Kirchhoff's Circuit Rules
 

Junction Rule:  At any junction point in a circuit, the sum of the current entering the junction point must equal the sum of the currents leaving the junction.  

            I in = I out

 Example:  In the figure below, at junction point a, the current into the junction is I1 and the sum of the current leaving the junction is I2 + I3.  Using Kirchhoff's junction rule
 
            I1 = I2 + I3.