junction rule

Meaning: The junction rule (Kirchhoff's first rule) says that the total current flowing into a junction equals the total current flowing out. A junction is a point where three or more wires meet. It follows from conservation of charge: in a steady circuit, charge does not build up at a point, so what arrives must leave.

Equation: ΣIin = ΣIout, where I is current in amperes (A), equal to coulombs per second.

On the MCAT: A passage shows a circuit with parallel branches and gives some currents. You find a missing one. For example, if 6 A enters a junction and 4 A leaves in one branch, then 2 A leaves in the other. Parallel branches share the same voltage, so the larger current goes through the smaller resistance. The rule is often paired with the loop rule to solve for several unknowns.

Common confusion: Students think current splits equally at a junction or is used up by a resistor. Current splits equally only when the branch resistances are equal, and the branch currents still add back to the original total. The junction rule concerns current (charge); the loop rule concerns voltage (energy).