If there are two resistance values in parallel connected to an  AC voltage, as seen in  Figure 9-23,  impedance is equal to  the total resistance of the circuit. Once again, the calculations  would be handled the same as if it were a DC circuit and the  following would apply:  First, the capacitance, 80 μf, is changed to farads by dividing  80 by 1,000,000, since 1 million microfarads is equal to 1  farad. This quotient equals 0.000080 farad. This is substituted  in the equation: Since this is a pure resistive circuit R$_{T}$ = Z (Resistance =  Impedance) The total opposition to current flow in an AC circuit is known  as impedance and is represented by the letter Z. The combined  effects of resistance, inductive reactance, and capacitive  reactance make up impedance (the total opposition to current  flow in an AC circuit). In order to accurately calculate  voltage and current in AC circuits, the effect of inductance  and capacitance along with resistance must be considered.  Impedance is measured in ohms.  The rules and equations for DC circuits apply to AC circuits  only when that circuit contains resistance alone and no  inductance or capacitance. In both series and parallel circuits,  if an AC circuit consists of resistance only, the value of the  impedance is the same as the resistance, and Ohm’s Law  for an AC circuit, I = E/Z, is exactly the same as for a DC  circuit.  Figure 9-22  illustrates a series circuit containing a  heater element with 11 ohms resistance connected across a  110-volt source. To find how much current flows if 110 volts  AC is applied, the following example is solved: To determine the current flow in the circuit use the equation:  Impedance is the total opposition to current flow in an AC  circuit. If a circuit has inductance or capacitance, one must take  into consideration resistance (R), inductive reactance (X$_{L}$),  and/or capacitive reactance (X$_{C}$) to determine impedance (Z).  In this case, Z does not equal R$_{T}$. Resistance and reactance  (inductive or capacitive) cannot be added directly, but they  Figure 9-23.  Two resistance values in parallel connected to an AC  voltage. Impedance is equal to the total resistance of the circuit. Figure 9-22.  Ohm's Law applies to AC circuit only when circuit  consists of resistance only. Impedance (Z) = Resistance (R). 110V AC  R = 11Ω I = 10A 50V AC Power supply  R = 20Ω Ammeter I = 5A A R = 20Ω X$_{C}$ = 1 2πfC X$_{C}$ = 1 2π(400)(0.000080) X$_{C}$  = 4.97Ω R$_{T}$ = 1 1 + 1  R$_{1}$   R$_{2}$ R$_{T}$ = 1 1 + 1  20   20 R$_{T}$  = 10Ω Impedance  Z$_{T}$ = R$_{T}$ Z$_{T}$  = 10Ω I = E Z I = 50V  10Ω I = 5 amps I = E Z I = 110V  11Ω I = 10 amps 9-15