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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. It is the driving force that causes a current to flow in a circuit.
Definition: The electromotive force (e.m.f.) of a source is the work done by the source in transferring one coulomb of charge.
Symbol: The e.m.f. is represented by the letter E.
The equation for e.m.f. is:
$$E = \frac{W}{Q}$$Where:
The equation E = W / Q states that the e.m.f. is equal to the work done per unit charge. A higher e.m.f. means the source is capable of doing more work on the charge, resulting in a larger current flow.
A battery does 100 Joules of work when transferring 20 Coulombs of charge.
To find the e.m.f. of the battery, we use the equation:
$$E = \frac{W}{Q} = \frac{100 \, J}{20 \, C} = 5 \, V$$Therefore, the e.m.f. of the battery is 5 Volts.
Potential difference (p.d.) is the difference in electric potential between two points in a circuit. It is the 'push' that causes charges to flow.
Definition: Potential difference is the work done per unit charge to move a charge between two points.
Symbol: Potential difference is represented by the letter V.
In an ideal circuit, the e.m.f. of the source is equal to the potential difference across its terminals. However, in a real circuit, some of the e.m.f. is used to overcome internal resistance within the source. Therefore, the potential difference across the terminals of a real source is slightly less than its e.m.f.
| Concept | Definition | Symbol | Units |
|---|---|---|---|
| Electromotive Force (e.m.f.) | Work done per unit charge by a source. | E | Volts (V) |
| Potential Difference (p.d.) | Work done per unit charge to move a charge between two points. | V | Volts (V) |