Capacitance and Capacitors

Capacitance and Capacitors

Capacitors are devices that consist of two conducting plates separatated by an insulating material (a dielectric). When a voltage is applied to the two plates, positive charge builds up on one plate and negative charge builds up on the other. This building up of charge (charging) takes time. An electric field is then created between these two plates going from the positively charged plate to the negatively charged plate. In this way, capacitors store electrical charge (Q) and it's this capability to store charge that is capacitance (C). A diagram of a parallel plate capacitor is shown below.

Capacitance is measured in the units Farads (F). The quantities of charge, voltage, and acpacitance can be related with the following equation:

The capacitance of a parallel plate capacitor, like the one shown above, can also be calculated using the geometry of the capacitor with the following equation:

In the equation above A is the area of a plate in the capacitor, d is the distance between the plates, and the Greek letter epsilon represents the dielectric constant of the dielectric between the two plates. 

Typical capacitors used in household circuits have relatively small capacitances on the order of picofarads (10^-12 Farads) to microfarads (10^-6 Farads). A 1 F capacitor is huge! Some capacitors are shown in the image below. In these capacitors the two plates with the dielectric between them are rolled up so that the capacitor can have a smaller form factor. 

When capacitors are being used in an AC circuit, a parameter called capacitive reactance also needs to be considered. Capacitive reactive is the frequency dependent complex impedance associated with a capacitor. Impedance will be discussed more in the following page.