Components in Series & Parallel CircuitsEdexcel IGCSE Physics: Revision notes
Section 1
Why choose series or parallel for a given application?
The choice between a series circuit and a parallel circuit depends on what the application needs.
- In a series circuit, components share one single loop — if one component fails (breaks the loop), all components stop working
- In a parallel circuit, components are connected across separate branches — each branch can be switched independently, and if one branch fails, the others keep working
Domestic lighting is wired in parallel so that each light can be switched on/off independently, and one bulb blowing does not turn off every light in the house.
Domestic lighting uses parallel circuits: each room's light is on its own branch, so turning one off (or a bulb failing) does not affect the others.
Section 2
How does current behave in a series circuit?
In a series circuit, the current is the same at every point around the single loop. The current depends on:
- The applied voltage (a higher voltage drives a larger current, for a given resistance)
- The number and nature of other components — adding more resistive components in series increases total resistance, which decreases the current
So increasing the voltage increases the current, while adding more components (increasing total resistance) decreases the current.
Section 3
How does current vary with voltage for different components?
Different components show different current–voltage (I–V) relationships, which can be investigated experimentally by varying the voltage and recording the current:
| Component | I–V graph shape | Description |
|---|---|---|
| Wire / fixed resistor (constant temperature) | Straight line through origin | Current is directly proportional to voltage (obeys Ohm's law) |
| Filament lamp | Curve that flattens as voltage increases | As current increases, the filament heats up, resistance increases, so current increases less than proportionally |
| Diode | Almost no current one way; sharp rise in current the other way | Conducts easily in the forward direction, blocks current in the reverse direction |
Describe I-V graphs by shape AND explain the physical reason (e.g. filament heating increases resistance) for full marks.
Section 4
What is the effect of changing resistance on current?
For a fixed voltage, increasing resistance decreases current, and decreasing resistance increases current — this follows from V = IR. This qualitative relationship explains the behaviour of variable-resistance components:
- A light-dependent resistor (LDR) has resistance that decreases as light intensity increases (more light → lower resistance → more current)
- A thermistor has resistance that decreases as temperature increases (higher temperature → lower resistance → more current)
Lamps and LEDs light up when current flows through them, so they can be used to indicate the presence of current in a circuit.
Do not say LDR/thermistor resistance increases with light/temperature — for both components, resistance decreases as the relevant quantity increases.
Must Know
- Series circuits share one loop (same current everywhere); parallel circuits have independent branches (same voltage across each)
- Domestic lighting uses parallel circuits so appliances/lights work independently
- Current in a series circuit depends on applied voltage and the number/nature of components
- Wire/resistor: I proportional to V (straight line); filament lamp: curve flattens as resistance rises with temperature; diode: conducts one way only
- Increasing resistance decreases current (and vice versa) for a fixed voltage
- LDR resistance decreases as light increases; thermistor resistance decreases as temperature increases
- Lamps and LEDs indicate the presence of current by lighting up
That's the notes covered.
Carry on to the next subtopic.