What is a Capacitor?

Paper, plastic (film) and paper-plastic dielectric combination capacitors serve the broad and middle ground of most capacitor requirements. The "plastic" capacitor group consists of a wide variety of styles, voltages and temperature characteristics. There are continuing developments in dielectric materials, processes and techniques which promise to expand this family of products.

A capacitor is a passive electrical component consisting, in basic form, of two metal electrodes or plates separated by a dielectric (insulator). The electronic symbol for capacitance is "C" and the unit of measurement is the Farad. A capacitor has a "capacitance" of 1 Farad when application of 1 volt per second across the capacitor produces a current of 1 amp through it. Since the Farad is a large value, the most common units of measurement are, microfarad (10¯6), nanofarad (10¯9), and picofarad (10¯12). Capacitance is thought of as analogous to mechanical elasticity. Figure 1 shows the general mathematics associated with a "simple" capacitor.

From the basic formula, we note that (C) varies directly with the dielectric constant (K) and area (A); and inversely with the distance between the plates (d). Both (A) and (d) are geometrically controlled figures.

Figure 2 is a chart of the various dielectric materials and their associated dielectric constants. The dielectric constant (K) is the property of an insulating materials' ability to store electrostatic energy in a specific volume, when voltage is applied. The "K" factor relates to the same capacitance that is achieved when compared to air as the dielectric media. It should be noted that the constants will vary somewhat due to the purity of the dielectric, temperature, frequency, voltage, and treatment during the manufacture.

If a capacitor is manufactured with given "A" and "d" dimensions, and clean dry air is used as the dielectric, it will measure a certain value of capacitance. If another dielectric material is substituted for the air, the resulting capacitance will increase. If the capacitance value had doubled, for instance, this would mean that the second dielectric had a dielectric constant of 2 (two times air).

To fully understand just how the "capacitance" measures the ability of a capacitor to store energy, Figure 3 illustrates the direct relationship between energy and capacitance.