Knowledge Organiser: Series and Parallel Circuits
Series Rules
- Current: same everywhere
- Voltage: adds up to supply
- Resistance: R = R₁ + R₂ + R₃
- One breaks: all stop
Parallel Rules
- Voltage: same on each branch
- Current: I = I₁ + I₂ + I₃
- Resistance: 1/R = 1/R₁ + 1/R₂
- R always less than smallest branch R
Key Equations
- Series R: R = R₁ + R₂
- Parallel R: 1/R = 1/R₁ + 1/R₂
- 2 resistors parallel: R = (R₁×R₂)/(R₁+R₂)
Exam Tips
- Parallel R always less than smallest
- Solve parallel group first, then add series
- Houses = parallel (independence)
- Check: parallel answer must be small
Common Mistakes
- Adding parallel resistances directly: For parallel resistors, use 1/R_total = 1/R₁ + 1/R₂ — adding them directly gives the wrong (too large) answer
- Thinking parallel resistance is larger: Adding resistors in parallel always decreases total resistance — the total is always less than the smallest individual resistor
- Forgetting current splits in parallel: In a parallel circuit, current splits between branches — the branch with lower resistance carries more current
- Applying series voltage rule to parallel: In parallel, all branches have the same voltage; in series, voltage divides — don't mix these up
- Removing a component affects series but not parallel: In series, removing one component breaks the whole circuit; in parallel, other branches continue to work
Memory Aid
- Series vs Parallel rules — "SVSS / PVSS":
- Series: Same current, Voltages sum, Sum of resistances.
- Parallel: Same voltage, Currents sum, Smaller total resistance.
- Christmas lights trick:
- Old fairy lights = series (one breaks, all go dark). Modern house lights = parallel (one breaks, rest stay on). Match the circuit type to this real-world example to remember which rule applies.
- Parallel resistance check — "Always Less":
- Parallel total resistance is ALWAYS less than the smallest resistor in the group. If your answer is bigger — it's wrong. No exceptions.