tion states. Selenium was discovered
in 1817 by Jöns Berzelius.
A
• Information from the WebElements site
selenium cell Either of two types
of photoelectric cell; one type relies
on the photoconductive effect, the
other on the photovoltaic effect (see
photoelectric effect). In the photoconductive selenium cell an external
e.m.f. must be applied; as the selenium changes its resistance on exposure to light, the current produced is
a measure of the light energy falling
on the selenium. In the photovoltaic
selenium cell, the e.m.f. is generated
within the cell. In this type of cell, a
thin Ülm of vitreous or metallic selenium is applied to a metal surface, a
transparent Ülm of another metal,
usually gold or platinum, being
placed over the selenium. Both types
of cell are used as light meters in
photography.
self-assembly See self-organization.
self-consistent Üeld (SCF) A concept used to Ünd approximate solutions to the many-body problem in
*quantum mechanics. The procedure
starts with an approximate solution
for a particle moving in a singleparticle potential, which derives
from its average interaction with all
the other particles. This average interaction is determined by the *wave
functions of all the other particles.
The equation describing this average
interaction is solved and the improved solution obtained is used in
the calculation of the interaction
term. This procedure is repeated for
wave functions until the wave functions and associated energies are
not signiÜcantly changed in the
cycle, self-consistency having been
attained. In atomic theory the
*Hartree–Fock procedure makes use
of self-consistent Üelds.
self-organization The spontaneous order arising in a system when
certain parameters of the system
reach critical values. Self-organization occurs in many systems in
physics, chemistry, and biology. It
can occur when a system is driven
far from thermal *equilibrium. Since
a self-organizing system is open to its
environment, the second law of
*thermodynamics is not violated by
the formation of an ordered phase, as
entropy can be transferred to the environment. Self-organization is related to the concepts of broken
symmetry, complexity, nonlinearity,
and *nonequilibrium statistical mechanics. Many systems that undergo
transitions to self-organization can
also undergo transitions to *chaos. In
chemistry, self-assembly is one of the
features of *supramolecular chemistry.
Seliwanoff’s test A biochemical
test to identify the presence of ketonic sugars, such as fructose, in solution. It was devised by the Russian
chemist F. F. Seliwanoff. A few drops
of the reagent, consisting of resorcinol crystals dissolved in equal
amounts of water and hydrochloric
acid, are heated with the test solution and the formation of a red precipitate indicates a positive result.
semicarbazones Organic compounds containing the unsaturated
group =C:N.NH.CO.NH 2 . They are
formed when aldehydes or ketones react with a semicarbazide
(H 2 N.NH.CO.NH 2 ). Semicarbazones
are crystalline compounds with relatively high melting points. They are
used to identify aldehydes and ketones in quantitative analysis: the
semicarbazone derivative is made
and identiÜed by its melting point.
Semicarbazones are also used in sep477
semicarbazones
s
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in 1817 by Jöns Berzelius.
A
• Information from the WebElements site
selenium cell Either of two types
of photoelectric cell; one type relies
on the photoconductive effect, the
other on the photovoltaic effect (see
photoelectric effect). In the photoconductive selenium cell an external
e.m.f. must be applied; as the selenium changes its resistance on exposure to light, the current produced is
a measure of the light energy falling
on the selenium. In the photovoltaic
selenium cell, the e.m.f. is generated
within the cell. In this type of cell, a
thin Ülm of vitreous or metallic selenium is applied to a metal surface, a
transparent Ülm of another metal,
usually gold or platinum, being
placed over the selenium. Both types
of cell are used as light meters in
photography.
self-assembly See self-organization.
self-consistent Üeld (SCF) A concept used to Ünd approximate solutions to the many-body problem in
*quantum mechanics. The procedure
starts with an approximate solution
for a particle moving in a singleparticle potential, which derives
from its average interaction with all
the other particles. This average interaction is determined by the *wave
functions of all the other particles.
The equation describing this average
interaction is solved and the improved solution obtained is used in
the calculation of the interaction
term. This procedure is repeated for
wave functions until the wave functions and associated energies are
not signiÜcantly changed in the
cycle, self-consistency having been
attained. In atomic theory the
*Hartree–Fock procedure makes use
of self-consistent Üelds.
self-organization The spontaneous order arising in a system when
certain parameters of the system
reach critical values. Self-organization occurs in many systems in
physics, chemistry, and biology. It
can occur when a system is driven
far from thermal *equilibrium. Since
a self-organizing system is open to its
environment, the second law of
*thermodynamics is not violated by
the formation of an ordered phase, as
entropy can be transferred to the environment. Self-organization is related to the concepts of broken
symmetry, complexity, nonlinearity,
and *nonequilibrium statistical mechanics. Many systems that undergo
transitions to self-organization can
also undergo transitions to *chaos. In
chemistry, self-assembly is one of the
features of *supramolecular chemistry.
Seliwanoff’s test A biochemical
test to identify the presence of ketonic sugars, such as fructose, in solution. It was devised by the Russian
chemist F. F. Seliwanoff. A few drops
of the reagent, consisting of resorcinol crystals dissolved in equal
amounts of water and hydrochloric
acid, are heated with the test solution and the formation of a red precipitate indicates a positive result.
semicarbazones Organic compounds containing the unsaturated
group =C:N.NH.CO.NH 2 . They are
formed when aldehydes or ketones react with a semicarbazide
(H 2 N.NH.CO.NH 2 ). Semicarbazones
are crystalline compounds with relatively high melting points. They are
used to identify aldehydes and ketones in quantitative analysis: the
semicarbazone derivative is made
and identiÜed by its melting point.
Semicarbazones are also used in sep477
semicarbazones
s
www.AzShimi.ir www.AzShimi.com