Physics Dictionary

The time for radiative cooling to occur following the imposition of a small temperature gradient. From Holton (1979), the change in energy flux as a function of height z in an atmosphere is given by

(1)

where is the density, is the heat capacity at constant pressure in J K-1 kg-1, and T is the temperature. Rearranging and integrating gives

(2)

If and are constant with z, then

(3)

where is the column density. Energy balance requires that

(4)

Holton, J. R. An Introduction to Dynamic Meteorology, 2nd ed. New York: Academic Press, 1979.

The dielectric constant is a characteristic quantity of a given dielectric substance, sometimes called the relative permittivity (Bekefi and Barrett 1987, p. 418). In general, the dielectric constant is a complex constant, with the real part giving reflective surface properties (Fresnel reflection coefficients), and the imaginary part giving the radio absorption coefficient .

In cgs,

(1)

where is the electric susceptibility. In MKS,

(2)

where is the electric permittivity and is the permittivity of free space.

The corresponding quantity for magnetism is called the relative permeability .

Electric Permittivity, Electric Susceptibility, Fresnel Reflection Coefficients, Radio Absorption Coefficient, Relative Permeability

Bekefi, G. and Barrett, A. H. Electromagnetic Vibrations, Waves, and Radiation. Cambridge, MA: MIT Press, pp. 418-420, 1987.

A beamsplitter is an optical element that allows part of an electromagnetic wave to pass through while reflecting the other part. A simple dielectric can therefore serve as a beamsplitter.

Fabry-Perot Étalon, Fabry-Perot Interferometer, Fourier Transform Spectrometer, Lloyd’s Mirror, Multiple Reflection Interferometer

Accretion onto a stationary black hole has only been solved analytically under the assumption of spherical symmetry. Shapiro and Teukolsky (1983) give a Newtonian treatment of accretion, and Michel (1972) gives a full general relativistic one.

Blandford-Znajek Process

Abramowicz, M. A.; Bjornsson, G.; and Pringle, J. E. Theory of Black Hole Accretion Disks. Cambridge, England: Cambridge University Press, 1998.

Balbus, S. A. and Hawley, J. F. “Instability, Turbulence, and Enhanced Transport in Accretion Disks.” Rev. Mod. Phys. 70, 1-53, 1998.

Frank, J.; King, A. R.; and Raine, D. J. Accretion Power in Astrophysics, 2nd ed. Cambridge, England: Cambridge University Press, 1992.

Hayashi, C. “Growth and Decay of Magnetic Fields.” Prog. Theor. Phys. Supp. 70, 35-53, 1981.

Michel, F. C. “Accretion of Matter by Condensed Objects.” Astrophys. Space Sci. 15, 153-160, 1972.

Pringle, J. E. “Accretion Disks in Astrophysics.” Ann. Rev. Astron. Astrophys. 19, 137-162, 1981.

Shapiro, S. L. and Teukolsky, S. A. Black Holes, White Dwarfs, and Neutron Stars: The Physics of Compact Objects. New York: Wiley, 1983.

Weisstein, E. W. “Books about Accretion Disks.” http://www.ericweisstein.com/encyclopedias/books/AccretionDisks.html.

Weizsäcker, C. F. von. “The Rotation of Cosmic Gas Masses.” Z. Naturforsch. 3a, 524, 1948.

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