Kirchhoff's Second Law (CIE A Level Physics)

• 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
• The energy transferred into the circuit is equal to the energy transferred out of the circuit
• Below is a circuit explaining Kirchhoff’s second law with the sum of the potential differences across the components in the closed series circuit equal to the sum of the e.m.f’s:

Kirchhoff's second law

The sum of the potential difference across the individual components are equal to the sum of the e.m.f from 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