Combined Resistance (Cambridge (CIE) IGCSE Physics): Revision Note

Resistors in series & parallel

Combination of resistance in series

• When two or more components are connected in series:

  • The combined resistance of the components is equal to the sum of the individual resistances

Combination of resistance in parallel

• When resistors are connected in parallel, the combined resistance 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

Determining resistance in parallel

Extended tier only

• The combined resistance of two resistors connected in parallel is found using the equation:

$\frac{1}{R}=\frac{1}{R_1}+\frac{1}{R_2}$

• Where:

  • $R$ is the combined resistance in ohms, Ω

  • $R_1$ is the resistance of resistor 1 in ohms, Ω

  • $R_2$ is the resistance of resistor 2 in ohms, Ω

• The method to calculate the resistance:

  • First find the value of $\frac{1}{R}$ (by adding $\frac{1}{R_1}+\frac{1}{R_2}$)

  • Next find the value of $R$ by using the reciprocal button on a calculator (labelled either $x^{-1}$ or $\frac{1}{x}$, depending on the calculator)