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Science · Topic

Circuits, resistance and electrical power

Draw the circuit, label the values, then choose V = IR or P = VI.

3 original questions in the bank, worth 8 marks · above the two-per-topic floor

Drill this topic All circuits, resistance and electrical power questions

The questions, with the examiner reasoning

Calculate3 marksBGCSE

A 12 V battery is connected across a 6.0 Ω resistor. Calculate the current in the circuit and the power dissipated by the resistor.

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Answer: 2.0 A and 24 W

  1. I = V ÷ R = 12 ÷ 6.0 = 2.0 A
  2. P = VI = 12 × 2.0
  3. P = 24 W

Examiner technique: Two answers means two calculations and two units. An unlabelled "2.0, 24" answer is at the mercy of the marker, and there is an easier way to live.

Calculate1 markBoth boards

Two 6.0 Ω resistors are connected in parallel. What is their combined resistance?

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Answer: 3.0 Ω

  1. 1/R = 1/6.0 + 1/6.0 = 2/6.0
  2. 1/R = 0.333…, so R = 3.0 Ω
  3. Sanity check: less than either single resistor
  • A. 3.0 Ω — Correct: two equal resistors in parallel halve, and the formula 1/R = 1/6 + 1/6 gives 3.0 Ω.
  • B. 12 Ω — That is what you get in series. In parallel there are more paths, so the total resistance falls.
  • C. 6.0 Ω — The value was copied without combining. Two paths carry more current than one.
  • D. 0.33 Ω — That is 1/R, not R. The final step is to invert.

Examiner technique: Two equal resistors in parallel always give half the value. Knowing that lets you check every answer in one second.

Compare4 marksBoth boards

A student has two lamps and one battery. Describe what happens when the lamps are connected in series, then compare that with connecting them in parallel, referring to current, p.d. and brightness in each case.

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Answer: Series: one path, same current through both lamps, p.d. shared, both dimmer. Parallel: two paths, same p.d. across each lamp, larger total current from the battery, each lamp at normal brightness.

  1. Series: a single path, so the current through both lamps is the same (1 mark)
  2. Series: the battery p.d. is shared between the lamps, so each lamp is dimmer (1 mark)
  3. Parallel: each lamp is connected across the battery, so each has the full p.d. and normal brightness (1 mark)
  4. Parallel: the total current from the battery is the sum of the branch currents, so it is larger and the battery drains faster (1 mark)

Examiner technique: Write comparisons as pairs — one sentence with "whereas" in the middle scores two marks that two separate paragraphs do not.