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How do current, potential difference and resistance behave in series and parallel circuits?

Series and parallel circuits, the current and potential difference rules for each, total resistance, circuit symbols, and the I-V characteristics of components.

A focused answer to WJEC GCSE Physics topic 1.1 on series and parallel circuits, covering the current and potential difference rules, total resistance, circuit symbols, and the I-V characteristics of resistors, lamps and diodes.

Generated by Claude Opus 4.89 min answerUpdated 2026-06-14

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this topic is asking
Circuit symbols
Series circuits
Parallel circuits
I-V characteristics
Try this

What this topic is asking

WJEC wants you to recognise circuit symbols, state and use the current and potential difference rules for series and parallel circuits, find total resistance, and interpret the I-V characteristics of common components. This is part of topic 1.1 Electric circuits in Unit 1 of WJEC GCSE Physics (3420).

Circuit symbols

You must recognise and draw the standard symbols: cell and battery, switch, lamp, resistor, variable resistor, ammeter (a circle with A, in series), voltmeter (a circle with V, in parallel), diode, light-dependent resistor (LDR), thermistor and fuse. A circuit diagram uses these symbols joined by straight lines that represent connecting wires of negligible resistance.

Series circuits

Because the same current flows through each component, a larger resistor takes a larger share of the voltage. If one component in a series circuit breaks, the whole circuit stops.

Parallel circuits

This is why household circuits are wired in parallel: every appliance gets the full mains voltage, can be switched on and off on its own, and keeps working if another fails.

I-V characteristics

Investigating the I-V characteristics of a resistor, a filament lamp and a diode is a specified practical: vary the supply with a variable resistor, record current and pd, and plot the graph.

Total resistance and current in a series circuit

A $3\,\Omega$ resistor and a $9\,\Omega$ resistor are connected in series across a $12\,\text{V}$ battery. Calculate the total resistance, the current, and the potential difference across the $9\,\Omega$ resistor.

step 1: Find the total resistance

In series, resistances add: $R_\text{total} = 3 + 9 = 12\,\Omega$ .

step 2: Find the current

The same current flows everywhere, so $I = \dfrac{V}{R_\text{total}} = \dfrac{12}{12} = 1.0\,\text{A}$ .

step 3: Find the pd across the 9 ohm resistor

The current through it is $1.0\,\text{A}$ , so $V = IR = 1.0 \times 9 = 9.0\,\text{V}$ .

step 4: Check

The two pds are $1.0 \times 3 = 3.0\,\text{V}$ and $9.0\,\text{V}$ , which add to the supply $12\,\text{V}$ , as expected in series.

Try this

Q1. Three $2\,\Omega$ resistors are connected in series. State the total resistance. [1 mark]

Cue. In series they add: $2 + 2 + 2 = 6\,\Omega$ .

Q2. State why the current is the same at every point in a series circuit. [1 mark]

Cue. There is only one path, so charge cannot build up or split.

Exam-style practice questions

Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

WJEC 20183 marksTwo resistors of

4\,\Omega

and

6\,\Omega

are connected in series to a

10\,\text{V}

supply. Calculate the current in the circuit.

Show worked answer →

A topic 1.1 series question. In series the total resistance is the sum: $R_\text{total} = 4 + 6 = 10\,\Omega$ (1 mark). The current is the same everywhere, so $I = \dfrac{V}{R} = \dfrac{10}{10} = 1.0\,\text{A}$ (2 marks for the equation and answer with units). Markers reward adding the resistances first and then using $V = IR$ with the total. A common error is to use one resistor's value only, or to share the current.

WJEC 20224 marksExplain how the current and potential difference behave in a parallel circuit with two identical lamps, and why household lighting is wired in parallel.

Show worked answer →

A topic 1.1 Explain question. In parallel, each branch gets the full supply voltage, so both lamps have the same pd across them (1 mark). The current from the supply splits between the branches and recombines, so the total current is the sum of the branch currents (1 mark). Household lighting is wired in parallel so that each lamp gets the full mains voltage and can be switched independently (1 mark), and if one lamp fails the others keep working because each has its own path (1 mark). Markers reward the full-voltage point, the splitting of current, independent switching and continued operation.

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Sources & how we know this

WJEC GCSE Physics specification (3420) from 2016 — WJEC (2016)