Resistors in Series & Parallel (AQA A Level Physics): Revision Note

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

The most common mistake in questions about parallel resistors is to forget to find the reciprocal of R_T (1/R_T) instead of R_T. 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

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)