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Series and Parallel CircuitsCambridge IGCSE Physics: Revision notes

Section 1

Current in Series Circuits

In a series circuit, all components are connected in a single loop, one after another.

  • The current at every point in a series circuit is the same, because there is only one path for charge to flow.
  • The total p.d. across all the components in a series circuit equals the sum of the individual p.d.s across each component.
  • Knowing how to construct and use series circuits is an essential practical skill.
Key termsseries circuit
Exam tip

If one component in a series circuit breaks or is removed, the whole circuit stops working because there is no alternative path for the current.

Section 2

Combining e.m.f. and Resistance in Series

When cells or resistors are connected in series, their effects add together.

  • The combined e.m.f. of several sources connected in series is the sum of their individual e.m.f. values.
  • The combined resistance of two or more resistors in series is the sum of their individual resistances.
Example

Two cells of 1.5 V connected in series in the same direction give a combined e.m.f. of 3 V. Two resistors of 4 ohm and 6 ohm in series give a combined resistance of 10 ohm.

Section 3

Current and Resistance in Parallel Circuits

In a parallel circuit, components are connected across separate branches.

  • The current from the source is larger than the current in each individual branch, since the total current splits between the branches.
  • The combined resistance of two resistors connected in parallel is less than the resistance of either resistor by itself, because adding a parallel path gives current more routes to flow through.
  • The p.d. across a parallel arrangement of resistors is the same as the p.d. across any one branch.
Key termsparallel circuit
Common mistake

Don't assume combined resistance in parallel is found by simply adding the resistances — adding a parallel resistor always reduces the total resistance, never increases it.

Section 4

Kirchhoff's Current Rule at a Junction

At any junction in a circuit, charge is conserved.

  • The sum of the currents entering a junction equals the sum of the currents leaving the junction.
  • This is why connecting lamps in parallel (rather than series) in a lighting circuit is an advantage: each lamp gets the full supply p.d., and if one lamp fails the others stay lit because the circuit is not broken.
Think of it like this

Think of a junction like a fork in a river — the total amount of water flowing in must equal the total amount flowing out through the different branches.

Section 5

Calculating Combined Resistance in Parallel

The combined resistance of two resistors in parallel can be calculated and is always smaller than either individual resistance.

For two resistors R1 and R2 in parallel, the combined resistance R is found from: 1/R = 1/R1 + 1/R2.

For equal resistors, combined resistance = individual resistance divided by the number of resistors.

Example

Two 6 ohm resistors in parallel: 1/R = 1/6 + 1/6 = 2/6, so R = 3 ohm — half of a single resistor's value, as expected for two equal resistors.

Must Know

  • Series circuits: current is the same everywhere; total p.d. = sum of individual p.d.s; combined e.m.f. and combined resistance both add.
  • Parallel circuits: source current is larger than each branch current; p.d. is the same across each branch.
  • Combined resistance in parallel is always less than the smallest individual resistance.
  • At a junction, the sum of currents in equals the sum of currents out.
  • Lamps in parallel are an advantage because each lamp gets full p.d. and one failing does not affect the others.

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