Series and Parallel Circuits
Understand Series and Parallel Circuits for GCSE Physics with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. In series the current is the same everywhere and resistances add; in parallel the potential difference is shared and the total resistance falls.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
In a series circuit there is only one path for the current, so if one component breaks the whole circuit stops working. In a parallel circuit each component is on its own branch.
In series: the current is the same everywhere; the potential difference is shared between components; and the total resistance is the sum of the individual resistances.
In parallel: the potential difference across each branch is the same; the current is shared between branches and the branch currents add to the total; and the total resistance is less than the smallest individual resistance, because adding branches gives the current more paths to flow through.
Revision notes
Series circuits
One path only. Current is the same at every point.
The potential differences across the components add up to the supply potential difference. Total resistance is the sum: R = R₁ + R₂. Adding a resistor in series increases the total resistance.
Parallel circuits
Each component has its own branch. The potential difference across each branch equals the supply potential difference.
The currents through the branches add up to the total current from the supply. If one branch breaks, the others keep working.
Why parallel resistance is lower
The total resistance of two resistors in parallel is less than the smaller of the two.
Adding a branch gives the current more paths to flow through, so more current flows for the same potential difference — which means the total resistance has fallen.
Key points
- Series circuits have one path only.
- Current is the same everywhere in series.
- Potential difference is shared in series.
- Total series resistance is the sum.
- Parallel branches share the same potential difference.
- Total parallel resistance is less than the smallest resistor.
Worked examples
Example 1
Two resistors of 4 Ω and 6 Ω are connected in series. Calculate the total resistance. [2 marks]
Model answer
Total resistance in series is the sum of the individual resistances: 4 + 6 (1 mark) = 10 Ω (1 mark).
Example 2
Explain why adding a resistor in parallel decreases the total resistance of a circuit. [3 marks]
Model answer
Adding a resistor in parallel creates another branch (1 mark), giving the current more paths to flow through (1 mark). More current therefore flows for the same potential difference, which means the total resistance has decreased (1 mark).
Example 3
Explain one advantage of connecting lamps in parallel rather than in series. [2 marks]
Model answer
Each lamp receives the full supply potential difference, so all shine at full brightness (1 mark). Also, if one lamp breaks the others continue to work, because each has its own branch (1 mark).
Common mistakes
Adding resistances in parallel as if in series.
Total parallel resistance is LESS than the smallest individual resistance.
Saying current is shared in series.
Current is the same everywhere in series; potential difference is shared.
Saying potential difference is shared in parallel.
Each branch gets the full supply potential difference.
Not explaining why parallel resistance falls.
The reason is more paths for the current to flow through.
Exam tips
- Learn the three series rules and the three parallel rules as separate sets.
- Explain falling parallel resistance through extra current paths.
- Give both parallel advantages when asked.
- Check whether the question describes series or parallel before answering.
Key terms
- Series
- Components connected in a single loop.
- Parallel
- Components connected on separate branches.
- Branch
- One path in a parallel circuit.
- Total resistance
- The combined resistance of a circuit.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.