Capacitance

WHAT IS CAPACITANCE?

An electrical parameter which physically is most easy to define, electrically often misconstrued.

Physically, a capacitor is two electrical conductors separated by a non-conducting (or very high resistance) medium between the conductors. Consider the two plates of area (A) in Figure 1. The plates are metallic and they are separated by a distance (D).

Figure 1.

C=KA/D

If we fill the space between the plates with a conductor (water, acid, etc.) and close the switch (S), we get a current dictated by ohms law, I=v/r (where V is voltage, and R is the resistance between the plates). If the space is a non-conductor, no current will flow but the voltage will exist across the plates. The plus side and the negative side attract, and electrical charges will exist on the plates; thus an electrical field will exist in the space between. Clearly, the larger the plates, the more charges will exist and the closer the plates (D), the stronger the electrical attraction between the plates will be.

If we now reverse the polarity of the battery, the plus plate is now negative and the negative plate plus. For the electrical charge to reverse, ¢ electrons¢ must have flowed, reversing their position. Since ¢ electron flow¢ is current, we have current flow when we reverse the polarity.

If we now substitute the battery with an alternating current source, the polarity will reverse every 1/2 cycle and we will, hence, get continuous current flow. Therefore, while a capacitor will not allow DC current to flow through, AC current can pass. The amount of current will depend on the supply voltage, the capacity of the plates to hold charges and the distance (D), which determines the leakage (or electrical field) in the space and the material between the plates.

Figure 2.

The formula for current flow through a capacitor with an alternating voltage applied is I=V2 FKA/D. K is defined as the dielectric of material or ability to store electrons.

Note: - the higher the area A, the larger the current

- the smaller the D, the larger the current

- K is determined by the material having a high value (eg. water = 80) and non-conducting materials having a lower value (eg. air = 1). Current is increased with the supply frequency (f), as you are reversing the charges more frequently and current increases with supply voltage.