CIE A Level Physics复习笔记10.1.4 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