Dielectric constant

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Dielectric constants of some materials at room temperature
Material	Dielectric constant
Vacuum	1 (by definition)
Air	1.00054
Polyethylene	2.25
Paper	3.5
PTFE (Teflon(TM))	2.1
Polystyrene	2.4-2.7
Pyrex glass	4.7
Rubber	7
Silicon	11.68
Methanol	30
Concrete	4.5
Water (20°C)	80.10
Barium titanate	1200

The relative dielectric constant of a material under given conditions is a measure of the extent to which it concentrates electrostatic lines of flux. It is the ratio of the amount of stored electrical energy when a potential is applied, relative to the permittivity of a vacuum. It is also called relative permittivity.

The dielectric constant is represented as ε_r or sometimes $κ$ or K or Dk. It is defined as

$\varepsilon_{r} = \frac{\varepsilon_{s}}{\varepsilon_{0}}$

where ε_s is the static permittivity of the material, and ε₀ is vacuum permittivity. Vacuum permittivity is derived from Maxwell's equations by relating the electric field intensity E to the electric flux density D. In vacuum (free space), the permittivity ε is just ε₀, so the dielectric constant is 1.

The relative permittivity of a medium is related to its electric susceptibility, $χ e$ by

$\varepsilon_r = 1 + \chi_e.$

1 Measurement
2 Practical relevance
3 Chemical applications
4 See also
5 References
6 External links

[edit] Measurement

The relative dielectric constant ε_r can be measured for static electric fields as follows: first the capacitance of a test capacitor C₀ is measured with vacuum between its plates. Then, using the same capacitor and distance between its plates the capacitance C_x with a dielectric between the plates is measured. The relative dielectric constant can be then calculated as:

$\varepsilon_{r} = \frac{C_{x}} {C_{0}}$

For time-varying electromagnetic fields, the dielectric constant of materials becomes frequency dependent and in general is called permittivity.

[edit] Practical relevance

The dielectric constant is an essential piece of information when designing capacitors, and in other circumstances where a material might be expected to introduce capacitance into a circuit. If a material with a high dielectric constant is placed in an electric field, the magnitude of that field will be measurably reduced within the volume of the dielectric. This fact is commonly used to increase the capacitance of a particular capacitor design. The layers beneath etched conductors in Printed Circuit Boards (PCBs) also act as dielectrics.

Dielectrics are used in RF transmission lines. In a coaxial cable, polyethylene can be used between the center conductor and outside shield. It can also be placed inside waveguides to form filters.

Optical fibers are examples of dielectric waveguides. They consist of dielectric materials that are purposely doped with impurities so as to control the precise value of ε_r within the cross-section. This controls the refractive index of the material and therefore also the optical modes of transmission. Doped fiber can also be configured to form an optical amplifier.

[edit] Chemical applications

The dielectric constant of a solvent is a relative measure of its polarity. For example, water (very polar) has a dielectric constant of 80.10 at 20°C while n-hexane (very non-polar) has a dielectric constant of 1.89 at 20°C.¹ This information is of great value when designing separation, sample preparation and chromatography techniques in analytical chemistry.