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Fundamentals of Electric Capacitors

1.1 Introduction
An electric capacitor has a sandwich structure containing two conductive plates (normally made of metal) surrounding a dielectric or insulator
as shown in Figure 1.1a. Common dielectrics include air, oiled paper, mica,
glass, porcelain, or titanate. An external voltage difference is applied across
the two plates, creating a charging process. During charging, the positive
charges gradually accumulate on one plate (positive electrode) while the
negative charges accumulate on the other plate (negative electrode). When
the external voltage difference is removed, both the positive and negative
charges remain at their corresponding electrodes. In this way, the capacitor
plays a role in separating electrical charges. The voltage difference between
the two electrodes is called the cell voltage of the capacitor. If these electrodes
are connected using a conductive wire with or without a load, a discharging
process occurs—the positive and negative charges will gradually combine
through the wire. In this way, the capacitor plays a role for charge storage
and delivery. Before we start a deeper discussion about capacitors, explanations of their history and some fundamental concepts may be useful.
1.1.1 History
Thales of Miletus, a philosopher, discovered electric charges when he rubbed
amber with a cloth and observed magnetic particle attraction. Since then, the
act of rubbing two non-conducting materials together to induce a charge has
been treated as a demonstration of the triboelectric effect. In 1745, a better
understanding of electrostatics and electrochemistry led to the invention
of a condenser, as shown in Figure 1.1b. In the first condenser, two important factors determined charge separation and charge storage: the dielectric
thickness and the surface area of the conductive materials. With technological advancements in materials and manufacturing, condensers evolved into
modern capacitors that are now used in electrical systems.
Early capacitors were created from a Leyden jar as shown in Figure 1.1b. A
capacitor consisted of a glass vessel whose interior and exterior were coated
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