Kirchhoff's Second Law (Cambridge (CIE) A Level Physics): Revision Note

Kirchhoff's second law

• Kirchhoff's second law states that:

The total e.m.f in a closed circuit is equal to the sum of the potential differences across the components

• This is a consequence of conservation of energy

• The energy transferred into the circuit is equal to the energy transferred out of the circuit

• The circuit below illustrates Kirchhoff’s second law:

Kirchhoff's second law

The sum of the potential difference across the individual components is equal to the sum of the e.m.f supplied by the batteries

• In a series circuit, the potential difference is split across all components depending on their resistance

  • The sum of the potential difference across each component is equal to the total e.m.f of the power supply

• In a parallel circuit, the potential difference is the same across each closed loop

  • The sum of the potential difference in each closed circuit loop is equal to the total e.m.f of the power supply:

Kirchhoff's second law in a series circuit

The total potential difference across the components is the sum of the potential difference across each individual component

• 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 a parallel circuit

Each circuit loop 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 potential difference

• If one light breaks, current can still flow through the rest of the lights and appliances