dentical to Eq. (A.14). A.8 ALTERNATING CURRENTS 2 radians in 360° or IT radians in 180°. It follows that 1 radian = 180° /, where IT can be approximated by 3.14159, or 1 radian is 57.296°. We can look up the value of cos O with our scientific calculator, given the value of A, which will vary between 0° and 90°. Between these two values, cos O will vary between 0 and 1. Note that when the power factor has a value of 1, cos O is 1 and O°. This tells us that the voltage and current are completely in phase under these circumstances. This leads to a discussion of impedance, which in most texts and reference books is expressed by the letter Z. In numerous places in our text we have used the notation Z. This is the characteristic impedance, which is the impedance we expect a circuit or device port to display. For example, we can expect the characteristic impedance of coaxial able to be 75 S2, of a subscriber loop to be either 600 So or 900 S2, and SO forth. A.8.2 Ohm's Law Applied to ac Circuits We can freely use simple Ohm's law relationships [Eqs. (A.14)-(A.16)], when ac current and voltage are completely in phase. For example: R = E/I, where R is expressed in S2, E in V, and I in A. Otherwise, we have to use the following variants: Z = E/I, (A.31) where Z is expressed in S. One must resort to the use of Eq. (A.31) if an ac circuit is reactive. A circuit is reactive when we have to take into account the effects of capacitance and/or inductance in the circuit to calculate the effective value of Z. Under these circumstances, Z is calculated at a specified frequency. Our goal here is to reduce to a common expression in ohms a circuit's resistance in ohms, its inductance expressed in henrys, and its capacitance expressed in microfarads. Once we do this, a particular circuit or branch can be handled simply as though it were a direct current circuit. We define reactance as the effect of opposing the flow of current in an ac circuit due to its capacitance and/or inductance. There are two types of reactance: inductive reacta