Capacitors and direct current

Suppose that you have two big, flat metal plates, both of which are excellent electrical conductors. Imagine that you stack them one on top of the other, with only air in between. What will take place if you connect a source of dc across the plates (Fig. 1)? The plates will become electrically charged, and will reach a potential difference equal to the dc source voltage. It won’t matter how big or small the plates are; their mutual voltage will always be the same as that of the source, although, if the plates are monstrously large, it might take awhile for them to become fully charged. The current, once the plates are charged, will be zero.

Fig. 1: A capacitor connected across a source of dc.

If you put some insulating material, such as glass, between the plates, their mutual voltage will not change, although the charging time might increase.
If you increase the source voltage, the potential difference between the plates will follow along, more or less rapidly, depending on how large the plates are and on what is between them. If the voltage is increased without limit, arcing will eventually take place. That is, sparks will begin to jump between the plates.