Electric circuits

O Level / IGCSE Physics · Electricity and magnetism. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

In a series circuit there is only one path. The current is the same everywhere, and the supply p.d. is shared between the components, with the bigger resistance taking the bigger share.

In a parallel circuit the current splits at a junction. Each branch has the same p.d. across it, and the branch currents add up to the current from the supply. Adding a branch lowers the combined resistance.

A potential divider is two resistors in series used to give a chosen fraction of the supply p.d. If one of them is a thermistor or an LDR, the output p.d. changes with temperature or light. An NTC thermistor has less resistance when hotter, and an LDR has less resistance in brighter light.

Rules to remember

Common mistake

Adding parallel resistors as if they were in series, or forgetting to turn 1/R upside down at the end. Check your answer: the combined resistance of resistors in parallel is always less than the smallest one.

Worked example

A 5.0 Ω resistor is in series with a parallel pair of resistors, 20 Ω and 30 Ω. The supply is 8.5 V. What is the p.d. across the 5.0 Ω resistor?

  1. Parallel pair: R = (20 × 30) ÷ (20 + 30) = 600 ÷ 50 = 12 Ω.
  2. Total resistance = 5.0 + 12 = 17 Ω.
  3. Current from the supply = V ÷ R = 8.5 ÷ 17 = 0.50 A.
  4. p.d. across the 5.0 Ω resistor = I × R = 0.50 × 5.0 = 2.5 V.

Answer: 2.5 V (the other 6.0 V is across the parallel pair)

Practice questions

Try each one, then open the answer.

1. Two identical lamps are connected in parallel to a battery. The filament of one lamp breaks. What happens to the other lamp and to the current from the battery?

  1. A
    the other lamp goes out and the current falls to zero
  2. B
    the other lamp gets brighter and the current stays the same
  3. C
    the other lamp gets dimmer and the current doubles
  4. D
    the other lamp is just as bright and the current halves
Show answer

Answer: D. Each parallel branch has the full battery p.d., so the working lamp keeps the same p.d. and current and is just as bright. The battery now supplies only one branch, so its current halves.

2. What happens to the resistance of a light-dependent resistor (LDR) when the light falling on it gets brighter?

  1. A
    it increases
  2. B
    it decreases
  3. C
    it stays the same
  4. D
    it becomes infinite
Show answer

Answer: B. An LDR's resistance falls as light intensity rises. An NTC thermistor behaves the same way with temperature.

3. A potential divider uses resistors R1 = 2 kΩ and R2 = 6 kΩ in series across 12 V. What is the p.d. across R2?

  1. A
    3 V
  2. B
    4 V
  3. C
    8 V
  4. D
    9 V
Show answer

Answer: D. The p.d. divides in the ratio of the resistances: V2 = 12 × 6/(2 + 6) = 9 V.

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