A student sets up a series circuit containing a battery, a lamp, and a resistor. She then sets up a second circuit where the same lamp and resistor are connected in parallel with each other, and the parallel combination is connected to the same battery. Explain how the current and voltage behave differently in the series circuit compared to the parallel circuit.

Eduqas A-Level Physics — 2.2 DC circuits · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

In a series circuit the components are connected one after another in a single loop. In a parallel circuit the components are connected across separate branches, each connected directly to the battery.

Model answer (5 marks)

In a series circuit the same current flows through the battery, lamp and resistor, so the current is identical in each component. The battery voltage is divided between the lamp and the resistor; each component receives only part of the total voltage.

In a parallel circuit the battery current splits at the junction: the lamp and resistor each receive a separate branch current, which is generally different (usually smaller) than the series current. Each branch is connected directly to the battery, so the full battery voltage appears across both the lamp and the resistor.

Because the lamp in the parallel circuit receives the full battery voltage, it dissipates more power and is brighter than when it is in series, where it receives only a fraction of the battery voltage.

Examiner tips

  • Show the current is the same in series, but splits in parallel; mention voltage division vs equal voltage.
  • Use the phrase "full battery voltage" for parallel branches and "voltage is shared" for series.
  • Explain brightness by relating power to voltage and current.
  • Keep answer concise and use correct terminology (series, parallel, current, voltage).

Common mistakes

  • Confusing current in series with voltage; writing that current is higher in parallel.
  • Saying the lamp is dimmer in parallel instead of brighter.
  • Using UK spelling incorrectly (e.g., "voltage" is correct, not "voltagee").

Mark scheme (5 marks)

  1. In a series circuit the current is the same through every component (lamp and resistor have the same current).
  2. In a parallel circuit the current from the battery splits between the branches, so the lamp and resistor carry different (or smaller) currents.
  3. In a series circuit the battery voltage is shared (divided) between the lamp and the resistor.
  4. In a parallel circuit each branch has the same voltage across it, equal to the battery voltage (both lamp and resistor have the full battery voltage across them).
  5. Because each parallel branch receives the full battery voltage (whereas the series components share a lower voltage), the lamp is brighter in the parallel circuit than in the series circuit.

Key terms in this question

series circuit · parallel circuit · current · voltage

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