loop rule

Meaning: The loop rule, also called Kirchhoff's voltage law, says the potential changes around a closed path in a circuit add to zero. A charge that returns to its starting point has the same potential energy, so the energy a source supplies equals the energy lost in the resistors. Crossing a battery from − to + is a gain (+ε). Crossing a resistor in the direction of conventional current is a drop (−IR).

Equation: ΣΔV = 0 around a closed loop, for example ε − IR₁ − IR₂ = 0. Here ΔV is a potential change (V), ε is emf (V), I is current (A), and R is resistance (Ω).

On the MCAT: You pair it with the junction rule (current in = current out) to find unknown currents in circuits with more than one loop. It also shows that parallel branches share the same voltage. For a battery with internal resistance r, it gives terminal voltage V = ε − Ir.

Common confusion: Students mix it up with the junction rule. The junction rule is about current at a point and reflects charge conservation. The loop rule is about voltage around a path and reflects energy conservation. The signs depend on the direction you choose to traverse the loop, not on the true current direction. A negative current in your answer only means the current flows opposite to the direction you assumed.