IGCSE Physics Series & Parallel Circuits Notes
Series and parallel circuits follow different rules. Learn the three rules for each and circuit questions become quick marks.
Key concepts
- Series circuit
- One loop: current is the same everywhere, voltage is shared, total resistance is the sum of resistances.
- Parallel circuit
- Separate branches: voltage is the same across each branch, current splits, total resistance is less than the smallest resistor.
- Series: I constant, V adds, R_total = R₁ + R₂.
- Parallel: V constant across branches, I splits, R_total < smallest branch resistance.
- Household circuits are wired in parallel so each appliance gets the full mains voltage.
Key formulas
Series resistance
R_total = R₁ + R₂ + R₃
Ohm's law
V = I × R
Worked example
Two 3 Ω resistors are connected in series to a 12 V supply. Find the current.
- 1Total resistance: R = 3 + 3 = 6 Ω.
- 2Use Ohm's law rearranged: I = V ÷ R.
- 3Substitute: I = 12 ÷ 6.
- 4Calculate: I = 2 A.
Answer: I = 2 A
Common mistakes
✗ Adding parallel resistances like series ones.
✓ Parallel total resistance is less than the smallest resistor — never simply add them.
✗ Thinking current is used up around a series loop.
✓ Charge is conserved; current is the same at every point in series.
✗ Assuming voltage is shared equally in parallel.
✓ Each parallel branch gets the full supply voltage.
Quick check
Try these, then reveal the answers.
In a series circuit, what stays the same everywhere?Reveal answer
The current.
Two 5 Ω resistors in series — total resistance?Reveal answer
10 Ω
In parallel, what is the same across every branch?Reveal answer
The voltage.
Exam tips
- Decide series or parallel first — it decides which rules apply.
- Label current and voltage on the diagram before calculating.
- Check units: amps, volts, ohms.
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Frequently asked questions
Why are homes wired in parallel?+
So each appliance receives the full mains voltage and can be switched on or off independently.
Why does adding parallel resistors lower total resistance?+
Each extra branch gives charge another path, so more current flows for the same voltage — lower overall resistance.
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