CIE A Level Physics (9702) 2019-2021

Revision Notes

12.1.5 Kirchhoff's Second Law

Kirchhoff's Second Law

  • Kirchhoff’s second law states that:
    • The sum of the e.m.f’s in a closed circuit equals the sum of the potential differences
  • This is a consequence of conservation of energy
  • Below is a circuit explaining Kirchhoff’s second law with the sum of the voltages in the closed series circuit equal to the sum of the e.m.f’s:

Kirchoff's second law in a series circuit, downloadable AS & A Level Physics revision notes

The sum of the voltages are equal to the sum of the e.m.f from the batteries

 

  • In a series circuit, the voltage is split across all components depending on their resistance
    • The sum of the voltages is equal to the total e.m.f of the power supply
  • In a parallel circuit, the voltage is the same across each closed loop
    • The sum of the voltages in each closed circuit loop is equal to the total e.m.f of the power supply:

Kirchoff's second law in a parallel circuit, downloadable AS & A Level Physics revision notes

  • A closed circuit loop acts as its own independent series circuit and each one separates at a junction. A parallel circuit is made up of two or more of these loops

Loops in parallel circuit, downloadable AS & A Level Physics revision notes

Each circuit loops acts as a separate, independent series circuit

  • This makes parallel circuits incredibly useful for home wiring systems: a single power source supplies all lights and appliances with the same voltage
  • If one light breaks, voltage and current can still flow through for the rest of the lights and appliances

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