# 12.1.4 Kirchhoff's First Law

### Kirchhoff's First Law

• Kirchhoff’s first law states that:
• The sum of the currents entering a junction always equal the sum of the currents out of the junction
• This is a consequence of conservation of charge – current shouldn’t decrease or increase in a circuit when it splits
• In a circuit:
• A junction is a point where at least three circuit paths meet
• A branch is a path connecting two junctions
• If a circuit splits into two branches, then the current before the circuit splits should be equal to the current after it has split

I1 = I2 + I3  , where I1 represents the current in the circuit before it branches, and I2 and I3 represent the current in the respective two branches

• The charge is conserved on both sides of the junction
• In a series circuit, the current is the same at any point The current is the same at each in a series circuit

• In a parallel circuit, the current divides at the junctions and each branch has a different value. Kirchhoff’s first law applies at each junction The current divides at each junction in a parallel circuit

#### Exam Tip

Junctions only appear in parallel circuits and as circuits become more complex, it can be confusing as to which currents are into the junction and which are out.

Drawing arrows on the diagram for the current flow (making sure it’s from positive to negative) at each junction like in the worked example will help with this. ### Author: Ashika

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.
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