AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Current, resistance and potential difference: combinations of resistors in series and parallel.
Resistors can be connected in series or in parallel, and the total resistance depends on the arrangement.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Resistors can be connected in series or in parallel, and the total resistance depends on the arrangement. In series, the same current flows through each resistor, the potential differences add to give the supply potential difference, and the total resistance is the sum of the individual resistances: R_total = R₁ + R₂. In parallel, the potential difference across each branch is the same, and the currents in the branches add to give the total current. Adding a parallel branch gives more paths for charge, so total resistance falls. The total resistance of two resistors in parallel is less than the resistance of the smallest individual resistor. Students are not required to calculate the exact total resistance of parallel circuits using the reciprocal formula.
What earns the marks
- States that in series the current is the same through each resistor and the potential differences add to equal the supply potential difference.
- Calculates total resistance in series using R_total = R₁ + R₂ + ... .
- States that in parallel the potential difference across each branch is the same and the branch currents add to give the total current.
- States that the total resistance of two resistors in parallel is less than the resistance of the smallest individual resistor.
- Explains the parallel result in terms of additional paths for charge carriers, which increases total current for the same potential difference and therefore lowers resistance.
Common mistakes and how to put them right
Adding resistances directly for parallel resistors.
Put it right: Adding resistors in parallel decreases the total resistance, making it less than the smallest individual resistor.
Believing current is used up as it passes through series resistors.
Put it right: Current is conserved in a series circuit, so the same current flows through each component.
Thinking that adding a resistor in parallel increases total resistance.
Put it right: Adding a parallel branch provides another path, so total resistance decreases.
Exam tips
- Sketch the circuit and label the current through and potential difference across each resistor before substituting numbers into a formula.
- For parallel circuits, a quick check is that the total resistance must be smaller than the smallest branch resistance.
- Remember that you do not need to calculate the exact total resistance of parallel resistors, just know it is less than the smallest resistor.
Try the lesson
The start of the Current, resistance and potential difference: combinations of resistors in series and parallel. lesson, made from checked teaching notes for this specification point.
Have a go first
Two resistors of 3 Ω and 5 Ω are connected in series. What is the total resistance of the combination?
Key words
Say what each word means before you turn the card.
Series
Try answering aloud or in your head before turning the card.
Parallel
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Total resistance
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Paths for charge
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See every definition
- Series
- The same current flows through each resistor and the potential differences add to give the supply potential difference.
- Parallel
- The potential difference across each branch is the same and the currents in the branches add to give the total current.
- Total resistance
- In series it is the sum of the individual resistances, and for two resistors in parallel it is less than the smallest individual resistor.
- Paths for charge
- Adding a parallel branch gives more paths for charge, so total resistance falls.
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