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How do current and voltage behave differently in series and parallel circuits?

The rules for current and potential difference in series and parallel circuits, and how total resistance changes with each arrangement.

A CCEA GCSE Physics answer on the rules for current and potential difference in series and parallel circuits, and how the total resistance changes when components are added in series or in parallel.

Generated by Claude Opus 4.811 min answer

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
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What this dot point is asking

CCEA wants you to know the rules for current and potential difference in series and parallel circuits, and how the total resistance changes when components are added in each arrangement. These rules let you solve multi-component circuits.

The answer

Series circuits

In a series circuit the components are connected in a single loop, one after another.

Parallel circuits

In a parallel circuit the components are connected on separate branches across the supply.

Why houses use parallel circuits

Household appliances are wired in parallel so each gets the full mains voltage, each can be switched independently, and one failing does not switch off the others.

Worked example: a parallel branch

Examples in context

Example 1. Old fairy lights
Cheap fairy lights wired in series all go out if one bulb fails, because the single loop is broken. Modern sets use parallel wiring so the rest stay lit.
Example 2. Car headlights
Headlights are wired in parallel so both get the full battery voltage and shine at full brightness, and one bulb blowing leaves the other working.
Example 3. A torch with two cells
Two cells in series add their voltages, giving a bigger push and a brighter bulb. This is why many torches stack cells end to end in series, while the bulbs in a room are wired in parallel so each runs at full mains voltage.

A reliable way to avoid mixing up the rules is to remember the single-loop picture for series (one path, so the current cannot change, but the voltage is divided up among the components) and the branching picture for parallel (several paths, so each gets the full voltage, but the current splits between them). Sketching the circuit and labelling the current and voltage at each point before calculating prevents most errors.

Try this

Q1. What happens to the current at different points in a series circuit? [1 mark]

  • Cue. It is the same at every point.

Q2. Two 5.0 Ω5.0\ \Omega resistors are in series. State the total resistance. [1 mark]

  • Cue. 5.0+5.0=10 Ω5.0 + 5.0 = 10\ \Omega.

Q3. In a parallel circuit, what is true about the potential difference across each branch? [1 mark]

  • Cue. It is the same as (equal to) the supply voltage.

Exam-style practice questions

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

CCEA style4 marksTwo resistors of 4.0 ohms and 6.0 ohms are connected in series to a 10 V battery. Calculate the total resistance, the current in the circuit, and the potential difference across the 6.0 ohm resistor.
Show worked answer →

Total resistance in series is the sum:

R=4.0+6.0=10 Ω.R = 4.0 + 6.0 = 10\ \Omega.

Current (the same everywhere in series):

I=VR=1010=1.0 A.I = \dfrac{V}{R} = \dfrac{10}{10} = 1.0\ \text{A}.

Potential difference across the 6.0 ohm resistor:

V=IR=1.0×6.0=6.0 V.V = I R = 1.0 \times 6.0 = 6.0\ \text{V}.

Markers reward the total resistance 10 ohms, current 1.0 A, and pd 6.0 V.

CCEA style3 marksState the rules for current and potential difference in a parallel circuit, and explain one advantage of wiring lamps in parallel in a house.
Show worked answer →

In parallel, the potential difference across each branch is the same as the supply. The total current is shared between the branches, so the branch currents add up to the supply current.

Advantage: each lamp can be switched on or off independently, and if one lamp fails the others stay on (each branch has the full supply voltage).

Markers reward: same pd across each branch; branch currents add to the total; and a valid advantage (independent control or others stay on if one fails).

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