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Cambridge IGCSE Physics · 0625

Chapter 4: Electricity and magnetism — Part 8

Topic 4.3.2 · Series and parallel circuits

Series circuits

The current is the same at every point in a series circuit.

Combined e.m.f. of several sources in series: add the e.m.f.s (same orientation).

Combined resistance of resistors in series: R = R1 + R2 + …

The total p.d. across components in series equals the sum of the individual p.d.s.

A 4.0 Ω resistor and a 6.0 Ω resistor are connected in series across a 12 V supply. Calculate the current in the circuit.

Worked example: series resistance 10 ohms, current 1.2 amperes
R = 10 Ω, then I = V / R = 1.2 A (same everywhere in series).

Parallel circuits

The current from the source is larger than the current in each branch.

The sum of currents into a junction equals the sum of currents out of the junction.

The p.d. across parallel branches is the same.

The combined resistance of two resistors in parallel is less than either resistor.

1/R = 1/R1 + 1/R2 (two resistors in parallel).

Calculate the combined resistance of a 6.0 Ω resistor and a 3.0 Ω resistor connected in parallel.

Worked example: parallel combination is 2.0 ohms, less than either resistor
1/R = 1/6 + 1/3, so R = 2.0 Ω — smaller than either branch.

Lamps in a lighting circuit are connected in parallel so that each lamp gets the full supply p.d., they can be switched independently, and if one lamp fails the others stay on.

Series circuit: same current at every point, e.m.f.s adding, resistances adding. Parallel lamps: each lamp has the source p.d., currents in the branches adding at the junction.
Diagram 1: Series circuit: same current at every point, e.m.f.s adding, resistances adding. Parallel lamps: each lamp has the source p.d., currents in the branches adding at the junction.
Two resistors in parallel: combined resistance less than either resistor, with 1/R = 1/R1 + 1/R2 shown beside a numerical example.
Diagram 2: Two resistors in parallel: combined resistance less than either resistor, with 1/R = 1/R1 + 1/R2 shown beside a numerical example.

Exam Traps

  • Do not add parallel resistances as if they were in series. Combined parallel R is smaller than the smallest branch.

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