A student has a battery connected to a single resistor in a series circuit. The student then adds a second identical resistor in series with the first. Explain what happens to the potential difference across the original resistor and the current in the circuit after the second resistor is added.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
The total resistance of the circuit increases when the second identical resistor is added. Because the battery voltage is fixed, Ohm’s law (V=IR) means that the increased resistance reduces the current in the circuit. With the current lower, the voltage drop across each resistor is smaller. Since the two resistors are identical, the supply voltage is divided equally, so the potential difference across the original resistor becomes half the battery voltage, i.e. less than it was before.
Examiner tips
- Show the chain: R_total↑ → I↓ → V_drop per resistor↓
- Use V=IR to link resistance and current
- Mention equal division of voltage for identical resistors
- Keep answer concise, use correct terminology
Common mistakes
- Saying the voltage across the resistor stays the same
- Confusing total voltage with voltage drop across each resistor
- Not linking the increase in resistance to the decrease in current
Mark scheme (5 marks)
- Total resistance of the circuit increases
- Current in the circuit decreases
- Correct reasoning linking increased resistance to decreased current (e.g. greater resistance opposes current flow more / V = IR so for fixed V, greater R means smaller I)
- The potential difference of the supply is shared between / split across both resistors
- The potential difference across the original resistor is less than before / is half the supply voltage (because the resistors are identical)
Key terms in this question
series · potential difference · 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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