might take awhile for them to become fully charged. The current, once the plates arecharged, will be zero.248 Capacitive reactance14-2A capacitor connectedacross a source of dc.If you put some insulating material, such as glass, between the plates, their mutualvoltage will not change, although the charging time might increase.If you increase the source voltage, the potential difference between the plates willfollow along, more or less rapidly, depending on how large the plates are and on what isbetween them. If the voltage is increased without limit, arcing will eventually take place.That is, sparks will begin to jump between the plates.Capacitors and alternating currentSuppose that the source is changed from direct to alternating current (Fig. 14-3). Imag-ine that you can adjust the frequency of this ac from a low value of a few hertz, to hun-dreds of hertz, to many kilohertz, megahertz, and gigahertz.14-3 A capacitor across asource of ac.At first, the voltage between the plates will follow just about exactly along as the acsource polarity reverses over and over. But the set of plates has a certain amount of ca-pacitance, as you have learned. Perhaps they can charge up fast, if they are small and ifthe space between them is large, but they can’t charge instantaneously.