Physics Dictionary (Contd...)

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.

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.

Total no. of atoms present in one molecule of an element of a substance is called as atomicity.

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”.

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