Kirchhoff's Laws in Circuits (OCR A Level Physics)

Revision Note

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Kirchhoff's First & Second Law in Circuits

  • Kirchhoff’s laws can be used to solve simple circuit problems

Using Kirchhoff's First Law

  • Remember:

The sum of the currents entering a junction always equal the sum of the currents out of the junction

Worked example

For the circuit below, state the readings of ammeters A1, A2 and A3.WE - Kirchoff's first law question image, downloadable AS & A Level Physics revision notes

Worked example - Kirchoff_s first law (2), downloadable AS & A Level Physics revision notes

Using Kirchhoff's Second Law

  • Remember:

The sum of the e.m.f’s in a closed circuit equals the sum of the potential differences

Worked example

For the circuit below, state the readings of the voltmeters V1, V2  and V3.All the lamps and resistors have the same resistance.WE - kirchoff's second law question image, downloadable AS & A Level Physics revision notes

Worked example - kirchoff_s second law (2), downloadable AS & A Level Physics revision notes

Examiner Tip

The best way to practice these calculations is to understand why you have made a mistakeCommon mistakes are:

  • Thinking the current is the same through every branch in a parallel circuit
  • Thinking the voltage is the same through all components in a series circuit
  • Not taking into account multiple resistors
  • Not calculating the total resistance using the appropriate parallel or series resistance equation
  • The sum of the voltages of all the components in a series circuit not adding up to the e.m.f of the supply
  • The current into a junction not being equal to the current out of a junction (Kirchhoff's First Law)

Don't be afraid to annotate circuit diagrams to help with this. The more information you have about all the components, the easier it is to calculate values that are missing

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Ashika

Author: Ashika

Expertise: Physics Project Lead

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.