At every node, the sum of all currents entering a node must equal zero. What this law means physically is that charge cannot accumulate in a node; what goes in must come out. In the example, Figure 1, below we have a threenode circuit, and thus have three KCL equations.
Given any two of these KCL equations, we can find the other by adding or subtracting them. Thus, one of them is redundant and, in mathematical terms, we can discard any one of them. The convention is to discard the one for the (unlabeled) node at the bottom of the circuit.
Figure 1  


Exercise 1
In writing KCL equations, you will find that in an
Solution
KCL says that the sum of currents entering or leaving a node must be zero. If
we consider two nodes together as a "supernode", KCL applies as well to currents
entering the combination. Since no currents enter an entire circuit, the sum of
currents must be zero. If we had a twonode circuit, the KCL equation of one
must be the negative of the other, We can combine all but one
node in a circuit into a supernode; KCL for the supernode must be the negative of
the remaining node's KCL equation. Consequently, specifying