1.1.3Electricity, Energy and Waves
Voltage in Series and Parallel Circuits — WJEC GCSE Science (Double Award) - Physics Revision Notes & Topical Questions
5 min read 7 parts Lesson 4 of 36
Section 1 of 7What voltage is
Source of voltage
- Source of voltage The terminals of a cell are one positive and one negative, which produces a voltage across the circuit.
- Voltage is also called potential difference (p.d.) and has the symbol V.
What it does
- What it does The voltage drives current round the circuit and transfers energy from the supply to the components.
- It is measured in volts (V).
Section 2 of 7Rules for voltage
| Circuit type | Rule for voltage | Example |
|---|---|---|
| Series | The supply voltage is shared between the components. The voltages across the components add up to the supply voltage. | A 6 V supply with two lamps of equal resistance gives 3 V across each lamp. |
| Parallel | Each branch has the same voltage as the supply. | Two lamps in parallel with a 6 V supply each have 6 V across them. |
| Series components within a branch | The branch voltage is shared between the components in that branch, following the series rule. | A 6 V branch with two equal lamps gives 3 V across each. |
Section 3 of 7Series circuit step by step
1
Identify the supply voltage, for example a 9 V battery.
2
Note that the components are all in one loop, so the voltage is divided between them.
3
If the resistances are equal, divide the voltage equally: with three identical lamps each has 3 V.
4
If one component has a larger resistance, it takes a larger share of the voltage.
5
Check by adding the voltages across the components; they must total the supply voltage.
Section 4 of 7Why parallel circuits are useful
Parallel circuits
Independent branches
- Independent branches If one component fails, the other branches keep working because each has its own path.
Full voltage on each branch
- Full voltage on each branch Every lamp receives the full supply voltage,
- so lamps shine more brightly than the same lamps in series,
- where they must share it.
Easy to extend
- Easy to extend New loops can be added to an existing circuit, which is how household lighting circuits are built.
Safer
- Safer A fuse can be placed on each loop, protecting each branch separately.
Section 5 of 7Measuring voltage
1Choose the meter
- Choose the meter Voltage is measured with a voltmeter, with units of volts.
2Connect in parallel
- Connect in parallel A voltmeter is connected on a branch across the component, not in the main loop.
3Read across
- Read across The reading is the voltage across that component, since the two ends of the meter touch the two ends of the component.
4Interpret
- Interpret Compare readings with the rules above: in series they should add up to the supply;
- in parallel they should match it.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Voltage | Another name for potential difference; the energy transferred per unit charge, measured in volts. |
| Potential difference | The voltage between two points, which drives current through a component. |
| Voltmeter | A meter that measures the potential difference across a component and is connected in parallel with it. |
| Series circuit | A circuit with a single loop in which the voltage is shared between components. |
| Parallel circuit | A circuit with branches in which each branch has the same voltage as the supply. |
Section 7 of 7Exam technique
1
Saying voltage is 'used up' or flows: it is current that flows; voltage is the push that drives it.
2
Assuming the voltage in a series circuit is the same across every component, regardless of their resistance.
3
Thinking parallel branches share the supply voltage between them: each gets the full voltage.
4
Connecting the voltmeter in series, which blocks the circuit and gives a misleading reading.
5
Forgetting to check that voltages around a series circuit add up to the supply voltage.
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