Capacitors & Relative Permittivity (εr) & Absolute Permittivity (ε)

Chapter 3.7 Capacitors

Fundamental Electrical and Electronic Principles Third Edition Book
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Fundamental Electrical and Electronic Principles Third Edition Book

  • 84Fundamental Electrical and Electronic Principles (a) EEAnsVd volt/metreso, kV/m 11125403253. (b) DQAAQDA cou mb/metreso, metrethus, lo2242053301111.62233m or mm 1 .Ans (c) QCVCQVC coulombso, faradthus, F 202564111.Ans 3.7 Capacitors A capacitor is an electrical component that is designed to have a specifi ed value of capacitance. In its simplest form it consists of two parallel plates separated by a dielectric; i.e. exactly the system we have been dealing with so far. In order to be able to design a capacitor we need to know what dimensions are required for the plates, the thickness of the dielectric (the distance of separation d ), and the other properties of the dielectric material chosen. Let us consider fi rst the properties associated with the dielectric. 3.8 Permittivity of Free Space ( εo ) When an electric fi eld exists in a vacuum then the ratio of the electric fl ux density to the electric fi eld strength is a constant, known as the permittivity of free space. The value for F/m.oε8 85410 12. Since a vacuum is a well defi ned condition, the permittivity of free space is chosen as the reference or datum value from which the permittivity of all other dielectrics are measured. This is a similar principle to using Earth potential as the datum for measuring voltages. 3.9 Relative Permittivity (εr) The capacitance of two plates will be increased if, instead of a vacuum between the plates, some other dielectric is used. This difference in capacitance for different dielectrics is accounted for by the relative