GCSE Physics Series & Parallel Circuits Notes
Series and parallel circuits behave differently. Learn the three rules for each and circuit questions become quick marks.
Key concepts
- Series circuit
- Components in a single loop. Current is the same everywhere; potential difference is shared; total resistance is the sum of resistances.
- Parallel circuit
- Components on separate branches. Potential difference is the same across each branch; current splits between branches; total resistance is less than the smallest resistor.
- Series: I is constant, V adds up, R_total = R₁ + R₂.
- Parallel: V is constant across branches, I splits, R_total < smallest branch resistance.
- Adding resistors in parallel decreases total resistance because there are more paths for charge.
Key formulas
Series resistance
R_total = R₁ + R₂ + R₃
Ohm's law
V = I × R
Worked example
Two 6 Ω resistors are connected in series to a 12 V supply. Find the current.
- 1Total resistance in series: R = 6 + 6 = 12 Ω.
- 2Use Ohm's law rearranged: I = V ÷ R.
- 3Substitute: I = 12 ÷ 12.
- 4Calculate: I = 1 A.
Answer: I = 1 A
Common mistakes
✗ Adding resistances in parallel the same way as in series.
✓ In parallel the total resistance is smaller than the smallest resistor — never just add them.
✗ Thinking current is used up as it goes round.
✓ Charge is conserved — in series the current is the same at every point.
✗ Assuming p.d. is shared equally in parallel.
✓ In parallel each branch gets the full supply p.d.
Quick check
Try these, then reveal the answers.
In a series circuit, what stays the same everywhere?Reveal answer
The current.
Two 4 Ω resistors in series — what is the total resistance?Reveal answer
8 Ω
In parallel, what is the same across every branch?Reveal answer
The potential difference.
Exam tips
- Label current and p.d. on the circuit diagram before you calculate.
- State whether the circuit is series or parallel first — it decides which rules apply.
- Watch your units: amps for current, volts for p.d., ohms for resistance.
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Frequently asked questions
Why does total resistance fall when resistors are added in parallel?+
Each extra branch gives charge another path to flow through, so more current flows for the same p.d., meaning lower overall resistance.
Is current shared in a series circuit?+
No — current is the same at every point in a series circuit; it is potential difference that is shared between components.
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