How do current, potential difference and resistance behave in series and parallel circuits?
The rules for current and potential difference in series and parallel circuits, and how resistance combines in each.
A focused answer to the WJEC GCSE Science Double Award Unit 3 topic on circuits, covering the rules for current and potential difference in series and parallel circuits, and how the total resistance changes in each.
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What this dot point is asking
WJEC Double Award Unit 3 wants you to state the rules for current and potential difference in series and parallel circuits, and describe how resistance combines in each.
Series circuits
Because all the components are in one loop, the same current flows through each, and the supply voltage is shared between them. If one component breaks, the whole circuit stops.
Parallel circuits
Because each branch is connected directly across the supply, each gets the full voltage, and the current splits between the branches. If one branch breaks, the others keep working.
Comparing the two
| Quantity | Series | Parallel |
|---|---|---|
| Current | Same everywhere | Splits between branches |
| Potential difference | Shared (adds up) | Full supply on each branch |
| Total resistance | Increases with more components | Decreases with more branches |
| If one breaks | Whole circuit stops | Others keep working |
Why brightness differs
Two identical lamps are brighter in parallel than in series. In series, they share the supply voltage, so each gets less and a smaller current flows. In parallel, each gets the full supply voltage, so a larger current flows through each lamp, making them brighter. This is also why household circuits are wired in parallel.
Resistance and current
Adding resistors in series increases the total resistance, so the total current from the supply falls. Adding components in parallel lowers the total resistance, so the total current from the supply rises, even though each branch carries its own current. Thinking about how the total resistance changes helps you predict what happens to the current when a circuit is altered.
Reading circuit symbols
Exam questions show circuits as diagrams using standard symbols, so you must recognise them. A cell is a long thin line and a short thick line; a battery is two or more cells; a switch is a gap with a lever; a lamp is a circle with a cross; a fixed resistor is a rectangle; a variable resistor is a rectangle with an arrow; an ammeter is a circle with an A and a voltmeter a circle with a V. Being able to read a circuit diagram lets you decide whether components are in series (one loop) or in parallel (separate branches), which is the first step in applying the rules.
Why homes use parallel circuits
Household wiring is in parallel rather than series for two important reasons. First, each appliance gets the full mains voltage (about 230 V), so it works at its proper brightness or power. Second, each appliance can be switched on and off independently, and if one breaks the others keep working, because a break in one branch does not stop the others. A series house circuit would dim every appliance and switch them all off together if one failed, so parallel wiring is far more practical.
Try this
Q1. In a parallel circuit, what voltage is across each branch? [1 mark]
- Cue. The full supply (battery) voltage.
Q2. What happens to the total resistance when you add a resistor in series? [1 mark]
- Cue. It increases.
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 style4 marksTwo identical lamps are connected first in series, then in parallel, to the same battery. Explain why they are brighter in parallel.Show worked answer →
A Unit 3 explain question worth 4 marks. Reward: in series, the lamps share the battery's potential difference, so each gets less voltage and the current is smaller (1), so they are dimmer (1); in parallel, each lamp gets the full battery voltage (1), so a larger current flows through each and they are brighter (1). Markers credit the sharing of voltage in series, the full voltage in parallel, and the link to brightness. A common error is to say the current is shared in series.
WJEC style3 marksIn a series circuit, the battery provides 12 V and there are two resistors. If one has 8 V across it, what is the potential difference across the other, and what is the current rule in series?Show worked answer →
A Unit 3 series question. The potential differences add up to the battery voltage, so the other resistor has across it (1 mark for method, 1 for 4 V). The current is the same at every point in a series circuit (1). Markers credit the voltage subtraction and the current rule. A common error is to say the current is shared between the resistors.
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