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AQA GCSE Physics

Circuits, current and resistance — AQA GCSE Physics revision

Free revision notes, key terms, common exam traps and 5 practice questions with answers. About 5 minutes to read.

The basics

  • Current is the rate of flow of charge: I = Q ÷ t. Measured with an ammeter in series.
  • Potential difference is energy transferred per coulomb: V = W ÷ Q. Measured with a voltmeter in parallel.
  • Ohm's law: V = I × R. Resistance is measured in ohms.

Series vs parallel

  • Series: same current everywhere, potential difference shares out, total resistance adds up.
  • Parallel: same potential difference across each branch, currents add, total resistance falls.
  • Adding a resistor in parallel always reduces the total resistance.

Key terms

Current
Rate of flow of electrical charge, in amperes.
Potential difference
Energy transferred per unit charge, in volts.
Resistance
How much a component opposes current, in ohms.
Ohmic conductor
A component with constant resistance, giving a straight-line I-V graph through the origin.
Diode
A component that lets current flow easily in one direction only.
Live wire
The wire that carries alternating current at high voltage in mains wiring.

Common exam traps

  • Putting the ammeter in parallel — it always goes in series.
  • Saying current 'is used up' round a circuit — it isn't.
  • Forgetting that a filament lamp's resistance rises as it heats up.

Practice questions with answers

  1. 1. A 12 V supply drives 3 A through a resistor. Calculate the resistance.

    Answer: 4 Ω

    R = V ÷ I = 12 ÷ 3.

  2. 2. In a series circuit, what stays the same at every point?

    • • Potential difference
    • • Current
    • • Resistance
    • • Power

    Answer: Current

    There is only one path, so current is identical throughout.

  3. 3. Adding a second identical resistor in parallel makes total resistance:

    • • Double
    • • Halve
    • • Stay the same
    • • Zero

    Answer: Halve

    More paths for the current means less total resistance.

  4. 4. Why does the resistance of a filament lamp increase with current?

    Answer: The filament gets hotter, ions vibrate more and electrons collide with them more often.

    Link temperature to increased collisions.

  5. 5. What is the standard unit of charge?

    • • Ampere
    • • Volt
    • • Coulomb
    • • Ohm

    Answer: Coulomb

    Charge is measured in coulombs (C).

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