During this charging process, a charging current (i) will flow to the capacitor opposite any voltage change at a rate equal to the charge rate of the electrical charge on the plates.
This charging current can be defined as: i = CdV / dt. Once the capacitor is “fully charged”, the capacitor blocks the flow of other electrons on its plates as they become saturated. However, if we apply an alternative power or alternating current source, the capacitor will alternately charge and discharge at a rate determined by the frequency of the power supply. Then the capacitance in the AC circuits varies in frequency, because the capacitor is constantly charged and discharged.
We know that the flux of electrons on the plates of a capacitor is directly proportional to the rate of change of voltage on these plates. Then we can see that the capacitors in the AC circuits want to pass current when the voltage on their plates changes constantly over time, such as AC signals but does not like to pass current when the applied voltage has a constant value such as DC signals. Consider the circuit below.
In the purely capacitive circuit above, the condenser is directly connected through the AC supply voltage. When the supply voltage increases and decreases, the capacitor charges and discharges with respect to this change. We know that the charge current is directly proportional to the rate of variation of the voltage across the plates with this maximum variation speed when the supply voltage passes from its positive semiperiod to its negative semiperiod or vice versa at points 0o and 180o along the wave sinusoidal.
Consequently, the minimum voltage change occurs when the CA sinusoidal wave crosses its maximum or minimum peak voltage level (Vm). In these loop positions, the maximum or minimum currents flow through the condenser circuit and this is shown below.
At 0 °, the switching of the supply voltage is increasing in a positive direction resulting in a maximum load current at that time over time. Since the applied voltage reaches a maximum of 90 ° for a short period of time, the supply voltage is neither increasing nor decreasing, so there is no current flowing through the circuit.
When the applied voltage begins to drop to zero at 180 °, the slope of the voltage is negative, so the condenser is discharged in the negative direction. At 180 ° along the line, the voltage variation rate is again at its maximum so that the maximum current flows at that time, and so on.
So we can say that for capacitors in AC circuits, the instantaneous current is at idle or zero whenever the applied voltage is at its maximum and also the instantaneous value of the current is at its maximum or maximum when the applied voltage is at idle or zero.
From the waveform above, we can see that the current drives the voltage to 1/4 or 90o as shown in the vector scheme. So we can say that in a purely capacitive circuit, the alternating voltage drops the current by 90 degrees.
We know that the current flowing through the capacitance of the AC circuits is in contrast to the applied voltage velocity but, like the resistors, the capacitors also provide a form of resistance against current through the circuit but with capacitors in the AC circuits this AC resistance is known under the name of Reaction or more frequently in capacitor circuits, the capacity, so that capacity in the AC circuits suffers from capacitive reactivity.