Basic DC Theory
Applying Kirchhoffs Voltage Law
For a simple series circuit, Kirchhoffs voltage law corresponds to Ohms Law. To find the
current in a circuit (Figure 33) by using Kirchhoffs voltage law, use equation (2-15).
SE source = SIR
Figure 33 Using Kirchhoffs Voltage Law to find Current with one Source
80 = 20(I) + 10(I)
80 = 30(I)
I = 80/30 = 2.66 amperes
In the problem above, the direction of current flow was known before solving the problem.
When there is more than one voltage source, the direction of current flow may or may not be
known. In such a case, a direction of current flow must be assumed in the beginning of the
problem. All the sources that would aid the current in the assumed direction of current flow are
then positive, and all that would oppose current flow are negative. If the assumed direction is
correct, the answer will be positive. The answer would be negative if the direction assumed was
wrong. In any case, the correct magnitude will be attained.