A torch uses a 6 V battery and contains a filament lamp connected in series with two identical resistors. The potential difference across each resistor is 1.5 V. Explain how the potential differences across the lamp and the two resistors are related to the battery voltage, and state the potential difference across the lamp.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A torch contains a 6 V battery connected in series with a filament lamp and two identical resistors. The potential difference across each resistor is 1.5 V.
Model answer (4 marks)
In a series circuit the potential differences across the components add to the supply voltage. The two identical resistors each have 1.5 V, so together they drop 1.5 V + 1.5 V = 3 V. The remaining voltage is across the filament lamp: 6 V – 3 V = 3 V. The lamp therefore has a greater potential difference than each resistor because it has a larger resistance and, in series, the component with the greatest resistance takes the greatest share of the total voltage.
Examiner tips
- State that the voltages in series add to the supply voltage.
- Calculate the total drop across the two resistors and subtract from the battery voltage.
- Explain that the lamp has a larger resistance, so it takes the larger share of the voltage.
Common mistakes
- Saying the lamp has the same voltage as a resistor, ignoring the series rule.
- Using the wrong sign when subtracting (e.g., 3 V – 6 V).
- Failing to mention the reason the lamp has a larger voltage drop.
Mark scheme (4 marks)
- In a series circuit, the potential differences across the components add up to / are shared to equal the total potential difference of the supply (6 V).
- The total potential difference across both resistors is 1.5 + 1.5 = 3 V.
- The potential difference across the lamp is 6 − 3 = 3 V.
- The lamp has a greater potential difference across it than each individual resistor because it has a greater resistance (in a series circuit, a component with greater resistance has a larger share of the potential difference).
Key terms in this question
Related
- All Pearson Edexcel International GCSE Physics (4PH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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