Capacitors And Ac:
Assume that the power source connected to a capacitor is changed from dc to ac. visualize that you can regulate the frequency of this ac from a low initial value of a little hertz up to hundreds of hertz, then to many kilohertz, and lastly to many megahertz or gigahertz.
Firstly, the voltage among the plates obeys along with the voltage of the power source as the source polarity reverses over and over. Though, the set of plates has a certain quantity of capacitance. The plates can charge up rapid when they are small and when the space between them is very large, though they cannot charge instantly. As you raise the frequency of the ac source, there comes a point in which the plates do not get charged up very much before the source polarity opposes. The set of plates becomes sluggish. The charge does not have time to get recognized with each ac cycle.
At high ac frequencies, the voltage among the plates has trouble following the current which is charging and discharging them. Just as the plates start to get a good charge, the ac current passes its peak and begins to discharge them, pull electrons out of the negative plate and pumping electrons into the positive plate. As the frequency is increased, the set of plates begins to act more and more like a short circuit. Ultimately, whenever you keep on raising the frequency, the time period of the wave is much shorter than the charging-discharging time, and current flow in and out of the plates in similar way as it would flow if the plates were shorted out.
Capacitive reactance is a quantitative measure of the opposition that the set of plates gives to ac. It is measured in ohms, just similar to inductive reactance and just similar to resistance. Though, by convention, it is assigned negative values instead of positive ones. Capacitive reactance, symbolized by XC in mathematical formulas, can differ from near zero (whenever the plates are huge and close altogether and/or the frequency is very high) to a few negative ohms to many negative kilohms or megohms.
The Capacitive reactance differs with frequency. It acquires larger negatively as the frequency goes down and smaller negatively as the frequency goes up. This is the reverse of what occurs with inductive reactance that gets larger (positively) as the frequency goes up. At times capacitive reactance is talked about in terms of its absolute value, with the minus sign eliminated. Then you may say that XC rises as the frequency reduces or that XC reduces as the frequency is raised. Though, it is best when you learn to work with negative XC values from the beginning.