Current, Potential Difference and Resistance — AQA GCSE Physics
These three quantities describe how electricity flows in a circuit and are linked by Ohm's law.
The three quantities
- Current (I) — the rate of flow of electric charge, measured in amperes (A). Measured with an ammeter in series.
- Potential difference (V) — the energy transferred per unit charge ("the push"), measured in volts (V). Measured with a voltmeter in parallel.
- Resistance (R) — how much the component opposes the current, measured in ohms (Ω).
Charge, current and time
$$Q = I \times t$$ (charge in coulombs = current × time in seconds)
Ohm's law
$$V = I \times R$$
Rearrange to find any quantity (I = V/R, R = V/I). For current to flow, there must be a closed circuit and a source of potential difference.
Resistance in a circuit
The greater the resistance of a component, the smaller the current for a given potential difference. Adding resistors in series increases the total resistance.
Exam tips
- Learn the definitions and units of current, p.d. and resistance.
- Ammeter in series, voltmeter in parallel.
- Learn Q = It and V = IR, and practise rearranging them.
- More resistance → smaller current for the same p.d.