Capacitor (Glossary)

Just like Batteries, capacitors too are components that can store electrical energy. There are similarities and differences between the two as well as fringe regions with flowing transitions.

The primary form of the capacitor was created in 1745 with the Leyden jar. This is in principle two flat current conductors (e.g. metal plates or films) that are separated by an insulator (the so-called dielectric, e.g. air, glass, mica, paper or oil). Electrons collect at a current conductor when a voltage is applied to this and an electric field builds up so that electrical energy is stored. The stored charge (Q) hereby depends on the voltage that is applied (U) and the capacity of the capacitor (C):

The capacity is expressed by the unit farad (F). this rises with the surface area of the conductive plates (A) and drops with their distance apart (d). It is also determined by the properties of the insulator (relative permittivity epsilon):

Capacitors are found in practically all electronic and electrotechnical fields, some in locomotives weigh many tons and are metres high, others are absolutely minuscule and can be found in computer processors. They are made, for example, from a tightly wound sandwich of metal and insulating paper films that are cast in a housing.

Comparison between a battery and capacitor

Whereas a capacitor only saves energy physically (electrostatically) and for only a short period of time on account of its high spontaneous discharge, the energy is chemically or electrochemically bound in a battery and available for a medium to longer period of time.

In a battery the polarity is usually fixed on account of the design, a capacitor on the other hand can periodically change polarity extremely quickly in an alternating current system.

The specific energy of capacitors is far below that of batteries, but they do have high power densities on account of their short charging and discharge times. However, high power densities up to 100 kW/kg at pulses of 1 ms to 10 s can also be realised with lithium-iron sulphide batteries.

Supercapacitors

In order to increase the specific energy, capacitors (supercapacitors or Super Caps or Supercaps) were developed that combine electrostatic and electrochemical features and thus represent a link between the capacitor and battery world, not just in terms of the functional principle and energy density but also with respect to the temporal availability of the electrical energy, that can be hours or even days with some Super Caps.

© Marc Stenzel

	See also:
		Glossary overview Batteries - Accumulators - Cells Battery-Packs

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