The Laws of Circuit - you can learn and practice by just reading

Copyright. Charles Kim 2006

How to become a circuit lawyer?

 

To become a lawyer, you study on law, and passing a bar exam would be necessary, to practice numrous laws. In the circuit theory, we have only 3 laws: Ohm's law (OHM), Kirchhoff's current law (KCL), and Kirchhoff's voltage law (KVL). Of course, knowing the 3 laws does not make us good circuit problem solvers: we need to practice the laws. Let's go over OHM: V=IR ("voltage across a resistor is product of the resistance of the resistor and the current flows through the resistor"). OHM seems too trivial to consider formally. A high school science course seemes more than enough, and redundantly, the second part of the physics again covers the OHM. And the bad news is students still are not competent in applying OHM in circuit problem. One thing we have to remember in OHM application is (i) OHM applies to a passive element, i.e., resistor R; (ii) the voltage V in the OHM equation is the voltage across the resistor (not any other voltage in the circuit), and (iii) the current I is the current flowing through the resistor. My experience tells me that students do not have any problem with (i) and (iii) above. However, there are enough mistakes and problems involving with (ii). Remember that the V in OHM is the voltage across an element in which you apply the law. Don't assume that you can find V across or I though an element by a simple observation of a circuit. It may involve writing an equation of two with 1 or 2 variables. If you do not know the voltage across, then define it as, for example, Vx, and solve for Vx first applying KCL/KVL to other parts of the circuit. Then find, for example, I of the circuit.

 

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