In a sine-wave ac circuit, the voltage across a capacitance lags its charge and discharge current by 90°.
Therefore, capacitive reactance XC is a phasor quantity out of phase with its series resistance by -90° because iC = C(dv/dt). This fundamental fact is the basis of all the following relations.
The combination of XC and R in series is their total impedance ZT. These three types of ohms of opposition to current are compared in Table 18–4.
The opposite characteristics for series and parallel circuits with XC and R are summarized in Table 18–5.
Two or more capacitors in series across a voltage source serve as a voltage divider. The smallest C has the largest part of the applied voltage.
A coupling capacitor has XC less than its series resistance by a factor of one-tenth or less to provide practically all the ac applied voltage across R with little across C.
In sine-wave circuits, IC = VC/XC. Then IC is out of phase with VC by 90°.
For a circuit with XC and R in series, tan θz = -(XC/R), and in parallel, tan θ1 = IC/IR. See Table 18–5.
When the voltage is not a sine wave, iC = C(dv/dt). Then the waveshape of iC is different from that of the voltage.
