Edexcel IGCSE Physics · Spec 2.10-2.12
Series & Parallel Circuit Rules
The rules for current, voltage and resistance in series and parallel circuits.
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Series & Parallel Circuit Rules
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Explained
Series and parallel, and the rules that follow from the paths
All six rules in this topic come from one question: how many paths can the charge take? Answer that and you do not need to memorise the rest.
Series
A series circuit is a single loop, so there is only one path.
The current is therefore the same at every point. Charge is not used up as it goes round, so whatever flows through one component flows through all of them.
The potential difference is shared between the components, and the individual voltages add up to the supply voltage. Each component takes a share according to its resistance.
Total resistance is found by adding the resistances together. Adding another component adds another obstacle to the only path, so the total resistance rises and the current falls. That is why series fairy lights dim as you add bulbs, and why one broken bulb stops all of them.
Parallel
A parallel circuit has branches, so there is more than one path.
Every branch is connected directly across the supply, so each branch gets the full supply voltage. This is why the lights in a house are wired in parallel: each one gets the mains voltage, and switching one off leaves the others working.
The current splits between the branches and the branch currents add up to the total current leaving the supply. More current flows through the branch with less resistance.
Total resistance falls as you add branches, which surprises people. Adding a branch adds another route for the charge, so more current can flow altogether, and more current for the same voltage means less resistance.
Where the meters go
An ammeter measures the current through a component, so it goes in series with it, in the same loop.
A voltmeter measures the potential difference across a component, so it goes in parallel with it, connected to each side. The words through and across are doing the work here, and they tell you where each meter belongs.
What the mark scheme accepts and rejects
An Edexcel International GCSE Physics mark scheme for a circuit drawing question sets out four separate requirements: a complete circuit with correct symbols for the cell, ammeter and voltmeter; the variable resistor in series with the component; the ammeter in series with the component; and the voltmeter in parallel with it.
It then rejects the voltmeter drawn in parallel with the variable resistor and the component together. A voltmeter placed across two things measures the pair, not the one you were asked about, so the position has to be exact.
The notes also say the circuit symbol must be correct for the variable resistor mark, while allowing more than one cell to be drawn and ignoring the orientation of the component relative to the cell. Some details are marked strictly and others are not, and the symbols are among the strict ones.
On the calculation that followed, the same mark scheme allows standard symbols and any rearrangement, such as R equals V over I, and ignores a lower case c written for current.
Answering a comparison question
Take current, then voltage, then resistance, and give both circuit types for each. Then, if the question asks why, give the path. The current is the same everywhere in series because there is only one route; the voltage is the same across each branch in parallel because each branch is joined straight to the supply.
The reason is usually worth as much as the rule, and it is the part most often left out.
Spec 2.10-2.12
What you need to know
- State the current and voltage rules in series
- State them in parallel
- Compare the two types of circuit
Active recall
Quick check
Answer each question before opening the answer.
How do current and voltage behave in a series circuit?
Current is the same everywhere; the voltage is shared between the components.
How do current and voltage behave in a parallel circuit?
Voltage is the same across each branch; the current is shared between the branches.
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