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Capacitance

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A capacitor can be formed by placing two metal plates close together separated by a small air gap. This arrangement concentrates the electric field in the space between the plates.

Capacitance is the ability of a capacitor to store charge. Capacitance is directly proportional to the area of the plates of the capacitor and inversely proportional to the distance between them. The unit of capacitance is the farad. We shall define this a little later on.

Capacitance also depends on the dielectric material between the plates. In some cases this is air but, more often, it is polyester, polycarbonate, mica or ceramic (all of which are insulators). Such materials are able to support the existence of an electric field and we refer to this as permittivity. Some materials, such as ceramics, support a more intense electric field than other materials. These materials have a relatively high value of permittivity. Air itself supports the existence of an electric field. We often specify the permittivity of a material relative to air.

The absolute permittivity of air (or vacuum), e₀ = 8.854 x 10^-12 F/m whilst the relative permittivity of air (or vacuum), e_r = 1. The relative permittivity of ceramic, e_r = 500 (i.e. it performs 500 times better than air in terms of supporting an electric field).

For any given material, the permittivity of the material is given by the product of the absolute permittivity of air (or vacuum), e₀, and the relative permittivity of the material, e_r. Hence, the resulting permittivity of the material, k = e₀ × e_r

It's now worth summarizing what we know about capacitance. The capacitance, C, of a capacitor is:

• Directly proportional to the permittivity, k, of the dielectric material between the plates,
• Directly proportional to the area, A, of the plates,
• Inversely proportional to the distance, d, between the plates.

Hence we arrive at the relationship:

C = k A/d

Where C is the capacitance in farads, k is the permittivity (in farad per metre), A is the area of the plates (in square metres), and d is the distance between the plates (in metres).

Start Increase Area Decrease Distance Change Dialectic

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