6.1.1: Introduction

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This chapter introduces another passive device, the capacitor. Capacitors are fundamentally different from resistors in terms of both their construction and their operation. For starters, when placed in DC circuits, capacitors are not ohmic, unlike resistors. Their current-voltage characteristic does not respond to Ohm's law. Instead, their current-voltage characteristic is dynamic in nature. Further, in the ideal case, capacitors do not dissipate power. Indeed, capacitors are energy storage devices. In a way, you could imagine them to be a little like rechargeable batteries, but unlike batteries these devices could be charged or discharged in tiny fractions of a second and last for billions upon billions of cycles.

Capacitors are an integral part of modern electronic systems. They are used in AC-to-DC power supplies to help smooth and stabilize the output voltage. In audio and communications systems they are used in filters, for example to control the high and low frequency response of amplifiers and similar equipment, or for tuning purposes. Essentially, in any application that needs to smooth out a varying voltage, store electric charge or filter a signal; a capacitor is likely to be used.

Much of the original work on capacitors was done in the 18th century. One of the earliest examples of a capacitor was the Leyden jar, which consists of a glass bottle or jar lined with metal foil on the inside and the outside. Benjamin Franklin's famous kite experiment made use of a Leyden jar. In the early days, capacitors were known as condensers (not to be confused with the condensers used in air conditioning systems).