Series & Parallel Circuits (AQA GCSE Combined Science: Synergy: Physical Sciences): Revision Note

Series & Parallel Circuits

• 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:

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

• A switch in a branch turns off only that branch, because the other branches still form a complete circuit

• A switch in the part of the circuit shared by all branches turns off every branch

• If two switches are in parallel, closing either switch completes the circuit

• If two switches are in series, both must be closed to complete the circuit

Series Circuits

• In a series circuit:

  • The current is the same at all points i.e. 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

• In the above circuit:

  • The current from the power supply is the same as the current in both lamps I = I₁ = I₂

  • The two lamps are identical, so the potential difference is shared equally. If the battery is marked 12 V, each lamp has 12 ÷ 2 = 6 V

  • The potential difference is shared in proportion to resistance – the component with the larger resistance gets the larger share

  • If each lamp has a resistance of R, then the total resistance in the circuit is equal to R + R = 2R

Parallel Circuits

• 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

• 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

  • Each lamp has a resistance of R, so the total resistance in the circuit will be less than R

• 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 is identical

• Current behaves in this way because it is the flow of electrons:

  • Current is the rate of flow of charge, and charge is not created or destroyed in a circuit

  • So the charge flowing into a junction each second equals the charge flowing out each second

  • At a junction the current splits between the branches, so I = I₁ + I₂