CIE AS Physics (9702) 2019-2021

Revision Notes

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

Kirchoff's first law, downloadable AS & A Level Physics revision notes

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

Current in a series circuit, downloadable AS & A Level Physics revision notes

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

Current in a parallel circuit, downloadable AS & A Level Physics revision notes

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.

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