A student connects two resistors in parallel with each other, and this parallel combination is connected in series with a battery and an ammeter. The student measures the current from the battery. She then removes one of the resistors from the parallel combination, leaving only a single resistor connected in series with the battery and ammeter. Explain what happens to the current reading on the ammeter and the voltage across the remaining resistor when one resistor is removed.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student is investigating how the arrangement of resistors in a circuit affects current and voltage.
Model answer (5 marks)
Removing one resistor reduces the number of parallel paths, so the total resistance of the circuit increases.
Because the battery voltage (e.m.f.) is constant, a higher resistance means the total current from the battery decreases.
The ammeter reading therefore falls.
With only one resistor left, the full battery voltage now appears across that resistor – the voltage across it increases (or remains the same as the supply, but is no longer shared with a second resistor).
In the original parallel set‑up both resistors shared the same battery voltage; after removal the remaining resistor has a higher voltage across it than before, equal to the supply voltage.
Because the battery voltage (e.m.f.) is constant, a higher resistance means the total current from the battery decreases.
The ammeter reading therefore falls.
With only one resistor left, the full battery voltage now appears across that resistor – the voltage across it increases (or remains the same as the supply, but is no longer shared with a second resistor).
In the original parallel set‑up both resistors shared the same battery voltage; after removal the remaining resistor has a higher voltage across it than before, equal to the supply voltage.
Examiner tips
- Use the command word ‘Explain’ – describe the cause (increased resistance) and the effect (lower current, higher voltage).
- Show the relationship V=IR and note that V is constant, so I∝1/R.
- Mention that the voltage across the remaining resistor equals the battery voltage because no other drop occurs.
- Keep the answer concise – 5 points can be covered in 5 short sentences.
Common mistakes
- Confusing current with voltage – writing that the voltage drops when a resistor is removed.
- Failing to recognise that the battery voltage is constant, not the current.
- Saying the voltage across the remaining resistor stays the same without explaining why it equals the supply voltage.
Mark scheme (5 marks)
- Removing one resistor reduces the number of parallel paths, so the total resistance of the circuit increases.
- Because the battery voltage (e.m.f.) stays the same / is constant, a higher resistance means the total current from the battery decreases.
- The ammeter reading decreases (the current falls).
- The full battery voltage now acts across the single remaining resistor (voltage across it increases).
- In the original parallel combination both resistors shared the same voltage (equal to the battery voltage), whereas in the new series-only circuit there is no voltage drop elsewhere, so the remaining resistor has a higher voltage across it than before / the same as the supply.
Key terms in this question
Related
- All Eduqas A-Level Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More DC circuits questions
- A student has a circuit containing a battery and two lamps connected in series. …
- A student builds a circuit containing a battery, a switch, an ammeter, and three…
- A student sets up a series circuit containing a battery, a lamp, and a resistor.…
- A student has a battery connected to three identical lamps. She first connects a…