Basic DC Theory
The mathematics involved becomes more difficult as the circuits become more complex.
Therefore, the discussion here will be limited to solving only relatively simple circuits.
Kirchhoffs Voltage Law
Kirchhoffs first law is also known as his "voltage law." The voltage law gives the relationship
between the "voltage drops" around any closed loop in a circuit, and the voltage sources in that
loop. The total of these two quantities is always equal. In equation form:
Esource = E1 + E2 + E3 + etc. = I1R1 + I2R2 + I3R3 + etc.
SEsource = SIR
where the symbol S (the Greek letter sigma) means "the sum of."
Kirchhoffs voltage law can be applied only to closed loops (Figure 32). A closed loop must
meet two conditions:
It must have one or more voltage sources.
It must have a complete path for current flow from any point, around the loop,
and back to that point.
You will remember that in a simple series circuit, the sum of the voltage drops around the circuit
Figure 32 Closed Loop
is equal to the applied voltage. Actually, this is Kirchhoffs voltage law applied to the simplest
case, that is, where there is only one loop and one voltage source.