Atmospheric Electricity
The
conductivity of atmosphere arises from ions created by cosmic rays. The
conductivity is variable; as we go to higher altitudes, conductivity gradually
increases and at about 50km, atmosphere is pretty good conductor. This
electrical phenomenon takes place between two conducting spherical surfaces,
earth and top of stratosphere. Each of these surfaces is equipotential.
Atmospheric Pressure
The
atmospheric air exerts normal pressure upon all surfaces with which it is in
contact, and is known as atmospheric pressure.
Atom Bomb
Nuclear
weapon which harnesses the forces that hold the nucleus of an atom together, by
using the energy released when the nucleus splits into nucleons. The atom bomb
works on principle of nuclear fission. Nuclear reactions occur when neutrons
are fired at closely packed atoms with heavy fissile nuclei like uranium-235 / Plutonium
isotopes. This leads to nuclear fission which further converts to chain
reaction.
Atom Percent (at %)
It
is defined as concentration specification on basis of number of moles (or
atoms) of a particular element relative to the total number of moles (or atoms)
of all elements within an alloy.
Atom
It
is tiny basic unit of matter which is neutral and from which any substance gets
constructed. It has positively charged nucleus constituting nucleons (Protons
& Neutrons) surrounded by electrons revolving round the nucleus.
Atomic Clock
It is
accurate type of time device in the world, which is controlled by vibrations of
an atomic or molecular system such as Cesium or Ammonia. The precision of an
atomic clock depends on fact that an atom, caused to oscillate, will always
vibrate at same frequency. NIST-F1, the United States standard atomic clock, is
said to be so accurate that it would neither gain nor lose a second in over 30
yrs. They are used as primary standard in global navigation satellite system
such as GPS.
Atomic Mass Unit (amu)
It is
defined as 1/12 of mass of Carbon-12 atom. It is average of Proton & Neutron
rest mass i.e. 1.67377 x 10-24 gm. The unit is used to express
relative masses of various isotopes of element.
Atomic Number
The
number of protons found inside the nucleus of a particular atom. The modern
periodic table is structured based on this number.
Atomic Orbital
A
quantum state attributed to electron around atom in quantum mechanics to
describe wave characteristic of electron.
It is expressed by mathematical function which is used to calculate
probability of finding any electron in any specific region around atom’s
nucleus.
Atomic Packing Fraction
It
is the ratio of volume of atoms per unit cell to volume of unit cell in a
crystal structure.
Atomic Physics
It
is study of structure of atom, arrangement of electrons around nucleus i.e.
energy states, interaction of atoms with other particles and with electric and
magnetic fields.
Atomic Radius
It
is defined as half the distance between neighboring atoms in a crystal of a pure
element.
Atomic Structure
It
is study of configuration of atom which is essential for knowing many
phenomenon involving electrons. Bohr’s atomic model is early model which has
explained structure of atom to a great extent. Later the limitations of that model
overcome by attributing dual nature to particle. The atomic structure
represents the manner in which electron states are occupied.
Atomic Weight
It
is the weighted average of the atomic masses of an atom’s naturally occurring
isotopes. It is usually expressed in amu.
Atomicity
Total
no. of atoms present in one molecule of an element of a substance is called as
atomicity.
Atto
Prefix
in metric system denoting a factor of 10-18.
Audio Frequency
It
is the frequency range of periodic vibrations audible to human ear. The range
is 20Hz to 20 KHz.
Audio Meter
It
is an instrument for measuring hearing activity for pure tones of normally
audible frequencies.
Aufbau’s Principle
Principle
which states that sub-shell of lower energy in an atom is filled up first while
that of higher energy is filled up later.
Auger Effect
See
Auger electron.
Auger Electron
In
photoelectric effect, following expulsion of photoelectron, another electron
from high energy state may occupy its location emitting X-ray photon. This
photon may in turn eject electron from loosely bound outer electron shell, called
auger electron and such photoelectric effect is called “Auger Effect”.