Practical capacitors

Chapter Practical capacitors

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • Energy will be stored in this electric field. The ability of the plates, and of the spacebetween them, to store this energy is the property of capacitance. It is denoted by theletter C.Practical capacitorsIt’s out of the question to make a capacitor of the above dimensions. But two sheets, orstrips, of foil can be placed one on top of the other, separated by a thin, nonconductingsheet such as paper, and then the whole assembly can be rolled up to get a large effec-tive surface area. When this is done, the electric flux becomes great enough so that thedevice exhibits significant capacitance. In fact, two sets of several plates each can bemeshed together, with air in between them, and the resulting capacitance will be sig-nificant at high ac frequencies.In a capacitor, the electric flux concentration is multiplied when a dielectric of acertain type is placed between the plates. Plastics work very well for this purpose. Thisincreases the effective surface area of the plates, so that a physically small componentcan be made to have a large capacitance.The voltage that a capacitor can handle depends on the thickness of the metalsheets or strips, on the spacing between them, and on the type of dielectric used.In general, capacitance is directly proportional to the surface area of the conduct-ing plates or sheets. Capacitance is inversely proportional to the separation betweenconducting sheets; in other words, the closer the sheets are to each other, the greaterthe capacitance. The capacitance also depends on the dielectric constant of the mate-rial between the plates. A vacuum has a dielectric constant of 1; some substances havedielectric constants that multiply the effective capacitance many times.The unit of capacitanceWhen a battery is connected between the plates of a capacitor, it takes some time be-fore the electric field reaches its full intensity. The voltage builds up at a rate that de-pends on the capacitance: the greater the capacitance, the slower the rate of change ofvoltage in the plates.The unit of capacitance is an expression of the ratio between the amount of currentflowing and the rate of voltage change across the plates of a capacitor. A capacitance ofone farad, abbreviated F, represents a current flow of one ampere while there is a po-tential-difference increase or decrease of one volt per second. A capacitance of onefarad also results in one volt of potential difference for an electric charge of onecoulomb.The farad is a huge unit of capacitance. You’ll almost never see a capacitor with avalue of 1 F. Commonly employed units of capacitance are the microfarad (µF) andthe picofarad (pF). A capacitance of 1 µF represents a millionth (10-6) of a farad, and1 pF is a millionth of a microfarad, or a trillionth of a farad (10-12 F).Some quite large capacitances can be stuffed into physically small components.Conversely, some capacitors with small values take up large volumes. The bulkiness ofa capacitor is proportional to the voltage that it can handle, more than it is related to thecapacitance. The higher the rated voltage, the bigger, physically, the component will be.The unit of capacitance201