Series & Parallel Circuits (AQA GCSE Physics: Combined Science)

• There are two ways of joining electrical components, in series and in parallel
  • Some circuits include both series and parallel parts

• A series circuit consists of a string of two or more components, connected end to end:

  

Diagram showing two bulbs connected in series

• A parallel circuit consists of two or more components attached along separate branches of the circuit

Diagram showing two bulbs connected in parallel

• In a series circuit:
  • The current is the same at all points ie. through each component
  • The total potential difference of the power supply is shared between the components
  • The total resistance of two components is the sum of the resistance of each component

Lamps connected in a series circuit

• In the above circuit:
  • The current from the power supply is the same as the current in both lamps I = I₁ = I₂
  • If the battery is marked 12 V, then the potential difference would be 12 ÷ 2 = 6 V across each lamp
  • If each lamp has a resistance of R, then the total resistance in the circuit is equal to R + R = 2R

• In a parallel circuit:
  • The total current through the whole circuit is the sum of the currents through the separate components
  • The potential difference across each component is the same
  • The total resistance of two resistors is less than the resistance of the smallest individual resistor

Lamps connected in a parallel circuit

• In the above circuit:
  • Because the current splits up, the sum of currents in each branch will equal the current from the power supply I = I₁ + I₂
  • If the battery is marked 12 V, then the potential difference would be 12 V across each lamp
  • If each lamp has a resistance of R, then the total resistance in the circuit will be less than 2R

• Note that the current does not always split equally – often there will be more current in some branches than in others
  • The current in each branch will only be identical if the resistance of the components along each branch are identical

• Current behaves in this way because it is the flow of electrons:
  • Electrons are physical matter – they cannot be created or destroyed
  • This means the total number of electrons (and hence current) going around a circuit must remain the same
  • When the electrons reach a junction, however, some of them will go one way and the rest will go the other

Current is split at a junction into individual branches