- Relative static permittivity
The relative static permittivity (or static relative permittivity) of a material under given conditions is a measure of the extent to which it concentrates
electrostaticlines 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. The relative static permittivity is the same as the relative permittivity evaluated for a frequencyof zero.
The relative static permittivity is represented as "εr" or sometimes or "K" or Dk. It is defined as
where "εs" is the static permittivity of the material, and "ε0" is the
electric constant. (The relative permittivity is the complex frequency-dependent , which gives the static relative permittivity for .)
Other terms for the relative static permittivity are the dielectric constant, or relative dielectric constant, or static dielectric constant. These terms, while they remain very common, are ambiguous and have been deprecated by some standards organizations.citation |last=Braslavsky |first=S.E. | url=http://iupac.org/publications/pac/2007/pdf/7903x0293.pdf |title=Glossary of terms used in photochemistry (
IUPACrecommendations 2006) | journal=Pure and Applied Chemistry | volume=79 |year=2007 |pages=p. 293-465; see p. 32] [cite web |author= IEEEStandards Board|url=http://ieeexplore.ieee.org/iel4/5697/15269/00705931.pdf?arnumber=705931|title=IEEE Standard Definitions of Terms for Radio Wave Propagation|year=1997 |pages=p. 6] The reason for the potential ambiguity is twofold. First, some older authors used "dielectric constant" or "absolute dielectric constant" for the absolute permittivity rather than the relative permittivity. [cite book
last = King
first = Ronold W. P.
title = Fundamental Electromagnetic Theory
publisher = Dover
year = 1963
location = New York
pages = p. 139] Second, while in most modern usage "dielectric constant" refers to a
relative permittivity[cite book|last=Jackson|first=John David|title=Classical Electrodynamics, 3rd edition|publisher=Wiley|year=1998|location=New York|pages=p. 154] , it may be either the static or the frequency-dependent relative permittivity depending on context.
By definition, the linear relative
permittivity of vacuum, where , is equal to 1,cite book
author=John David Jackson
url=http://worldcat.org/isbn/047130932X] although there are theoretical nonlinear quantum effects in vacuum that have been predicted at high field strengths (but not yet observed).Mourou, G. A., T. Tajima, and S. V. Bulanov, "Optics in the relativistic regime," "Reviews of Modern Physics" vol. 78 (no. 2), 309-371 (2006).]
The static relative permittivity of a medium is related to its static
electric susceptibility, by
The relative static permittivity "εr" can be measured for static
electric fields as follows: first the capacitanceof a test capacitor "C0" is measured with vacuum between its plates. Then, using the same capacitor and distance between its plates the capacitance "Cx" with a dielectricbetween the plates is measured. The relative dielectric constant can be then calculated as
electromagnetic fields, this quantity becomes frequencydependent and in general is called "relative permittivity".
The dielectric constant is an essential piece of information when designing
capacitors, and in other circumstances where a material might be expected to introduce capacitanceinto 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
RFtransmission lines. In a coaxialcable, polyethylenecan be used between the center conductor and outside shield. It can also be placed inside waveguides to form filters. Optical fibersare examples of "dielectric waveguides". They consist of dielectric materials that are purposely doped with impurities so as to control the precise value of "εr" within the cross-section. This controls the refractive index of the material and therefore also the optical modes of transmission. However, in these cases it is technically the relative permittivity that matters, as they are not operated in the electrostatic limit.
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. [D.R. Lide, Ed. "CRC Handbook of Chemistry and Physics, 85th Ed." (2004). CRC Press. Boca Raton. p.8–141.] This information is of great value when designing separation, sample preparation and chromatographytechniques in analytical chemistry.
Linear response function
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