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Inductive Reactance Summary
Capacitive Reactance

Electrical Science Volume 3 of 4
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Basic AC Reactive Components CAPACITANCE CAPACITANCE There are many natural causes of capacitance in AC power circuits, such   as   transmission   lines,   fluorescent   lighting,   and   computer monitors. Normally,   these   are   counteracted   by   the   inductors previously discussed.  However, where capacitors greatly outnumber inductive devices, we must calculate the amount of capacitance to add or subtract from an AC circuit by artificial means. EO 1.5 DEFINE capacitive reactance (XC). EO 1.6 Given  the  operating  frequency  (f)  and  the  value  of capacitance (C), CALCULATE the capacitive reactance (XC) of a simple AC circuit. EO 1.7 DESCRIBE  the  effect  on  phase  relationship  between current (I) and voltage (E) in a capacitive circuit. EO 1.8 DRAW   a   simple   phasor   diagram   representing   AC current (I) and voltage (E) in a capacitive circuit. Capacitors The variation of an alternating voltage applied to Figure 3   Voltage, Charge, and Current in a Capacitor a capacitor, the charge on the capacitor, and the current    flowing    through    the    capacitor    are represented by Figure 3. The    current    flow    in    a    circuit    containing capacitance  depends  on  the  rate  at  which  the voltage changes.   The current flow in Figure 3 is greatest at points a, c, and e.  At these points, the voltage  is  changing  at  its  maximum  rate  (i.e., passing through zero). Between points a and b, the  voltage  and  charge  are  increasing,  and  the current flow is into the capacitor, but decreasing in   value. At   point   b,   the   capacitor   is   fully charged, and the current  is zero.   From points b to c, the voltage and charge are decreasing as the capacitor  discharges,  and  its  current  flows  in  a direction opposite to the voltage.   From points c to   d,   the   capacitor   begins   to   charge   in   the opposite direction, and the voltage and current are again in the same direction. Rev. 0 Page 5 ES-08







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