Energy Conservation in Circuits (Edexcel International A Level Physics)

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Energy Conservation in Circuits

The Electrical Voltages Rule (Kirchhoff's Second Law)

  • Energy is never used up or lost in a circuit, since everything follows the Law of Conservation of Energy
  • The electrical voltages rule is defined as:

The sum of the e.m.f.s in a closed circuit loop is equal to the sum of the potential differences around that loop

  • Each closed circuit loop can be treated like a series circuit
  • A typical circuit might have a setup where  E1 + E2 = V1 + V2 where:
    • E1 and E2 represent the e.m.f.s in the closed loop
    • V1  and V2 represent the potential differences in the closed loop


The sum of the voltages is equal to the total 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:


The sum of the p.ds in each closed loop is equal to the total e.m.f of the power supply

  • A closed-circuit loop acts as its own independent series circuit
  • Each loop 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

Exam Tip

The Electrical Voltages Rules is sometimes known as Kirchhoff's Second Law.

Drawing the loops in different colours, as in the example above, can be a helpful way of identifying the different loops.

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