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
- In the circuit below, I = I1 + I2+ I3, where I represents the current in the circuit before it branches, and I1, I2 and I3 represent the current in the respective three branches:
The current I into the junction is equal to the sum of the currents out of the junction
- 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 all points 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
Examiner 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.
