A scientist is investigating how two resistors can be connected in a circuit. Explain why connecting two identical resistors in parallel gives a smaller total resistance than connecting just one of those resistors alone in the circuit.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Two identical resistors, each with a resistance of 6 Ω, are available. The scientist considers connecting them in two different ways: using only one resistor, or connecting both resistors in parallel.
Model answer (5 marks)
In a parallel circuit the applied voltage is the same across each resistor, but the current splits into two branches. Only part of the charge flows through each resistor, so not all the charge must pass through a single resistor. Adding a second branch gives an extra path for the charge, reducing the overall opposition to flow. The total resistance of a parallel arrangement is always less than the smallest individual resistance, so here it is less than 6 Ω. With only one resistor, every charge carrier must travel through that single 6 Ω resistor, offering more opposition and therefore a higher resistance.
Examiner tips
- Use the word "parallel" to show you understand the circuit type.
- Explain that current splits and that each branch offers a path.
- State that the total resistance is less than the smallest resistor.
- Show why a single resistor gives higher resistance.
Common mistakes
- Confusing series with parallel, saying resistance adds up.
- Claiming the voltage changes in parallel.
- Forgetting to mention that current splits.
Mark scheme (5 marks)
- In a parallel circuit, the current splits into two separate branches
- Only some of the charge flows along each branch (not all charge passes through both resistors)
- Adding a second parallel branch provides an additional path for current, making it easier for charge to flow overall
- The total resistance in a parallel arrangement is less than the smallest individual resistance
- With only one resistor, all the charge must pass through that single resistor, making it harder for current to flow, so resistance is higher
Key terms in this question
Related
- All OCR A-Level Physics B: Advancing Physics (H557) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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