If an AC power supply is connected to a resistor, then the current and voltage in the circuit at any point in the timing diagram will be proportional to each other. This means that the current and voltage curves will reach the "peak" value at the same time. In doing so, we say that the current and voltage are in phase.
Now consider how a capacitor will behave in an AC circuit.
If a capacitor is connected to an AC voltage source, the maximum voltage across it will be proportional to the maximum current flowing in the circuit. However, the peak of the voltage sine wave will not occur at the same time as the peak of the current.
In this example, the instantaneous value of the current reaches its maximum value a quarter of a period (90 el.deg.) Before the voltage does. In this case, they say that "the current leads the voltage by 90◦".
Unlike the situation in the DC circuit, the V/I value here is not constant. Nevertheless, the ratio V max / I max is a very useful value and in electrical engineering is called capacitance(Xc) component. Since this value still represents the ratio of voltage to current, i.e. in the physical sense it is resistance, its unit of measure is the ohm. The Xc value of a capacitor depends on its capacitance (C) and AC frequency (f).
Because the rms voltage is applied to the capacitor in an AC circuit, the same AC current flows in that circuit, which is limited by the capacitor. This limitation is due to the reactance of the capacitor.
Therefore, the value of current in a circuit containing no components other than a capacitor is determined by an alternative version of Ohm's Law
IRMS=URMS / XC
Where URMS is the rms (rms) voltage value. Note that Xc replaces R in the DC version of Ohm's Law.
Now we see that a capacitor in an AC circuit behaves very differently from a fixed resistor, and the situation here is correspondingly more complicated. In order to better understand the processes occurring in such a chain, it is useful to introduce such a concept as a vector.
The basic idea of a vector is the notion that the complex value of a time-varying signal can be represented as the product of a complex number (which is independent of time) and some complex signal that is a function of time.
For example, we can represent the function Acos(2πνt + θ) just as a complex constant A∙ejΘ.
Since vectors are represented by magnitude (or modulus) and angle, they are graphically represented by an arrow (or vector) rotating in the XY plane.
Given that the voltage on the capacitor is "lag" in relation to the current, the vectors representing them are located in the complex plane as shown in the figure above. In this figure, the current and voltage vectors rotate in the opposite direction of the clockwise direction.
In our example, the current on the capacitor is due to its periodic recharge. Since the capacitor in the AC circuit has the ability to periodically accumulate and discharge an electric charge, there is a constant exchange of energy between it and the power source, which in electrical engineering is called reactive.
