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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

📚 Combined Science 0653 - Physics Hub / Topical Revision
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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/R2
    What 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

QuantitySeriesParallel
CurrentSame everywhereSplits at junctions: I = I1 + I2
P.d.Shared: V = V1 + V2Same across each branch
Combined resistanceR = R1 + R2 (larger than each)1/R = 1/R1 + 1/R2 (smaller than each)
One lamp breaksAll go outThe 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

TermMeaning
Series circuitA circuit with one loop, so the same current passes through every component.
Parallel circuitA circuit with branches, in which the current divides and each branch has the same p.d.
JunctionA 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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