Charge Conservation in Circuits (Edexcel A Level Physics)

The Electric Current Rule (Kirchhoff's First Law)

• The electric current rule is defined as:

The algebraic sum of the currents entering and leaving a junction is equal to zero

• This is a consequence of conservation of charge - current shouldn’t decrease or increase in a circuit when it splits

Series and Parallel

• 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
• A typical circuit might have a setup where  I = I₁ + I₂+ I₃ where:
  • I represent the current in the circuit before it branches
  • 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. 
• The electric current rule applies at each junction
  • The sum of the currents before the junction will equal the sum of the currents after the junction

The current divides at each junction in a parallel circuit