Pearson Edexcel · GCSE · Physics
Course version: 1PH0 · Version from Sept 2016
Recall that current is conserved at a junction in a circuit
Full specification wording
Topic 10 – Electricity and circuits: 10.11 Recall that current is conserved at a junction in a circuit
At a junction in a circuit, the total current entering equals the total current leaving, because charge is not created or destroyed there.
Checked against the Pearson Edexcel 1PH0 specification (opens in a new tab) on .
- Point 11 of 39
Study notes
From the checked teaching notes for this specification point.
The idea
At a junction in a circuit, the total current entering equals the total current leaving, because charge is not created or destroyed there. If 0.30 A flows into a junction and one branch carries 0.12 A, the other branch must carry 0.30 A − 0.12 A = 0.18 A. The rule applies to any number of branches: sum the currents in and set them equal to the sum out. It holds for series paths (same current throughout) and for parallel branches, and it follows from conservation of charge. In questions, identify the junction, list the known currents with their directions, then solve for the unknown.
What earns the marks
- States that the total current entering a junction equals the total current leaving it.
- Applies conservation of charge to find an unknown branch current by subtracting known branch currents from the total entering.
- Recognises that the current is the same at all points in a single series loop.
- Uses the correct unit, the ampere (A), and keeps currents in the same unit before adding or subtracting.
Common mistakes and how to put them right
Adds branch currents that leave a junction to the current entering instead of equating them.
Put it right: Set total in equal to total out.
Assumes current is used up as it passes through components.
Put it right: Keep the current conserved at the junction.
Mixes milliamps and amps without converting.
Put it right: Convert all currents to the same unit first.
Exam tips
- Sketch the junction and label each current with an arrow showing direction before calculating.
- Write the balance as an equation, total in = total out, then substitute the values.
- Check the answer is smaller than the total entering when one branch is unknown.
Try the lesson
The start of the Topic 10 – Electricity and circuits: 10.11 Recall that current is conserved at a junction in a circuit lesson, made from checked teaching notes for this specification point.
Have a go first
A current of 3.0 A flows along a wire towards a junction, where the wire splits into two branches. What is the total current in the two branches combined?
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