Kirchhoff's rules

Meaning: Kirchhoff's rules analyze circuits that cannot be reduced by simple series and parallel combinations. The junction rule reflects conservation of charge: current flowing into a junction equals current flowing out. The loop rule reflects conservation of energy: the potential changes around a closed loop add to zero, so voltage gained from sources equals voltage dropped across resistors.

Equation: ΣIin = ΣIout at a junction, and ΣΔV = 0 around a closed loop. I is current in amperes (A) and ΔV is potential change in volts (V). Crossing a resistor in the direction of current, ΔV = −IR (R in ohms, Ω). Crossing a battery from its negative to its positive terminal, ΔV = +ε (emf in V).

On the MCAT: Expect a circuit with two or more branches. You may find an unknown branch current from the junction rule, or reason that parallel branches have the same voltage drop while their currents add to the total current. Series resistors share one current, and their voltage drops sum to the emf.

Common confusion: Students assume current splits equally at a junction. It does not; more current flows through the branch with less resistance. If an assumed current direction gives a negative answer, the current simply flows opposite to the guess.