal resistance), the electromotive force is equal to the potential difference across the terminals of the source.
| Concept | Definition | Unit |
|---|---|---|
| Electromotive Force (e.m.f.) | Energy supplied per unit charge | Volt (V) |
| Potential Difference (p.d.) | Difference in electric potential between two points | Volt (V) |