A student connects two resistors in a parallel circuit. The student then reconnects the same two resistors in a series circuit using the same power supply. Explain how the total resistance of the circuit and the current drawn from the power supply differ between the parallel arrangement and the series arrangement.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The two resistors each have a resistance of 6 Ω and the power supply has a fixed potential difference.
Model answer (5 marks)
In a series circuit the total resistance is the sum of the individual resistances, so R_total(series)=6 Ω+6 Ω=12 Ω, which is greater than either resistor alone.
In a parallel circuit the total resistance is given by 1/R_total=1/6 Ω+1/6 Ω, giving R_total(parallel)=3 Ω, which is less than the smallest individual resistance.
Thus the parallel arrangement has a lower total resistance than the series arrangement.
Because the potential difference of the supply is unchanged, Ohm’s law (I=V/R) shows that a lower resistance draws a greater current, so the current drawn from the supply is higher in the parallel circuit than in the series circuit.
The parallel circuit offers two paths for charge; each branch carries part of the total current, reducing the overall resistance experienced by the supply.
In a parallel circuit the total resistance is given by 1/R_total=1/6 Ω+1/6 Ω, giving R_total(parallel)=3 Ω, which is less than the smallest individual resistance.
Thus the parallel arrangement has a lower total resistance than the series arrangement.
Because the potential difference of the supply is unchanged, Ohm’s law (I=V/R) shows that a lower resistance draws a greater current, so the current drawn from the supply is higher in the parallel circuit than in the series circuit.
The parallel circuit offers two paths for charge; each branch carries part of the total current, reducing the overall resistance experienced by the supply.
Examiner tips
- State the formula for total resistance in series and parallel; calculate the values. Explain the effect of lower resistance on current using I=V/R. Mention that the supply voltage is constant. Use the exact wording from the mark scheme where possible.
Common mistakes
- Confusing the direction of current flow or using the wrong formula for parallel resistance. Forgetting to state that the supply voltage is unchanged. Claiming that the current is the same in both circuits.
Mark scheme (5 marks)
- In a series circuit the total resistance is the sum of the individual resistances, so total resistance is greater than either individual resistance alone.
- In a parallel circuit the total resistance is less than the smallest individual resistance.
- Therefore the total resistance in the parallel arrangement is lower than in the series arrangement.
- Since the potential difference is the same, a lower resistance means a greater current is drawn from the supply in the parallel circuit (compared to the series circuit).
- In the parallel circuit, charge has more than one branch/path to flow through, so only some of the charge flows through each branch, reducing the overall resistance experienced.
Key terms in this question
total resistance · parallel · series · current
Related
- All OCR A-Level Physics B: Advancing Physics (H557) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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