Explain why adding a second resistor in parallel to a circuit reduces the total resistance of the circuit, even though more components have been added.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
In a parallel circuit the current from the supply splits between the branches, so the charge has more than one path to take.
Adding a second branch allows more current to flow from the same supply; the total current increases.
Because the same potential difference now drives a larger current, the total resistance of the circuit must be lower (V = IR, resistance opposes current flow).
The resulting total resistance is always less than the resistance of the smallest individual branch, and therefore less than either of the individual resistors.
Adding a second branch allows more current to flow from the same supply; the total current increases.
Because the same potential difference now drives a larger current, the total resistance of the circuit must be lower (V = IR, resistance opposes current flow).
The resulting total resistance is always less than the resistance of the smallest individual branch, and therefore less than either of the individual resistors.
Examiner tips
- Use the word "splits" or "divides" to show understanding of current flow. Show the relationship V = IR to justify why increased current means lower resistance. Mention that the total resistance is less than the smallest resistor to hit the final point.
Common mistakes
- Confusing series and parallel behaviour – writing that resistance increases. Forgetting to explain that the same voltage is applied across each branch. Not recognising that the total resistance is less than the smallest individual resistor.
Mark scheme (4 marks)
- In a parallel circuit, the current splits between the branches / charge has more than one path to take
- Adding a second branch means more current can flow from the same supply / total current increases
- Greater total current from the same potential difference means the total resistance must be lower (V = IR / resistance opposes current flow)
- The total resistance is always less than the resistance of the smallest individual branch / less than either individual resistor
Key terms in this question
Related
- All AQA GCSE Physics (8463) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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