A level Physics · Edexcel 9PH0 · Topic 3C
Series and parallel circuits
Key points
- Series: the same current passes through each component; p.d.s add up to the supply; total resistance .
- Parallel: each branch has the same p.d.; currents add up; . Total resistance is always less than the smallest branch.
- These rules come from conservation of charge (currents at junctions) and conservation of energy (p.d.s around loops).
- For combination circuits, reduce parallel groups to a single resistance first, then add series parts.
- Adding a resistor in parallel lowers the total resistance and increases the current from the supply.
- An ideal ammeter has zero resistance and goes in series; an ideal voltmeter has infinite resistance and goes in parallel.
- Exam tip: when one component changes, work through the effect on total resistance, current, then each p.d. step by step.
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Most circuits combine components in series and in parallel. These notes show where the combination rules come from and how to analyse a network one step at a time.
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