1-5 Figure 1-3.—Effect of frequency on capacitive reactance. Effect of Frequency on Resistance In the expression for inductive reactance, X_{L} = 2pfL, and in the expression for capacitive reactance, both contain "f" (frequency). Any change of frequency changes the reactance of the circuit components as already explained. So far, nothing has been said about the effect of frequency on resistance. In an Ohm's law relationship, such as R = E/I no "f" is involved. Thus, for all practical purposes, a change of frequency does not affect the resistance of the circuit. If a 60-hertz a.c. voltage causes 20 milliamperes of current in a resistive circuit, then the same voltage at 2000 hertz, for example, would still cause 20 milliamperes to flow. NOTE: Remember that the total opposition to a.c. is called impedance (Z). Impedance is the combination of inductive reactance (X_{L}), capacitive reactance (X_{C}), and resistance (R). When dealing with a.c. circuits, the impedance is the factor with which you will ultimately be concerned. But, as you have just been shown, the resistance (R) is not affected by frequency. Therefore, the remainder of the discussion of a.c. circuits will only be concerned with the reactance of inductors and capacitors and will ignore resistance. A.c. Circuits Containing Both Inductive and Capacitive ReactancesA.c. circuits that contain both an inductor and a capacitor have interesting characteristics because of the opposing effects of L and C. XLand X_{C} may be treated as reactors which are 180 degrees out of phase. As shown in figure 1-2, the vector for X_{L} should be plotted above the baseline; vector for X_{C}, figure 1-3, should be plotted below the baseline. In a series circuit, the effective reactance, or what is termed the RESULTANT REACTANCE, is the difference between the individual reactances. As an equation, the resultant reactance is: