Revision Notes

4.3.3 Resistors in Series & Parallel

Resistors in Series

  • When two or more components are connected in series:
    • The combined resistance of the components is equal to the sum of individual resistances

combined-resistance, IGCSE & GCSE Physics revision notes

When several components are connected in series, their combined resistance is equal to the sum of their individual resistances


Resistors in Parallel

  • When resistors are connected in parallel, the combined resistance decreases and is less than the resistance of any of the individual components
  • If two resistors of equal resistance are connected in parallel, then the combined resistance will halve

Equal resistors parallel, IGCSE & GCSE Physics revision notes

The above resistors will have a combined resistance of 2 Ω − half the value of each resistor


Extended Only

Determining Resistance in Parallel

  • More generally, to determine the combined resistance of any combination of two resistors, you must use the equation:

resistors-parallel, IGCSE & GCSE Physics revision notes

Resistors in parallel equation, IGCSE & GCSE Physics revision notes

The above equation is not the same as R = R1 + R2  – a common, incorrect simplification that is made


  • To calculate the resistance:
    • First find the value of 1/R (by adding 1/R1 + 1/R2)
    • Next find the value of R by using the reciprocal button on your calculator (labelled either x-1 or 1/x, depending on your calculator)

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