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4.3.2Electricity and Magnetism

Series AND Parallel Circuits — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 11 parts Lesson 35 of 65 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

4.3.2 Series and Parallel Circuits — IGCSE PHYSICS notes: Compare current and p.d. in series and parallel circuits and use the rules for combined resistance.Single loop — a series circuit has one complete loop.Current — it is the same at every point.P.d.

  • What the symbols mean
  • Junction (Extended)
  • Two resistors in parallel
  • Equal resistors
  • Same brightness
  • Independent

Part 1 of 9Series circuits

Single loop
  • Single loop a series circuit has one complete loop.
Current
  • Current it is the same at every point.
P.d.
  • P.d. the p.d.s across the components add up to the e.m.f. of the supply.
Break
  • Break if one component breaks, every component stops.
Resistance
  • R = R1 + R2 + …
    What the symbols mean: RT = total resistance of resistors in series (Ω) · R1, R2 = the separate resistances (Ω)

Part 2 of 9Parallel circuits

Branches
  • Branches components are on separate loops between the same two points.
Current
  • Current the current from the source is larger than the current in each branch and the branch currents add up to the total.
P.d.
  • P.d. the p.d. across each branch is the same and equals the supply p.d.
Resistance
  • Resistance the combined resistance of resistors in parallel is less than that of either resistor alone.
Junction (Extended)
  • Junction (Extended) the current into a junction equals the current out, because charge is conserved.

Part 3 of 9Combined resistance (Extended)

1Two resistors in parallel
  • 1/R = 1/R1 + 1/R2,
    • or R = R1R2 / (R1 + 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)
2Equal resistors
  • Equal resistors two equal resistors in parallel give half the resistance of one.

Part 4 of 9Parallel lighting circuits

Same brightness
  • Same brightness every lamp has the full supply p.d.
Independent
  • Independent each lamp can be switched on and off separately.
Failure
  • Failure if one lamp fails the others stay lit.

Part 5 of 9Changing a circuit

ChangeSeries circuitParallel circuit
Add a bulbOther bulbs dimmerOther bulbs the same
Remove a bulbCircuit breaks, all offOthers stay on
Add a cellAll bulbs brighterAll bulbs brighter
Open a switchEverything offOnly that branch off

Part 6 of 9Cells in a circuit

Cells in a circuit
Cells in series
  • Cells in series add their e.m.f.s, so more cells give brighter bulbs.
Too many cells
  • Too many cells they can blow a bulb.
Christmas lights in series
  • Christmas lights in series all go out if one breaks.

Part 7 of 9Potential divider (Extended)

Ratio
  • Ratio a potential divider splits the supply p.d. in the ratio of the resistances: V1/V2 = R1/R2.
Sensors
  • Sensors using a thermistor or LDR as one resistor makes the output change with temperature or light.

Part 8 of 9Solving a mixed circuit

1
Identify which components are in series and which are in parallel.
2
Combine parallel resistors using 1/R = 1/R1 + 1/R2.
3
Add the series resistances to find the total resistance.
4
Calculate the total current with I = V / R.
5
Use V = I R for each part and apply the current and p.d. rules to find branch values.

Part 9 of 9More from the specification

1Electrical power distribution in parallel branches
  • Electrical power distribution in parallel branches parallel wiring ensures full supply voltage across each appliance, providing an independent means to control power.
2Series circuits
  • The same current flows through every component and the supply voltage is shared between them.
3Parallel circuits
  • Current splits between branches and then recombines, while each branch has the same potential difference as the supply.
4Potential difference
  • The difference across a component is the energy transferred per unit charge, measured with a voltmeter connected in parallel.
5Mains safety
  • Fuses melt when current is too high, and insulation on cables stops electrical contact with the live wire.
6Earth wire
  • The earth connection on a metal-cased appliance carries fault current away safely, so the fuse blows and the user is protected.
7Potential difference
  • Work done by a unit charge passing through a component.
8Mains supply
  • Domestic a.c. supply, rated 230 V rms at 50 Hz in the UK.
9Circuit diagram
  • Drawing of a circuit using standard symbols.
10Insulation
  • Material that reduces thermal energy transfer.

Key terms

TermMeaning
Series circuitCircuit with one loop; same current everywhere.
Parallel circuitCircuit with branches; same p.d. across each branch.
Combined resistanceSingle resistance that could replace several resistors.
JunctionPoint where wires meet and current splits or joins.
Potential dividerSeries resistors that share the supply p.d.

Avoid these mistakes

1
Saying the p.d. is the same across series components — the current is the same, the p.d.s add.
2
Thinking adding a parallel resistor increases resistance — it decreases.
3
Believing a break in one parallel branch stops the whole circuit — only that branch stops.
4
Adding currents of series components — they are equal.

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