 |
Business | Industries | Finance | Tax |
| Index: > A B C D E F G H I J K L M N O P Q R S T U V W X Y Z |
|
|||||
| First Prev [ 1 2 ] Next Last |
In electromagnetism, permittivity ε is a measure of how much a medium changes to absorb energy when subject to an electric field. It is defined as the ratio D / E where D is the electric displacement by the medium and E is the electric field strength.
In SI units, the displacement D is usually given in units of coulombs per square metre (C/m2), while the electric field E is given as volts per metre (V/m). Permittivity is then specified in farads per metre (F/m).
It can also be defined as a dimensionless relative permittivity, or dielectric constant, normalized to the absolute vacuum permittivity ε0 = 8.85419 10-12F/m.
In the common case of an isotropic medium, D and E are parallel vectors and ε is a scalar, but in more general anisotropic media this is not the case and ε is a rank-2 tensor (causing birefringence).
The permittivity ε and magnetic permeability μ of a medium together determine the velocity v of electromagnetic radiationElectromagnetic radiation is a combination of oscillating electric and magnetic fields in perpendicular orientation to each other, moving through space, effectively transporting energy from one place to another. Visible light is a form of electromagnetic through that medium:
In a vacuumThe article on the vacuum cleaner is located elsewhere. In physics, a vacuum is the absence of matter in a volume of space. A partial vacuum is expressed in units of pressure. The SI unit of pressure is the pascal (abbreviated to Pa in usage). It can also, these are given by
where
ε0 is the permittivity of free space, equal to 8.85419 10-12F/m
μ0 is the magnetic constant, or permeability of free space, equal to 4 πOr pi with a lower-case p . minuscule, or lower-case, pi The mathematical constant (written as pi when the Greek letter is not available) is ubiquitous in mathematics and physics. In Euclidean plane geometry, π may be defined as either the ratio of a c × 10-7 N·A-2
c is the speed of lightCherenkov effect in a "swimming pool" nuclear reactor. The effect is due to electrons moving faster than the speed at which light moves in water. The speed of light (denoted as c reputedly from the Latin celeritas "speed", and also known as Einstein's con in vacuum, 299,792,458 m/sMetre per second ( U. spelling: meter per second is an SI derived unit of both speed ( scalar) and velocity ( vector), defined by distance in metres divided by time in seconds. The symbol is m/s or equivalently, m s-1 . Some examples of speeds in m/s: Con.
When an electric field is applied, a currentIn electricity, current is the rate of flow of charges, usually through a metal wire or some other electrical conductor. Conventional current was defined early in the history of electrical science as a flow of positive charge, although we now know that, i flows. The total current flowing in a real medium is in general made of two parts: a conduction current and a displacement one. The displacement currentDisplacement current is a pseudocurrent invented in 1865 by James Clerk Maxwell when formulating what are today known as Maxwell's equations. It is defined by the flux of the electric field through the surface: : It is incorporated into Ampere's law, whos can be thought of as an elastic response which a material has to the applied electric field. As the electric field is increased, the displacement current is stored in the material, and when the electric field is decreased the material releases the displacement current. A perfect dielectric is a material that shows displacement current only, so it stores and returns electrical energy as if it were an ideal 'battery'.
In case of lossy medium (i.e. when the conduction currents are not negligible) the total current density flowing is:
where
σ is the conductivity (responsible for conduction current) of the medium
εd is the relative permittivity (responsible for displacement current).
The size of the displacement current is seen to be dependant on the frequency ω of the applied field E; there is no displacement current in a constant field.
In this formalism the complex permittivity ε* is defined as:
For realistic materials, both the real and imaginary parts of the permittivity are more complicated functions of frequency ω; since this leads to dispersion of signals containing multiple frequencies, such materials are called dispersive. This frequency dependence reflects the fact that a material's polarization does not respond instantaneously to an applied field—because the response must always be causal (come after the applied field), the dielectric function ε(ω) must have poles only for ω with positive imaginary parts, and ε(ω) therefore satisfies the Kramers-Kronig relations. However, in the narrow frequency ranges that are often studied in practice, the dielectric constants can often be approximated as frequency-independent.
At a given frequency, the imaginary part of ε leads to absorption loss if it is negative (in the above sign convention for frequency) and gain if it is positive. (More generally, one looks at the imaginary parts of the eigenvalues of the anisotropic dielectric tensor.)
| Politics | Business | Finance |
 |
 |
 |
|