Explain what happens to the current through each component and the total current from the battery when an additional resistor is added in parallel to an existing resistor already connected to a battery.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A simple circuit contains a battery connected to a single resistor. A second, identical resistor is then connected in parallel with the first.
Model answer (5 marks)
The potential difference across each resistor remains the same (equal to the battery voltage). The current through the original resistor is unchanged. The second (new) resistor also has a current flowing through it. The total current from the battery increases because the total current equals the sum of the currents in each branch. The total resistance of the circuit decreases (is less than the resistance of either individual resistor).
Examiner tips
- Use the command word ‘Explain’ – describe the change and give the reason (Ohm’s law and parallel resistance).
- Show that the voltage across each resistor is the same as the battery voltage.
- State that the current in the original branch is unchanged and that a new current flows through the added resistor.
- Explain that the total current is the sum of the branch currents and that the equivalent resistance falls.
Common mistakes
- Confusing the voltage across each resistor with the total voltage drop.
- Saying the current in the original resistor changes instead of remaining the same.
Mark scheme (5 marks)
- The potential difference across each resistor remains the same (equal to the battery voltage)
- The current through the original resistor is unchanged
- The second (new) resistor also has a current flowing through it
- The total current from the battery increases because the total current equals the sum of the currents in each branch
- The total resistance of the circuit decreases (is less than the resistance of either individual resistor)
Key terms in this question
parallel · current · total current
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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