ASPhysicsOCRRevision Notes4. Electrons, Waves & Photons4.3 Series & Parallel Circuits4.3.5 Resistors in Series & Parallel Circuits

Resistors in Series & Parallel Circuits (OCR AS Physics)

Revision Note

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Combining 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

  • For example, for three resistors connected in series:

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

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

Combined resistance of two or more resistors in series equation

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

Combining Resistors in Parallel

  • When two or more component are connected in parallel:

The reciprocal of the combined resistance is the sum of the reciprocals of the individual resistances

  • For example, for two resistors connected in parallel:

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

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

Combined resistance of two or more resistors in parallel equation

 
  • This means the combined resistance decreases and is less than the resistance of any 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

Examiner Tip

  • The most common mistake is to forget to find the reciprocal of RT
  • The reciprocal of a value is 1 / value
  • For example, the reciprocal of a whole number such as 2 equals ½
    • 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
  • In the case of the resistance R, this becomes 1/R
    • To get the value of R from 1/R, you must do 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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    Ashika

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