Series and Parallel Circuits
Series and parallel circuits
Components in a circuit can be connected in series (one loop) or parallel (separate branches). The way they're connected changes how current, potential difference and resistance behave.
Series circuits
All components are connected in a single loop, one after another.
- Current is the same everywhere in the circuit.
- Potential difference is shared between the components (the individual p.d.s add up to the supply p.d.).
- Total resistance = the sum of the individual resistances:
R = R₁ + R₂ + …(adding a resistor increases total resistance). - Drawback: if one component breaks, the circuit is broken and everything stops (like old fairy lights).
Parallel circuits
Components are connected on separate branches, each connected across the supply.
- Potential difference is the same across each branch (equal to the supply p.d.).
- Current is shared between the branches — the branch currents add up to the total current.
- Total resistance is less than the smallest individual resistance (adding branches gives current more paths, so total resistance decreases).
- Advantage: each branch works independently — if one breaks, the others keep working. This is why house wiring is parallel.
Summary table
| Current | Potential difference | If one breaks | |
|---|---|---|---|
| Series | Same everywhere | Shared (adds up) | All stop |
| Parallel | Shared (adds up) | Same on each branch | Others continue |
Worked example
Two 4 Ω resistors are connected in series with a 12 V supply. Find the total resistance and the current.
- Total resistance = 4 + 4 = 8 Ω.
- Current = V ÷ R = 12 ÷ 8 = 1.5 A (the same through both resistors). ✓
Common mistakes
- Swapping the rules: in series current is the same; in parallel p.d. is the same.
- Adding resistances in parallel the same way as series — parallel total is less than the smallest resistor.
- Forgetting that adding components in series increases resistance (and lowers current).
Exam tips
- Memorise the two summary rules (series: same current, shared p.d.; parallel: same p.d., shared current).
- Explain why house wiring is parallel (independent components + same voltage to each).
- For calculations, add series resistances directly; know parallel total decreases.
Key facts to remember
- Series: same current everywhere, p.d. shared, resistances add (R₁ + R₂), one break stops all.
- Parallel: same p.d. across each branch, current shared, total resistance decreases, branches independent.
- House wiring is parallel so appliances get full voltage and work independently.