Editing Relative permittivity (section)

==Applications==

=== Energy ===
The relative permittivity is an essential piece of information when designing [[capacitor]]s, and in other circumstances where a material might be expected to introduce [[capacitance]] into a circuit. If a material with a high relative permittivity 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 ([[printed circuit board|PCBs]]) also act as dielectrics.

=== Communication ===
Dielectrics are used in [[radio frequency]] (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 [[dielectric resonator filter|filters]]. [[Optical fibers]] are examples of ''dielectric [[waveguide]]s''. They consist of dielectric materials that are purposely doped with impurities so as to control the precise value of ''ε''<sub>r</sub> 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.

=== Environment ===
The relative permittivity of air changes with temperature, humidity, and barometric pressure.<ref>5×10<sup>−6</sup>/°C, 1.4×10<sup>−6</sup>/%RH and 100×10<sup>−6</sup>/atm respectively. See [http://repository.tudelft.nl/islandora/object/uuid:e2234250-950d-4eb5-9f2e-b5b8e67af2e5/datastream/OBJ/download A Low Cost Integrated Interface for Capacitive Sensors], Ali Heidary, 2010, Thesis, p. 12. {{ISBN|9789461130136}}.</ref> Sensors can be constructed to detect changes in capacitance caused by changes in the relative permittivity. Most of this change is due to effects of temperature and humidity as the barometric pressure is fairly stable. Using the capacitance change, along with the measured temperature, the relative humidity can be obtained using engineering formulas.

=== Chemistry ===
The relative static permittivity of a solvent is a relative measure of its [[chemical polarity]]. For example, [[water (molecule)|water]] is very polar, and has a relative static permittivity of 80.10 at 20&nbsp;°C while ''n''-[[hexane]] is non-polar, and has a relative static permittivity of 1.89 at 20&nbsp;°C.<ref>{{RubberBible86th}}</ref> This information is important when designing separation, [[sample preparation]] and [[chromatography]] techniques in [[analytical chemistry]].

The correlation should, however, be treated with caution. For instance, [[dichloromethane]] has a value of ''ε''<sub>r</sub> of [[dichloromethane (data page)|9.08]] (20&nbsp;°C) and is rather poorly soluble in water (13{{nbsp}}g/L or 9.8{{nbsp}}mL/L at 20&nbsp;°C); at the same time, [[tetrahydrofuran]] has its ''ε''<sub>r</sub> = [[tetrahydrofuran (data page)|7.52]] at 22&nbsp;°C, but it is completely miscible with water.  In the case of tetrahydrofuran, the oxygen atom can act as a [[hydrogen bond]] acceptor; whereas dichloromethane cannot form hydrogen bonds with water.<!-- The commonly known "like-dissolves-like" principle could be useful here, as the probable reason for the discrepancy is the specific interaction between the oxygen atoms, as the THF could be treated as a homologue of water. -->

This is even more remarkable when comparing the ''ε''<sub>r</sub> values of [[acetic acid]] (6.2528)<ref name="gaussian">AE. Frisch, M. J. Frish, F. R. Clemente, G. W. Trucks. Gaussian 09 User's Reference. Gaussian, Inc.: Walligford, CT, 2009.- p. 257.</ref> and that of [[iodoethane]] (7.6177).<ref name="gaussian" /> The large numerical value of ''ε''<sub>r</sub> is not surprising in the second case, as the [[iodine]] atom is easily polarizable; nevertheless, this does not imply that it is polar, too (electronic [[polarizability]] prevails over the orientational one in this case).