P4.3.2P4. Electricity and Magnetism
Series and parallel circuits — Combined Science 0653 - Physics Revision Notes & Topical Questions
5 min read 11 parts Lesson 9 of 19 1 quiz
Lesson summary
P4.3.2 Series and parallel circuits — COMBINED SCIENCE notes: State the rules for current, p.d.
- Series circuit
- Current rule
- Junction rule
- E.m.f. of cells in series
- Potential difference rule
- Several components on one branch
Part 1 of 9Current in a series circuit
Series circuit
Series circuit a circuit with a single complete loop
Current rule
- Current rule the current is the same at every point
- The same number of electrons per second passes every part of the loop.
- Charge cannot build up or be lost.
Factors that change the current
- Increasing the supply voltage drives more current round the circuit.
- Adding more components raises the total resistance, so less current flows.
Part 2 of 9Current in a parallel circuit
Parallel circuit
Parallel circuit a circuit with two or more loops (branches) containing components
Current rule
- Current rule the current splits at a junction, so the current from the source is larger than the current in any one branch
Junction rule
- Junction rule the current into a junction equals the current out of it
- This is because charge is conserved, as electrons cannot be created or destroyed.
- The branch currents are equal only if the branch resistances are equal.
- The branch with the lower resistance carries the larger current.
Part 3 of 9E.m.f. and p.d. in series
1E.m.f. of cells in series
- E.m.f. of cells in series the combined e.m.f. is the sum of the individual e.m.f.s
2Potential difference rule
- Potential difference rule the p.d.s across the components add up to the e.m.f. of the supply
- The components share the e.m.f. of the supply.
- The component with the larger resistance takes the larger share.
- EquationE = V1 + V2.
Part 4 of 9P.d. in parallel
P.d. in parallel
P.d. rule
- P.d. rule the p.d. across each branch equals the e.m.f. of the power source
Several components on one branch
- Several components on one branch the branch p.d. is shared between them
Part 5 of 9Combined resistance in series
- Series rule
- the combined resistance is the sum of the individual resistances
- EquationR = R1 + R2 + R3.What the symbols mean: RT = total resistance of resistors in series (Ω) · R1, R2 = the separate resistances (Ω)
- Adding resistors in series increases the total resistance and decreases the current.
Part 6 of 9Combined resistance in parallel
1Parallel rule
- Parallel rule the combined resistance is less than the smallest individual resistance
- Two equal resistors in parallel have a combined resistance equal to half of one resistor.
- Adding another resistor in parallel decreases the total resistance and increases the supply current.
2Equation for two resistors
- 1/R = 1/R1 + 1/R2What the symbols mean: RT = total resistance of resistors in parallel (Ω), always smaller than the smallest one · R1, R2 = the separate resistances (Ω)
- Add 1/R1 and 1/R2 to find 1/R.
- Use the reciprocal button on the calculator to find R.
Part 7 of 9Lighting circuits
- Lamps in a house are wired in parallel
- Each lamp has the full supply p.d., so identical lamps have the same brightness.
- Each lamp can be switched on and off independently.
- If one lamp breaks, the others keep working because each branch is independent.
Part 8 of 9Series and parallel compared
| Quantity | Series | Parallel |
|---|---|---|
| Current | Same everywhere | Splits at junctions: I = I1 + I2 |
| P.d. | Shared: V = V1 + V2 | Same across each branch |
| Combined resistance | R = R1 + R2 (larger than each) | 1/R = 1/R1 + 1/R2 (smaller than each) |
| One lamp breaks | All go out | The others stay on |
Part 9 of 9More from the specification
More from the specification
Circuit schematics as an engineering means
- Circuit schematics as an engineering means using standard graphical symbols provides an unambiguous universal means to document electrical circuit topologies.
Series circuit
- Series circuit components are in one loop, the same current flows through each, and the potential differences add up to the supply voltage
Parallel circuit
- Parallel circuit components are on separate branches, the current splits and recombines, and each branch has the full supply voltage
Mains safety
- Mains safety fuses melt when the current is too high, cutting off the appliance and preventing fires
Earth wire
- Earth wire connects the metal case of an appliance to earth so that a fault current flows safely and blows the fuse
Insulation
- Insulation plastic casing and double insulation stop live parts from being touched, preventing electric shock
Series circuit
- Series circuit components are in one loop, the same current flows through each, and the potential differences add up to the supply voltage
Parallel circuit
- Parallel circuit components are on separate branches, the current splits and recombines, and each branch has the full supply voltage
Mains safety
- Mains safety fuses melt when the current is too high, cutting off the appliance and preventing fires
Earth wire
- Earth wire connects the metal case of an appliance to earth so that a fault current flows safely and blows the fuse
Insulation
- Insulation plastic casing and double insulation stop live parts from being touched, preventing electric shock
Circuit diagram
- A diagram using symbols to show a circuit.
Insulation
- A material that reduces thermal energy loss.
Key terms
| Term | Meaning |
|---|---|
| Series circuit | A circuit with one loop, so the same current passes through every component. |
| Parallel circuit | A circuit with branches, in which the current divides and each branch has the same p.d. |
| Junction | A point where branches meet; the current entering equals the current leaving. |
| Combined resistance (series) | The sum of the individual resistances: R = R1 + R2 + ... |
| Combined resistance (parallel) | A value smaller than any single resistor, found from 1/R = 1/R1 + 1/R2. |
| E.m.f. | The electrical energy supplied per unit charge by the source; in series the p.d.s across components add up to it. |
Avoid these mistakes
1
Thinking current is used up as it passes round a series circuit — the current is the same at every point
2
Thinking parallel branches must carry equal currents — they are equal only when the branch resistances are equal
3
Thinking more resistors in parallel increase the total resistance — the combined resistance falls
4
Adding 1/R1 and 1/R2 and forgetting the final reciprocal — the sum gives 1/R, so R must be found by taking the reciprocal
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