Kirchhoff's rules and internal resistance

Real circuits often have several loops and batteries, and two rules named for Gustav Kirchhoff help you analyze even complicated ones. The junction rule: the total current flowing into any junction, a point where wires meet, equals the total current flowing out, because charge is conserved and does not pile up. If 3 amperes flow into a junction and one branch leaving it carries 1 ampere, the other branch must carry 2 amperes. The loop rule: around any closed loop in a circuit, the voltage rises (across batteries) and the voltage drops (across resistors and other components) add up to zero, because a charge that travels all the way around ends with the same energy it started with. Writing enough independent junction and loop equations, using Ohm's law for each resistor, and solving them together gives the unknown currents. Real batteries also have internal resistance, a small resistance inside the battery itself. A battery's emf is the voltage it supplies when no current flows; while it supplies a current I, some voltage is lost inside, so the terminal voltage equals the emf minus I times the internal resistance. That is why a battery's voltage sags when it powers a heavy load, like a car's starter motor, and why a battery warms up when it delivers a large current.

This explanation is AI-generated and can occasionally be wrong. 🚩 Report