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CAPACITORS ‎

الكلية كلية الهندسة     القسم  الهندسة الميكانيكية     المرحلة 1
أستاذ المادة فرح فاهم السرحان       05/07/2018 17:42:32
‎5.1 INTRODUCTION‎

‎ So far we have limited our study to resistive circuits. In this chapter, we ‎shall introduce two new and important passive linear circuit elements: the ‎capacitor and the inductor (the inductor is discussed in detail in Chapter 7). ‎Unlike resistors, which dissipate energy, capacitors and inductors do not ‎dissipate but store energy, which can be retrieved at a later time. For this reason, ‎capacitors and inductors are called storage elements. We begin by introducing ‎capacitors and describing how to combine them in series or in parallel. Later, we ‎do the same for inductors. ‎

‎5.2 CAPACITORS‎

‎ A capacitor is a passive element designed to store energy in its electric ‎field. Besides resistors, capacitors are the most common electrical components. ‎Capacitors are used extensively in electronics, communications, computers, and ‎power systems. For example, they are used in the tuning circuits of radio ‎receivers and as dynamic memory elements in computer systems.‎

A capacitor consists of two conducting plates separated by an insulator ‎‎(or dielectric).‎
In many practical applications, the plates may be aluminum foil while the ‎dielectric may be air, ceramic, paper, or mica.‎
The amount of charge stored, represented by q, is directly proportional to the ‎applied voltage v so that
‎ q = Cv ‎
where C, the constant of proportionality, is known as the capacitance of the ‎capacitor. The unit of capacitance is the farad (F), in honor of the English ‎physicist Michael Faraday (1791–1867). From Eq. (5.1), we may derive the ‎following definition.‎

Capacitance is the ratio of the charge on one plate of a capacitor to the voltage ‎difference between the two plates, measured in farads (F).‎

Note from Eq. (5.1) that 1 farad = 1 coulomb/volt. ‎
‎ Although the capacitance C of a capacitor is the ratio of the charge q per ‎plate to the applied voltage v, it does not depend on q or v. It depends on the ‎physical dimensions of the capacitor. The capacitance is given by
‎ C =?A/d
where A is the surface area of each plate, d is the distance between the plates, ‎and ? is the permittivity of the
dielectric material between the plates. Typically, ‎capacitors have values in the picofarad (pF) to microfarad (?F) range. Figure ‎‎5.1 shows the circuit symbols for fixed and variable capacitors. ‎


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