Lecture_7

Lecture_7 - ECE103 Spring 2008 Lecture 7 Capacitors and...

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ECE103 Spring 2008 Lecture 7 Capacitors and inductors These are two important passive linear circuit elements. Unlike the resistors, these elements do not dissipate energy. However they can store energy that can be retrieved at latter time. For this reason capacitors and inductors are called storage elements. Capacitors A capacitor is a passive element designed to storage energy in its electric field. Capacitors are used extensively in electric circuits and are the most common components besides resistors. A capacitor is composed by two electrodes with different geometry with a suitable insulating material (dielectric) between them. The figure represents a typical parallel plates capacitor and the photo corresponds to different capacitors (fixed and variable value) The plates can be aluminum foil while the dielectric can be air, ceramic, or mica. Capacitors can be built with different geometries (parallel plate, cylindrical, etc). The parallel plate capacitor described in this figure is just an example. When a voltage source is connected to the capacitor, the source deposits a positive charge in one of the plates and a negative charge in the other. In this case the charged capacitor is said to store the electric charge. The amount of charge stored is [] C qC V C F V == = C is a constant of proportionality between charge and voltage and is called the capacitance. The value of this constant depends on the physical characteristics of the Figure 1: Left scheme of a parallel plates capacitor. It is composed by two flat electrodes with a dielectric material between them. Right : photos of a fixed value and variable capacitors

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capacitor (size and materials used in the fabrication of the element). The units of the capacitance is the Farad (F) equals Coulomb/Volt For the specific case of a parallel plates capacitor, the capacitance is A C d ε = Where A is the area of the electrode, d is the distance between the two electrodes and ε is the permittivity (a constant factor that characterize the material) of the dielectric between the parallel plates.
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Lecture_7 - ECE103 Spring 2008 Lecture 7 Capacitors and...

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