Current in Circuits (Cambridge (CIE) IGCSE Physics): Revision Note

Current in series circuits

What is a series circuit?

• A series circuit contains a single complete loop

Current in a series circuit

• In a series circuit, the current is the same value at any point

  • This is because the number of electrons per second that passes through one part of the circuit is the same number that passes through any other part

• This means that all components in a closed-loop have the same current

Factors affecting current in a series circuit

• The amount of current flowing around a series circuit depends on two things:

  • The voltage of the power source

  • The resistance of the components in the circuit

• Increasing the voltage of the power source drives more current around the circuit

  • So, decreasing the voltage of the power source reduces the current

• Increasing the number of components in the circuit increases the total resistance

  • Hence less current flows through the circuit

Current in parallel circuits

What is a parallel circuit?

• A parallel circuit consists of multiple loops containing circuit components

What is the rule for current in a parallel circuit?

• In a parallel circuit, the current splits along each branch at a junction

  • Some of it goes down one branch and the rest goes down the other

• This means that the current from the source is larger than the current in each branch

Lighting circuits

• A lighting circuit is used to supply power to multiple light sources

  • Office lights are a common example, as multiple identical bulbs must have the same brightness

• Lighting circuits are constructed in parallel because:

  • Bulbs all have the same potential difference and therefore the same brightness

  • If one bulb breaks, the rest continue to function as current passes along each branch independently

  • This also means lamps can be switched off and on individually without breaking the whole circuit

Calculating current in parallel circuits

Extended Tier Only

• The current before a junction is equal to the sum of currents along each branch after the junction

• At a junction in a parallel circuit (where two or more wires meet) the current is conserved

  • This means the amount of current flowing into the junction is equal to the amount of current flowing out of it

• This is because charge is conserved

• 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:

  • 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