AQA A Level Physics

Revision Notes

5.3.1 Resistors in Series & Parallel

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

Resistors in series diagram, downloadable AS & A Level Physics revision notes

Resistors connected in series

  • To calculate the total resistance of resistors in series:

Resistors in series equation, downloadable AS & A Level Physics revision notes

Worked Example

The combined resistance R in the following series circuit is 60 Ω.

What is the resistance value of R2?

A.     100 Ω               B.     30 Ω               C.     20 Ω               D.     40 Ω

WE - Resistors in series answer image, downloadable AS & A Level Physics revision notes

Resistors in Parallel

  • When two or component are connected in parallel:
    • The reciprocal of the combined resistance is the sum of the reciprocals of the individual resistances

Resistors in parallel diagram, downloadable AS & A Level Physics revision notes

Resistors connected in parallel

  • To calculate the total resistance of resistors in parallel:

Resistors in parallel equation, downloadable AS & A Level Physics revision notes

  • This means as more resistors are added, their combined resistance decreases and is, therefore, less than the resistance of the individual components
    • For example, If two resistors of equal resistance are connected in parallel, then the combined resistance will halve

Worked Example

WE - Resistors in parallel question image, downloadable AS & A Level Physics revision notes

WE - Resistors in parallel answer image, downloadable AS & A Level Physics revision notes

Exam Tip

The most common mistake in questions about parallel resistors is to forget to find the reciprocal of RT (1/RT) instead of RT. Here is a maths tip to rejig your memory on reciprocals:

  • The reciprocal of a value is 1 / value
  • For example, the reciprocal of a whole number such as 2 equals ½
    • Conversely, the reciprocal of ½ is 2
  • If the number is already a fraction, the numerator and denominator are ‘flipped’ round

Reciprocals, downloadable AS & A Level Physics revision notes

The reciprocal of a number is 1 ÷ number

  • In the case of the resistance R, this becomes 1/R
  • To get the value of R from 1/R, you must calculate 1 ÷ your answer
  • You can also use the reciprocal button on your calculator (labelled either x-1 or 1/x, depending on your calculator)

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