When an alternating sinusoidal voltage is applied to the plates of a capacitor, the capacitor is first charged in one direction and then again in the opposite direction, changing polarity at the same rate as the AC supply voltage.
This instantaneous change in voltage across the capacitor is opposed by the fact that it takes a certain amount of time to deposit (or release) this charge onto the plates and is given by
V = Q/C
Capacitors oppose these changes in sinusoidal voltage with the flow of electrons through the capacitor being directly proportional to the rate of voltage change across its plates as the capacitor charges and discharges. Unlike a resistor were the opposition to current flow is its actual resistance,
The opposition to current flow in a capacitor is called Capacitive Reactance
Like resistance, reactance is measured in Ohm’s but is given the symbol “X” to distinguish it from a purely resistive ohmic R value and as the component in question is a capacitor, the reactance of a capacitor is called Capacitive Reactance, ( XC ) which is also measured in Ohms. In an AC capacitance circuit this capacitive reactance value,
XC is equal to 1/( 2πƒC ) or 1/( jωC ).
What is AC Capacitance?
In a pure AC Capacitance circuit, the voltage and current are both “out-of-phase” with the current leading the voltage by 900 (or π/2 rads). So for a pure capacitor, VC “lags” IC by 900, or we can say that IC “leads” VC by 900.