AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Series and parallel circuits: the total resistance of two resistors is less than the resistance of the smallest individual resistor.
When a second resistor is added in parallel, it provides an additional pathway for the current to flow.
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
When a second resistor is added in parallel, it provides an additional pathway for the current to flow. Because the potential difference across each branch is the same as the power supply, the total current leaving the supply increases. According to the equation R = V / I, if the total current increases for a constant potential difference, the total resistance of the circuit must decrease. Therefore, the combined resistance is always less than the resistance of the smallest individual resistor.
What earns the marks
- Identifies that adding resistors in parallel provides additional pathways for the current to flow.
- Explains that the total current drawn from the power supply increases when a parallel branch is added.
- Links the increase in total current to a decrease in total resistance for a given potential difference (using R = V / I).
- Concludes that the total resistance of a parallel combination is always less than the resistance of the smallest individual resistor.
Common mistakes and how to put them right
Adding the resistance values together for components in parallel.
Put it right: Resistances are only added together in series circuits; in parallel, the total resistance decreases.
Stating that the total resistance is the average of the parallel resistors.
Put it right: The total resistance is always strictly less than the smallest individual resistor in the parallel circuit.
Attempting to use the reciprocal formula and making a mathematical error.
Put it right: Calculations of parallel resistance are not required; explain the decrease qualitatively using current pathways.
Exam tips
- Use the concept of 'additional pathways' to explain why total current increases and total resistance decreases in parallel circuits.
- If asked to compare a 10 Ω and a 100 Ω resistor in parallel, you can immediately state the total resistance is less than 10 Ω without any calculation.
Try the lesson
The start of the Series and parallel circuits: the total resistance of two resistors is less than the resistance of the smallest individual resistor. lesson, made from checked teaching notes for this specification point.
Have a go first
Two resistors are connected in parallel. One resistor has a resistance of 4 Ω and the other has a resistance of 12 Ω. Which statement about the total resistance of this parallel combination is correct?
Key words
Say what each word means before you turn the card.
Parallel circuits and current pathways
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Qualitative resistance behaviour
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Relationship between current and resistance
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See every definition
- Parallel circuits and current pathways
- Adding a second resistor in parallel provides an additional pathway for the current to flow.
- Qualitative resistance behaviour
- Adding resistors in parallel decreases the total resistance, and the combined resistance is always less than the resistance of the smallest individual resistor.
- Relationship between current and resistance
- Because the potential difference across each branch is the same as the power supply, the total current leaving the supply increases, so according to R = V / I the total resistance must decrease.
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