Circuit Theorems

INTRODUCTION

In the practical analysis of electric networks, engineers often need to know the power absorbed by one particular load. The load may be a large machine or a lighting network over a factory floor. Such loads are often represented by a single resistor or, possibly, a general two-terminal network. Usually the load varies over time.
The analysis of such a situation is often greatly simplified by replacing the linear network, exclusive of the load, by a simple equivalent consisting of just one resistance and one independent source.

This chapter investigates the replacement of a network N by its Thevenin equivalent or by its Norton equivalent.

The Superposition Principle

The superposition principle states that “In any linear resistive network, the voltage across or the current through any branch may be calculated by adding algebraically all the individual voltages or currents caused by the separate independent sources acting alone, with all other independent voltage sources replaced by short circuits and all other independent current sources replaced by open circuits.
Thus, if there are N independent sources, we must perform N experiments, each having only one of the independent sources active and the others inactive/turned off. Note that dependent sources are in general active in every experiment.