Capacitor

A capacitor is a passive electronic component designed to store electrical energy.

It works by accumulating electric charges on two closely spaced surfaces that are insulated from one another. This ability to hold a charge is known as capacitance. While any two conductors near each other in a circuit possess some capacitance, a capacitor is a component built specifically to add a controlled amount of capacitance to a circuit.

The utility and function of the device depend entirely on its specific capacitance value. Historically, these devices were often called condensers. This term is still used in certain contexts, such as in the name of a condenser microphone.

To understand the mechanism, imagine two electrical conductors, such as metallic plates, separated by a non-conducting material. This material is called a dielectric.

Common dielectric materials include glass, ceramic, plastic film, paper, mica, air, or even oxide layers. When a voltage, or electric potential difference, is applied across the capacitor's terminals, an electric field develops across the dielectric. This field causes a net positive charge to collect on one plate and a net negative charge to collect on the other.

Crucially, no current actually flows through a perfect dielectric. Instead, there is a flow of charge through the source circuit. If the voltage is applied long enough, the current in the source circuit will eventually cease once the capacitor is charged.

Capacitors can be understood through a hydraulic analogy to visualize how they handle energy.

In this comparison, voltage is like water pressure, and electrical current is like water flowing through a pipe. A capacitor acts like an elastic diaphragm placed inside that pipe. While water cannot pass through the diaphragm, the pressure causes it to stretch or un-stretch. In an alternating current (AC) circuit, the diaphragm allows charge to move back and forth on both sides, even though no electrons cross the barrier. In a direct current (DC) circuit, the capacitor acts like a hydraulic accumulator, storing energy until the pressure is released.

There are many different types of capacitors used in modern technology.

Early versions used flat glass plates alternating with foil conductors. As the need for radio technology grew around 1900, engineers required more compact designs. They began using flexible dielectric sheets, such as oiled paper, sandwiched between metal foils and rolled into small packages. Other types include ceramic capacitors, which use porcelain, and mica capacitors, which were invented by William Dubilier in 1909. Electrolytic capacitors were also developed; Charles Pollak patented an aluminum version in 1896. Today, we also use supercapacitors, which were invented in 1957 using porous carbon electrodes to achieve extremely high capacity.

The history of the capacitor began in the 1740s with the discovery of the Leyden jar.

In 1745, Ewald Georg von Kleist found that charge could be stored in a glass jar filled with water. A year later, Pieter van Musschenbroek invented a similar device named after the University of Leiden. Benjamin Franklin later investigated these jars and reached a vital conclusion. He discovered that the charge was actually stored on the glass rather than in the water. Franklin also introduced the term "battery" to describe a row of these units used together to increase power.

Capacitors serve many essential roles in electrical systems. In analog filter networks, they are used to smooth the output of power supplies. In resonant circuits, they allow radios to tune into specific frequencies. They are also vital in electric power transmission systems to stabilize voltage and power flow.

In the world of computing, capacitors have been used for dynamic memory. Modern DRAM utilizes the property of energy storage in capacitors to function. Specifically, in 1966, Dr. Robert Dennard invented the modern DRAM architecture, which combines a single MOS transistor with a capacitor for each memory cell.

Nature also demonstrates the principles of capacitance on a massive scale. The most common natural example is the accumulation of static charges between clouds in the sky and the surface of the Earth. In this massive natural system, the air between the clouds and the ground serves as the dielectric. When the electrical pressure reaches a point where the breakdown voltage of the air is exceeded, the stored energy is released. This sudden discharge results in the bolts of lightning we see during storms.