Electrical Terminology
The basic quantities
Current (ampere) is the rate of charge flow, I = Q/t; one ampere is one coulomb per second. Electromotive force (emf) is the energy per unit charge supplied by a source; potential difference is the energy per unit charge converted by a load. Both are measured in volts, but emf is a property of the source while pd is a property of the circuit. Energy (joule) = V x Q; power (watt) = V x I, the rate of energy transfer.
Resistance R = V/I in ohms; conductance is its reciprocal. Impedance is the general AC term covering both resistance and reactance.
Measurement rules
Ammeter in series, low resistance. Voltmeter in parallel, high resistance. Ohmmeter on an isolated, de-energised component. An ideal voltmeter has infinite resistance; a real one slightly loads the circuit.
Traps
- The kilowatt-hour is energy, not power: power x time in hours.
- Terminal voltage falls below emf when current flows because of internal resistance: V = E − Ir.
- The term emf is used even where nothing is moving, for example a photocell.
- SI prefixes: 1 MΩ = 10⁶ Ω, 1 µF = 10⁻⁶ F, 1 nF = 10⁻⁹ F, 1 mH = 10⁻³ H.
Worked example
A 24 V battery delivers 3 A. Power = 24 x 3 = 72 W. Run for 2 hours: energy = 144 Wh = 0.144 kWh. If internal resistance is 0.5 Ω, terminal voltage = 24 − (3 x 0.5) = 22.5 V.