A student connects two resistors in a parallel circuit. Explain why the total resistance of two resistors connected in parallel is less than the resistance of either individual resistor.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student is building a circuit for a school project. They notice that when they connect two resistors in parallel, the total resistance of the combination is lower than the resistance of either resistor on its own. The student finds this surprising, as they expected adding more components to always increase resistance.
Model answer (5 marks)
In a parallel circuit the applied voltage is the same across each resistor, so the current splits and each resistor provides a separate path for charge to flow. Because charge can flow through more than one branch, the overall opposition to the flow of charge is reduced. The effective resistance is therefore less than the resistance of any single resistor in the pair, and in fact is always lower than the smallest individual resistance. In contrast, in a series circuit all the charge must pass through every resistor, so the resistances add and the total resistance is higher.
Examiner tips
- Use the phrase "current splits" and mention "multiple paths" to show understanding of parallel behaviour.
- State that the effective resistance is less than the smallest individual resistor – a key point in the mark scheme.
- Contrast with series to demonstrate full knowledge of both configurations.
- Include the reason "less overall opposition to charge flow" to hit the explanation mark.
Common mistakes
- Confusing series and parallel – writing that resistance adds in parallel.
- Failing to mention that the voltage is the same across each resistor.
- Claiming the total resistance equals the smallest resistor instead of being less than it.
Mark scheme (5 marks)
- In a parallel circuit, the current splits and takes more than one path (branch)
- Only some of the total charge flows through each individual resistor / branch
- This means there is less overall opposition to the flow of charge compared to a single resistor
- The total resistance is less than the smallest individual resistance in the parallel combination
- Contrast with series: in series all charge must pass through every resistor, so resistances add up and total resistance increases
Key terms in this question
Related
- All OCR A-Level Physics A (H556) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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