Capacitor in AC Circuit

The most important property of a capacitor is its ability to block a steady DC voltage, while passing AC signals.

 

In the illustration above, the capacitor is connected to a sinusoidal voltage source. Current is always leading the capacitor voltage by 90^o. If the source voltage has a constant amplitude value and its frequency is increased, the amplitude of the current increases accordingly. Further, when the frequency of the source decreases, the current amplitude decreases. Therefore, the capacitor offers opposition to current, which varies inversely with frequency.

Capacitive Reactance

The opposition to sinusoidal current in a capacitor is called capacitive reactance. The symbol is X_c, and its unit is the ohm. X_c varies inversely not only with frequency but with capacitance as well. When a sinusoidal voltage with a fixed amplitude and fixed frequency is applied to a capacitor with given value, there is a constant amount of AC current. When the capacitance value is increased, the current increases. The formula for X_c is shown above. Ohm's Law applies to capacitive circuits as follows: U = I X_c.