Current in Circuits (Cambridge O Level Physics)

• In a circuit that is a closed-loop, such as 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

Current in a Series Circuit

The current is the same at each point in a closed-loop

• 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

Increasing Potential Difference & Adding Components

Current will increase if the voltage of the power supply increases, and decreases if the number of components increases (because there will be more resistance)

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

Current in a Parallel Circuit

Diagram showing two bulbs connected in parallel

• The advantages of this kind of circuit are:
  • The components can be individually controlled, using their own switches
  • If one component stops working the others will continue to function

• In a parallel circuit, the current splits up - some of it going one way and the rest going the other
• This means that the current in each branch will be smaller than the current from the power supply

• 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 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 at Junctions

Current is conserved a junction; the total current is the sum of the current in the individual branches