Kirchhoff's Current and Voltage Laws

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Kirchhoff's Current Law and Nodal Analysis

Kirchhoff's Current Law says that the current going into a junction or node is equal to the current going out of a node. In other words, the sum of the currents entering the node must be zero (if we consider currents leaving the node to be a negative current entering the node). Consider the following diagram:

File:Kirchhoffs current law node diagram.jpg

For the node A in the center, i1 and i2 are entering the node, and i3 and i4 are leaving the node. We would write:

i1+i2+(-i3)+(-i4)=0\, which can also be written as i1+i2=i3+i4\,

Note that i7 is equal to i2; we can prove this by analyzing node B. We can also treat everything between node C and D as one big node, and conclude that i5 is equal to i6 without having to know the value of any of the currents within.

When solving for the currents in a real problem, we can choose arbitrarily in which direction the arrows point. If we guessed incorrectly, the value we obtain after solving for the current will be negative. However, if you draw i4 as leaving node A as in the diagram above, don't draw i4 as leaving node D when you are writing your node equations.

If we had drawn the currents for node A as following:

File:Kirchhoffs current law node2.jpg

Then our node equation looks like:

i1+i2+i3+i4=0\,

Unless all the currents are zero, one or more of the currents must turn out to be negative. The negative currents will flow in the direction opposite from that which the arrow is pointing.

Kirchhoff's Voltage Law

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