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).
Model answer (5 marks)
In a parallel circuit the current splits, so each resistor has a separate path for the charge to flow.
Only part of the total charge passes through each branch, not all of it.
Adding a second resistor gives the charge an additional path, making it easier for the charge to move.
Because there are more paths, the overall resistance is lower than that of any single resistor.
The potential difference across each branch is the same as the supply voltage, so the extra path does not change the voltage but reduces the resistance.
Only part of the total charge passes through each branch, not all of it.
Adding a second resistor gives the charge an additional path, making it easier for the charge to move.
Because there are more paths, the overall resistance is lower than that of any single resistor.
The potential difference across each branch is the same as the supply voltage, so the extra path does not change the voltage but reduces the resistance.
Examiner tips
- Use the word ‘parallel’ to show you understand the circuit type.
- Explain that current splits and each branch has the same voltage.
- Show that more paths mean less resistance.
- Mention that the total resistance is always less than the smallest individual resistor.
Common mistakes
- Saying the resistance is the same as one resistor.
- Confusing series and parallel relationships.
- Ignoring that the voltage across each resistor is equal to the supply voltage.
Mark scheme (5 marks)
- In a parallel circuit, current splits / has more than one path to travel through
- Only some of the charge flows through each branch / resistor
- Adding a second resistor in parallel gives the charge an additional path / makes it easier for charge to flow
- The overall / total resistance decreases as more parallel branches are added
- The potential difference across each branch remains the same / equal to the supply potential difference
Key terms in this question
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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