A student connects two identical resistors in a series circuit with a battery. The student then reconnects the same two resistors in a parallel circuit using the same battery. Explain how the current from the battery differs between the two circuits.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
In a series circuit the two identical resistors add, so R_total = R + R = 2R, which is larger than a single resistor.
In a parallel circuit the resistors share the voltage, giving R_total = 1/(1/R + 1/R) = R/2, which is smaller than either resistor.
Because the battery voltage is the same, the lower resistance of the parallel arrangement means a larger current, as I = V/R.
Thus the battery supplies a greater current in the parallel circuit than in the series circuit.
In a parallel circuit the resistors share the voltage, giving R_total = 1/(1/R + 1/R) = R/2, which is smaller than either resistor.
Because the battery voltage is the same, the lower resistance of the parallel arrangement means a larger current, as I = V/R.
Thus the battery supplies a greater current in the parallel circuit than in the series circuit.
Examiner tips
- State the formula for series and parallel resistance, then relate to current using I=V/R.
- Show the comparison of R_total values before concluding about current.
Common mistakes
- Confusing the direction of current flow; the battery supplies the same voltage in both cases.
- Failing to calculate the parallel resistance correctly (using R_total = R/2).
Mark scheme (5 marks)
- In a series circuit, the total resistance equals the sum of both resistors' resistances (so total resistance is greater than one resistor alone)
- In a parallel circuit, the total resistance is less than the resistance of either individual resistor
- The parallel circuit therefore has a lower total resistance than the series circuit
- The greater the resistance, the smaller the current for a given potential difference (or V = IR used as reasoning)
- The current from the battery is greater in the parallel circuit than in the series circuit
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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