IGCSE Physics / Current and voltage in circuits
Voltage in series and parallel circuits
Edexcel International GCSE Physics (4PH1), chapter 7: Current and voltage in circuits, specification point 2.18, 2.8. Free lesson.
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Voltage is shared between components in a series circuit, but each branch in a parallel circuit has the full supply voltage. These rules let you work out missing voltmeter readings and explain how voltage is distributed.
How voltage is shared in series
A series circuit has one loop, so the supply voltage is shared between its parts. The voltage drops across all the parts add up to the battery’s supply voltage. A part with more resistance gets a bigger share. With a 6.0 V supply, a lamp can have 2.5 V across it and a resistor 3.5 V across it. Check: 2.5 V + 3.5 V = 6.0 V.
How to find a missing voltage in series
Use the rule that the series voltages add up to the supply. If one voltage is missing, subtract the known voltage from the supply.
Voltage in parallel branches
A parallel circuit has branches connected between the same two junctions. Each branch is connected across the battery, so each gets the full supply voltage. Parallel branches do not share the supply voltage. With a 6.0 V supply, the voltage across the lamp is 6.0 V and the voltage across the resistor is also 6.0 V. Parts in parallel have the same voltage across them.
Compare the two circuit rules
The arrangement tells you how to explain a voltage reading. A voltage is shared only when the parts are in series. In parallel, every branch gets the full supply, even if its resistance is different. Series: each part gets a share, and the shares add up to the supply. More resistance means a bigger share. Parallel: each branch has the full supply voltage.
Use the series rule to answer a question
When a question asks for a missing series voltage, find what the known voltages must add up to. Give the value and the reason. For a 12 V supply with two voltage drops of 3 V and 4 V, the third voltage is 5 V because the three voltages must add up to 12 V.