Time taken by earth to complete one
revolution around the sun.
Yield
Point
The maximum stress level at which plastic
deformation begins.
Yielding
The phenomenon of
transformation of elastic deformation to plastic deformation is called as
yielding.
Young’s
Modulus
The ratio of
longitudinal stress to longitudinal strain with in elastic limit is called
young’s modulus.
X-ray
Astronomy
This branch of
physics was born in 1962 when Italian–American astronomer Riccardo Giacconi
discovered a cosmic x-ray source in the form of a compact star located in the
constellation of Scorpius. For this discovery Giacconi received the 2002 Nobel
Prize in Physics. X-ray astronomy is a relatively new branch of astronomy
dealing with the study of x-ray emission from celestial objects, such as
neutron stars, pulsars, and black holes. Since the X-rays emitted by celestial
objects have relatively low energies of the order of a few KeV, they cannot
penetrate through the Earth’s atmosphere to reach the surface of the Earth.
Thus, to study these celestial rays, detectors must be taken above the Earth’s
atmosphere. Methods used to achieve this involve mounting x-ray detectors on
rockets, balloons, or satellites. The x-ray detectors used for this purpose are
either special charge-coupled devices (CCDs) or micro calorimeters.
X
– Ray Crystallography
X- ray crystallography is a study of
crystal structures through the use of X-ray diffraction techniques. X-rays are
very suitable for this purpose because their wavelength in the 0.1 Ao
(~ 100KeV) to 1 Ao (~10 KeV) range is of the order of typical
crystalline lattice separations. An X-ray beam striking crystalline
lattice is scattered by the
spatial distribution of atomic electrons and the imaged diffraction pattern
provides information on the atomic or molecular structure of the crystalline
sample. In 1912, Max Von Laue established the wave nature of X rays and
predicted that crystals exhibit diffraction phenomena. Soon, there after
William H. Bragg and William L. Bragg analyzed the crystalline structure of
sodium chloride, derived the Bragg relationship and laid the
foundation for X- ray crystallography.
X
Ray Spectroscopy
X ray spectroscopy is an analytical technique for
determination of elemental composition of solid or liquid sample in many fields
Such as material science, environmental science, geology, biology and forensic
science. The technique is divided into three related categories. The most
common of them is the X-ray absorption spectrometry (also called as X-ray
fluorescence spectrometry). The other two are X-ray photoelectron spectrometry
and auger spectrometry. All three techniques rely on creation of vacancies in
atomic shells of the various elements in the sample under study as well as on
an analysis of the effects that accompany the creation of vacancies.
X-rays
Electromagnetic
radiation emitted by charged particles (usually electrons) in changing atomic
energy levels.
Wave
Front
The wave front of a wave is a surface at
each point of which the vibrating particles are in same phase.
Wave
Length
Distance travelled by the wave in the time
in which particle of medium completes one vibration.
or
Distance between two nearest particles in the
same phase.
Wave
Motion
It is a form of
disturbance which travels through the medium due to repeated periodic motion of
particles of medium about their mean positions. Wave motion, in general, refers
to transfer of energy from one point to another point of medium. It is not
matter that is propagated but it is only state motion of matter that is propagated.
It is a form of dynamic condition that is propagated from one point to the
other point in the medium.
Wave
Wave is pattern
and doesn’t have any physical existence. Wave motion comprises of patterns
repeating at regular intervals and thus wave travels with uniform velocity.
Weidmann-Franz
Law
The linear dependence of “ratio of thermal
conductivity to electrical conductivity” on absolute temperature is known as
Weidmann-Franz law. The ratio is directly proportional to absolute temperature.
Weight
Percent (wt%)
Weight of a
particular element relative to the total alloy weight.
Weld
Decay
Intergranular
corrosion that occurs in some welded stainless steels at regions adjacent to
the weld.
Welding
Process of permanently joining two similar
metals by localized fusion by applying suitable heat and pressure is called as
welding.
Wet
Bulb Temperature
Wet-bulb
temperature represents how much moisture the air can evaporate. This
temperature is often measured with a common mercury thermometer that has the
bulb covered with a water-moistened wick and with a known air velocity passing
over the wick.
White
Dwarf
They are stars
which are fainter, possess small diameter, and are very dense compared to other
stars of same mass. A white dwarf star is a mass of Helium at an extremely high
temperature and under external compression. At its central temperature, Helium
atoms are expected to be completely ionized and the star may be regarded as gas
composed of Helium nuclei and electrons.
Wiedmann
Franz Law
The
ratio of thermal conductivity(K) to electrical conductivity σ is a constant
(called Lorentz number) at a particular temperature for all the metals.
or
Weidmann–Franz
discovered in 1853 that all good electrical conductors are also good thermal
conductors. Ratio of K to σ is constant at a particular
temperature for all the metals. Later Lorentz showed that this ratio is
proportional to absolute temperature ‘T’.
K/σT= const=L; L is called as Lorentz number.
Wien’s
Displacement Law
The
wavelength of radiation emitted by a black body at any temperature is inversely
proportional to absolute temperature of the body.
or
At
any temperature, the energy of radiations of wavelength emitted by black body
is directly proportional to fifth power of its absolute temperature.
Work
If the applied
force on object leads to some displacement then it is said that work is done on
object.
Work-Energy
Theorem
The work done by the resultant net force
acting on a body is equal to change in its kinetic energy.
Wrought
Alloy
A metal alloy that is relatively ductile
and amenable to hot working or cold working during fabrication.
Vacancy
Diffusion
The diffusion
mechanism where in net atomic migration is from lattice site to an adjacent
vacancy.
Vacancy
A normally occupied lattice site in crystal
from which an atom or ion is missing.
Vacuum
Pressure
If pressure of a
fluid is below atmospheric pressure, it is designated as gauge pressure.
Vacuum
According to
definition of American vacuum society, “vacuum” refers to a given space filled
with gas at pressures below atmospheric i.e. having a density of molecules less
than about 2.5 x 1019 molecules /cm3.
Valence
Band
For solids, the electron energy levels band
that contains valence electrons. These electrons lie in outermost orbits of
atoms at absolute zero temperature.
Valence
Electrons
The electrons occupying outermost orbits
which usually participate in interatomic bonding are called as valence
electrons.
Van
De Graff Generator
It was invented by American physicist Robert J.
VandeGraff in 1929. It is a machine capable of building up potential difference
of few million volts, and fields close to breakdown field of air which is about
3x106 V/m. The potential is build up with help of static change by
friction.
Vanderwaal’s
Gas
See real gas
Vanderwalls
Bond
A secondary
interatomic bond between adjacent molecular dipoles, which may be permanent or
induced is called Vanderwall’s bond. They are formed as a result of dipole
attractions, the dipoles being formed as a result of unbalanced distribution of
electrons in asymmetrical molecules.
Vapor
Gaseous state below the critical
temperature of substance is known as vapor.
Vapor
Degreasing
The process in
which solvent is heated to boiling point, and parts to be cleaned are hung in
the chamber exposing to hot vapors. The vapors get condensed on metal surfaces
during which dissolves oil & grease and flows back to solvent condition.
Variance
The variance
gives us a sense of how closely concentrated a set of values is around its
average value, and is calculated in the following manner: Subtract the mean of
the distribution from each of the values. Square each result. Add all of the
squared results. Divide the result by the number of values minus 1.
Vector
Product
See cross product.
Vector
It is a physical quantity having both
magnitude and direction and which obey laws of vector addition.
Velocity
Addition Theorem
The addition of
any velocity to the velocity of light merely reproduces again the velocity of
light. Thus velocity of light is maximum range of velocity attainable in
nature.
Velocity
Gradient
It is rate of change of velocity of liquid
layers with distance, the distance being taken normal to the direction of flow.
Vibration
It is the to & fro motion of a particle
from one extreme position to the other and back again. It is motion of a
particle from mean position to one extreme position, then to other extreme
position and finally back to mean position.
Vibration-Rotational
Spectrum
Molecular spectra obtained in near infrared
region.
Virtual
Image
A
virtual mage is formed at a position where the paths of principal rays cross
when projected backward from their paths beyond the lens. Although a virtual
image does not form a visible projection a screen, it is no sense “imaginary”
i.e. it has a definite position and size and can be “seen” or imaged by the
eye, camera or other optical instrument.
or
When
a beam of light diverging from a point after reflection (or refraction) appears
to diverge from second point, then the second point is called as virtual image
of first. The virtual image cannot be formed on screen.
Ultra
Thin Films
Films
having thickness below 100 Ao.
Ultrasonic Waves
Sound waves of high frequency greater than 20,000 Hz
are called as ultrasonic waves. These frequencies are beyond audible limit.
Umbra
(Latin for
“shadow”) is the innermost and darkest part of a shadow, where the light source
is completely blocked by the occluding body.
Uniaxial
Crystal
It is a double
refracting crystal which has only one axis along which both the refracted rays
have same speed. Ex: Calcite, Quartz, etc.
Uniform
Circular Motion
The motion of an object with constant speed
in a circle.
Unit
Cell
Basic structural
unit or building block of crystal structure, which defines the crystal structure by virtue of
its geometry and the atom positions within it. A unit cell is chosen to
represent the symmetry of crystal structure, where in all the atom positions in
the crystal may be generated by translation of unit cell integral distances
along each of its edges.
Universal
Gravitational Constant
It is the force between two unit masses
separated by unit distance is called universal gravitational constant. The
value of ‘G’ is 6.67 x 10-11 Nm2Kg-2.
Unsaturated
Vapor
If a liquid is in open container, the vapor
molecules rapidly diffuse away from liquid, and in general produce unsaturated
vapor.