Kirchhoff’s First Rule
Kirchhoff’s First Rule
Kirchhoff’s first rule (the junction rule) is an application of the conservation of charge to a junction; it is illustrated in this figure. Current is the flow of charge, and charge is conserved; thus, whatever charge flows into the junction must flow out. Kirchhoff’s first rule requires that \({I}_{1}={I}_{2}+{I}_{3}\) (see figure). Equations like this can and will be used to analyze circuits and to solve circuit problems.
Making Connections: Conservation Laws
Kirchhoff’s rules for circuit analysis are applications of conservation laws to circuits. The first rule is the application of conservation of charge, while the second rule is the application of conservation of energy. Conservation laws, even used in a specific application, such as circuit analysis, are so basic as to form the foundation of that application.
The junction rule. The diagram shows an example of Kirchhoff’s first rule where the sum of the currents into a junction equals the sum of the currents out of a junction. In this case, the current going into the junction splits and comes out as two currents, so that \({I}_{1}={I}_{2}+{I}_{3}\). Here \({I}_{1}\) must be 11 A, since \({I}_{2}\) is 7 A and \({I}_{3}\) is 4 A.
This lesson is part of:
Circuits and DC Instruments