Figure.1: Circuit for Kirchhoff’s Current Law
We now present the Kirchhoff’s current law which is essentially the law of conservation of electric charge.
Since charge must be conserved and does not accumulate at a node, the amount of current flowing out of a node equals the amount flowing in at any instant. In other words, an electrical node acts like a junction of water pipes where the amount of water going out equals the amount coming in.
Specifically for the portion of the network shown in figure 1, by applying KCL we obtain the equation
An alternative, but equivalent, form of KCL can be obtained by considering currents directed into a node be positive in sense and currents directed out of a node to be negative in sense (or vice versa). Under this circumstance, the alternative form of KCL can be stated as follows:
Applying this form of KCL to the node in figure 1 and considering currents directed into be positive in sense, we get
A close inspection of last two equations, however, reveals that they are the same.
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Kirchhoff’s current Law Solved Example
Find Io using KCL
Using KCL at node a,
Application of KCL
A very simple application of KCL is to combine current sources in parallel.
Using KCL, we can convert above figure into single current source form.
Current I1 and I3 are in the same direction as IT total current, so we consider I1 and I3 as positive currents, while I2 is in opposite direction of I1 and I3 so will be considered as negative current. So, the total resultant current IT will be,