Current in Circuits (Edexcel GCSE Physics)

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Current in Circuits

  • A current will flow in a circuit if:
    • The circuit includes a source of potential difference
    • The circuit is closed - i.e. there are no gaps in the circuit

  • Sources of potential difference include:
    • A cell
    • Batteries (multiple cells)
    • Electrical generator

Charge Conservation

  • 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, downloadable AS & A Level Physics revision notes

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

Current at a Junction

  • At a junction in a 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

Junctions current splitting, IGCSE & GCSE Physics revision notes

The current coming into a junction must equal the current coming out of a junction

  • In electrical wires, the current is a flow of electrons
  • Electrons are negatively charged; they flow away from the negative terminal of a cell towards the positive terminal
  • Conventional current is defined as the flow of positive charge from the positive terminal of a cell to the negative terminal
    • This is the opposite to the direction of electron flow, as conventional current was described before electric current was really understood

Electric current flow, downloadable AS & A Level Physics revision notes

By definition, conventional current always goes from positive to negative (even through electrons go the other way)

Worked example

In the circuit below, ammeter A0 shows a reading of 10 A, and ammeter A1 shows a reading of 6 A.WE Current Conservation Question image, downloadable IGCSE & GCSE Physics revision notesWhat is the reading on ammeter A2?

Step 1: Recall that at a junction, the current is conserved

    • This means that the total amount of current flowing into a junction is equal to the total amount flowing out

Step 2: Consider the first junction in the circuit where current splits

    • The diagram below shows the first junction in the circuit

WE Current Conservation Solution image, downloadable IGCSE & GCSE Physics revision notes

Step 3: Calculate the missing amount of current

    • Since 10 A flows into the junction (the total current from the battery), 10 A must flow out of the junction
    • The question says that 6 A flows through ammeter A1 so the remaining current flowing through ammeter A2 must be:

10 A − 6 A = 4 A

    • Therefore, 4 A flows through ammeter A2

Examiner Tip

The direction of current flow is super important when considering junctions in a circuit, you should remember that current flows from the positive terminal to the negative terminal of a cell / battery

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Ashika

Author: Ashika

Expertise: Physics Project Lead

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.