Capacitors and Direct Current

CAPACITIVE REACTANCE IS THE NATURAL COUNTERPART OF INDUCTIVE REACTANCE. IT, LIKE INDUCTIVE reactance, can be represented as a ray. The capacitive-reactance ray goes in a negative direction and is assigned negative ohmic values. When the capacitive-reactance and inductive-reactance rays are joined at their endpoints (both of which correspond to a reactance of zero), a complete number line is the result, as shown in following figure. This line depicts all possible values of reactance.

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Inductive and capacitive reactance can be represented as numerical values (corresponding to ohms multiplied by j) along a number line

Capacitors and Direct Current

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A capacitor connected across a source of dc

Suppose 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. If you connect a source of dc across the plates (above figure), 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 huge, it will take awhile for them to become fully charged. Once the plates are fully charged, the current will drop to zero.

If you put some insulating material, such as glass, between the plates, their mutual voltage will not change, although the charging time will 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.