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# 9.1: Capacitance

Capacitors are important components of electrical circuits in many electronic devices, including pacemakers, cell phones, and computers. In this chapter, we study their properties, and, over the next few chapters, we examine their function in combination with other circuit elements. By themselves, capacitors are often used to store electrical energy and release it when needed; with other circuit components, capacitors often act as part of a filter that allows some electrical signals to pass while blocking others. You can see why capacitors are considered one of the fundamental components of electrical circuits.

• 9.1.1: Prelude to Capacitance
Lichtenberg figures, named for the German physicist Georg Christof Lichtenberg (1742–1799), are tree-like branch patterns with the “branches” are created by the dielectric breakdown produced by a strong electric field.
• 9.1.2: Capacitors and Capacitance
A capacitor is a device used to store electrical charge and electrical energy. It consists of at least two electrical conductors separated by a distance. (Note that such electrical conductors are sometimes referred to as “electrodes,” but more correctly, they are “capacitor plates.”) The space between capacitors may simply be a vacuum, and, in that case, a capacitor is then known as a “vacuum capacitor.” However, the space is usually filled with an insulating material known as a dielectric.
• 9.1.3: Capacitors in Series and in Parallel
Several capacitors can be connected together to be used in a variety of applications. Multiple connections of capacitors behave as a single equivalent capacitor. The total capacitance of this equivalent single capacitor depends both on the individual capacitors and how they are connected. Capacitors can be arranged in two simple and common types of connections, known as series and parallel, for which we can easily calculate the total capacitance.
• 9.1.4: Energy Stored in a Capacitor
The energy delivered by the defibrillator is stored in a capacitor and can be adjusted to fit the situation. SI units of joules are often employed. Less dramatic is the use of capacitors in microelectronics to supply energy when batteries are charged (Figure). Capacitors are also used to supply energy for flash lamps on cameras.
• 9.1.5: Capacitor with a Dielectric
The capacitance of an empty capacitor is increased by a factor of κ when the space between its plates is completely filled by a dielectric with dielectric constant κ Each dielectric material has its specific dielectric constant. The energy stored in an empty isolated capacitor is decreased by a factor of κκ when the space between its plates is completely filled with a dielectric with dielectric constant κ
• 9.1.6: Molecular Model of a Dielectric
All molecules can be classified as either polar or nonpolar. There is a net separation of positive and negative charges in an isolated polar molecule, whereas there is no charge separation in an isolated nonpolar molecule. In other words, polar molecules have permanent electric-dipole moments and nonpolar molecules do not.  Nonpolar molecules can become polar in the presence of an external electrical field, which is called induced polarization.