Topic explanation

Standard explanation

The algebraic sum of potential changes around a closed loop is zero.

Key points

• Kirchhoff's loop rule follows from conservation of energy.
• The algebraic sum of potential changes around a closed loop is zero.
• Across a resistor: −IR with current and +IR against current.
• Across a source: +ε from negative to positive and −ε from positive to negative.
• Clockwise or anticlockwise traversal gives the same physical solution when used consistently.
• Internal resistance contributes an Ir drop like any series resistance.
• Negative current means the actual direction opposes the assumed arrow.
• Multi-loop circuits require loop equations together with junction equations.

Labelled diagram