Combined Resistance (CIE IGCSE Physics)

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

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

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

 

Extended

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

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)