CHAPTER 14. ELECTRONICS 14.2
TimeCurrentVoltageFigure 14.1: The voltage and current are in phase when a resistor is connected to an
alternating voltage.
same time as the maximum current. The current has its maximum (it peaks) one quar-
ter of a cycle before thevoltage peaks. Engineers say that the ‘current leads the voltage
by 90 ◦’. This is shown in Figure 14.2.
TimeCurrentVoltageFigure 14.2: The current peaks (has its maximum) one quarter of a wave before the
voltage when a capacitor is connected to an alternating voltage.
For a circuit with a capacitor, the instantaneousvalue ofVIis not constant. However,
the value ofVImaxmaxis useful, and is called the capacitive reactance (XC) of the compo-
nent. Because it is stilla voltage divided by acurrent (like resistance),its unit is the
ohm. The value of XC(C standing for capacitor)depends on its capacitance (C) and
the frequency (f) of the alternating current (in South Africa 50 Hz).
XC=
Vmax
Imax=
1
2 πfC(14.1)
Inductors are very similar, but the current peaks 90 ◦after the voltage. This is shown
in Figure 14.3. Engineers say that the ‘current lags the voltage’. Again, the ratio of