If the AC power supply is connected to the resistor, the current and voltage in the circuit at any point in the waveform will be proportional to each other.This means that the current and voltage curves will reach the "peak" values simultaneously.At the same time we say that the current and voltage are in phase.
Consider now how will behave capacitor AC circuit.
If an AC voltage source connected to the capacitor, the maximum value of the voltage across it will be proportional to the maximum value of the current flowing in the circuit.However, the peak wave sine wave voltage will not advance at the same time as the current maximum.
In this example, the instantaneous value of the current reaches its maximum value by a quarter period (90 el.grad.) Before it will make the voltage.In this case we say that "the current leads the voltage by 90◦».
Unlike the situation in a circuit postoyanngo current value of V / I is not constant.However, the ratio of V max / I max value is very useful in electrical engineering as capacitive impedance (Xc) of the component.Since this value still indicates the ratio of voltage to current, i.e.in a physical sense is impedance, it is a unit of measurement of Ohms.The value Xc capacitor depends on the capacitance (C) and the AC frequency (f).
As to the capacitor in the AC circuit is supplied rms voltage in this circuit flows the same alternating current, which is limited to a capacitor.This limitation is due to the reactance of the capacitor.
Therefore, the value of current in the circuit, does not contain any components other than the capacitor is determined by an alternative version of Ohm's Law
IRMS = URMS / XC
Where URMS - root mean square (rms) voltage.Please note that replaces the R value Xc version of Ohm's Law for DC.
Now we see that the capacitor in the AC circuit behaves not as a fixed resistor, and the situation here, accordingly, is more complicated.In order to better understand the processes occurring in such a circuit, it is useful to introduce the notion of the vector.
basic idea vector - is the idea that the value of the complex time-varying signal can be represented as the product of a complex number (which is independent of the time) and some of the complex signal is a function of time.
For example, we can represent the function A cos (2πνt + θ) just as a complex constant A ∙ ejΘ.
Since the vectors represented value (or module) and the angle, they graphically represented by an arrow (or vector), rotating in the plane XY.
Given that the voltage across capacitor "lagging" in relation to the current vectors representing them are located in the complex plane as shown in the figure above.In this figure, the voltage and current vectors are rotated in the opposite direction to the clockwise.
In our example, the current in the capacitor due to its periodic recharge.As the capacitor in the AC circuit has the ability to accumulate and periodically reset the electric charge, between him and the power supply is a constant exchange of energy, which is called the electrical reactive.
