ASPhysicsCIERevision Notes10. D.C. Circuits10.2 Kirchhoff’s LawsResistors in Series

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First exams 2025

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Resistors in Series (CIE AS Physics)

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Deriving the equation for resistors in series

  • In a series circuit, the combined resistance of two or more resistors is the sum of the individual resistances
  • In a series circuit:
    • The current is the same through all resistors
    • The potential difference is shared between all the resistors

  • The equation for combined resistors in series is derived using Kirchhoff’s laws:

Derivation for resistors in series, downloadable AS & A Level Physics revision notes

Resistors 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

Resistors in series

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

Resistors connected in series each have their own separate potential difference

 

  • The equation for the combined resistance, R  of resistors in series is:

R space equals space R subscript 1 space plus thin space R subscript 2 space plus thin space...

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

Answer: C

Step 1: State the equation for the combined resistance for resistors in series

R space equals space R subscript 1 space plus thin space R subscript 2 space end subscript plus space... space equals space R subscript 1 space plus thin space R subscript 2 space plus thin space R subscript 3

Step 2: Rearrange for resistance R2

R subscript 2 space equals space R space minus space open parentheses R subscript 1 space plus space R subscript 3 close parentheses

Step 3: Substitute the values

R subscript 2 space equals space 60 space minus space open parentheses 30 space plus thin space 10 close parentheses space equals space 20 space straight capital omega

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    Katie M

    Author: Katie M

    Expertise: Physics

    Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.