Kirchoff's junction rule
Summary
Kirchhoff's junction rule says the total current flowing into a circuit junction equals the total current flowing out, which is conservation of charge. On the MCAT, use it to find a missing branch current: parallel branch currents add to the source current.
Detail
Meaning: Kirchhoff's junction rule (the current rule) says the total current into a junction equals the total current out. It expresses conservation of charge: in a steady circuit, charge cannot accumulate at a node, so what arrives must leave.
Equation: ΣIin = ΣIout, where I is current in amperes (A, coulombs per second). Equivalently, ΣI = 0 at a junction when currents entering count as positive and currents leaving count as negative.
On the MCAT: A passage shows a circuit node with several wires, gives some currents, and asks for the missing one, including its direction. Another common setup has parallel resistors across the same voltage: Ohm's law (V = IR) shows the smaller resistance carries the larger current, and the branch currents sum to the source current. Series elements with no junction between them carry the same current.
Common confusion: Students mix it up with the loop rule, which says potential changes around a closed loop sum to zero and reflects conservation of energy. The junction rule concerns current and charge. Another error is thinking a resistor uses up current; it dissipates energy, and the current leaving a resistor equals the current entering it.
Sources
- University Physics
- Fundamentals of Physics
- College Physics
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.