# Electrical capacity

the electrical **capacity** (adjective **capacitively**) in physics/electro-technology represents the ability of a body to store electrical charges. It is determined as relationship of the supplied charge quantity to the developed tension. Thus it is a measure for the charge capacitya body.

The term “capacity” is used straight in electro-technology and electronics frequently also synonymously for the electrical element condenser. With accumulators the designation “capacity” possesses colloquially another meaning. There it becomes directly as name for the charge quantityused.

The electrical capacity is measured in the SI-UNIT Farad.
A Farad (1 F) is the capacity *C* of a condenser, which stores a charge quantity of Q *of* 1 coulomb ( As) *when* putting on a tension U by 1 V:

<math> [C] = \ frac {[Q]} {[U]}< /math>

<math> 1 \, \ mathrm {F} =\ frac {1 \, \ mathrm {As}} {1 \, \ mathrm {V}}< /math>

The Farad is a quite large unit, which most in practice used condensers have nominal values between some Picofarad (pF) and to unite microfarads (μF).

The size of the capacity hangs of the dimensions and the material of the dielectric between thatCondenser plates off. For a plate capacitor (two metal plates of the surface* A* in the distance *D*) one computes in accordance with capacity:

*<math> C = \ frac {\ epsilon_r \ \ epsilon_0 \ A} {D}< /math>*

This formula applies only, if the spatial expansion of the condenser plates is very much largeras their distance, i.e. if the electrical field can be regarded as homogeneous.

The capacity of a free ball with the radius R amounts to:

*<math> C = 4 \ pi \ \ epsilon_r \ \ epsilon_0 \ R </math>*

This formula does not apply however any longer, ifitself articles in particular leaders at earth potential (e.g. the hand of the experimenter) in the proximity of the ball find.

In the formulas ε *0 _{}* the electrical field constant, math

*< \> epsilon_0 = 8 {, is cdot} 854187817 \ 10^ {- 12} \ mathrm {\ frac {F} {m}} </math>*

*ε _{ r}* is one for the isolation materialspecific dimensionless material constant, the Permittivitätszahl, also dielectric constant mentioned (see also: Permittivität).