Kirchhoff's First Law (OCR A Level Physics)

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

  • Charges cannot be created nor destroyed

  • Therefore, the 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