4.1 Linearity and proportionality
In Chapter 2 we defined a linear resistor as one that satisfied Ohm’s law
V = Ri
And we considered circuits that were made up of linear resistor and independent sources. We defined dependent sources in Chapter 3 and analyzed circuits containing both independent and dependent sources. The dependent sources that we considered all had source functions of the form
Y = kx (4.1)
Where k is a constant and the variables x and y circuit variables (voltages or currents). Clearly, Ohm’s law is a special case of (4.1). In (4.1) the variable y is proportional to the variable x, and the graph of y versus x is a straight line passing through the origin. For this reason authors refer to elements that are characterized by (4.1) as linear elements. For our purposes, we shall define a linear element in a more general way, which includes (4.1) as a special case. If x and y are circuit variables associated with a two-terminal element, then we shall say that the element is linear if multiplying x by a constant K results in the multiplication of y by the same constant K . This is called the proportionality property and evidently holds for all elements obeying (4.1) since, after multiplying both sides by K ,