PHYSICS DICTIONARY

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Avalanche Multiplication

Phenomenon observed in gas filled detectors as per which when high electric field is applied to anode wire, there occurs rapid multiplication of charges created in primary ionization event near the wire where electric field intensity is high. This process is called avalanche multiplication.

Average Binding Energy

It is obtained by dividing total binding energy of a nucleus by the number of nucleons in nucleus. This energy is a measure of strength of nuclear bonding. The more tightly bound the nucleons; the more stable the nucleus becomes and larger is its binding energy.

Avogadro Number

The number of atoms or molecules in a mole of a substance is called Avogadro’s number. 

Avogadro’s Law

Equal volumes of ideal gases existing under same conditions of temperature and pressure contain equal number of molecules.

Avogadro’s Principle

Whenever a body is fully or partially submerged in a fluid, a buoyant force acts on the body from the surrounding fluid. The force is directed upward and has a magnitude equal to weight of fluid that has been displaced by body.

Axial Chromatic Aberration

For an object at infinity, the difference in focal length of a lens for the red and violet light is called the axial chromatic aberration.

Axis of Precession

The axis about which the direction of rotation of body precesses is called as axis of precession.

Axis of Rotation

When a rigid body undergoes a combined translation and rotational motion, there is at least a point inside or outside the body whose velocity is zero. The body is said to have pure rotation about that point. Then axis passing through that point and perpendicular to plane of rotation is known as axis of rotation.

Axis

Vertical and horizontal lines that make up quadrants of coordinate plane are referred to as axis. 

Azimuthal Quantum Number

Azimuthal quantum number is a quantum number, an integer assigned to an atomic orbital. It is also called as “orbital angular momentum quantum number”. It helps in the determination of the orbital angular momentum, which further helps in elucidating the shape of the electron’s orbital. Azimuthal quantum number is the second set of the quantum numbers which determines the quantum state of an electron and assumes integral values from 0 to 1.

PHYSICS DICTIONARY

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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”.

Aurora

It is a luminous atmospheric phenomenon appearing as streamers or bands of light sometimes visible in the night sky in northern or southern regions of the earth. It is thought to be caused by charged particles from the sun entering the earth’s magnetic field and stimulating molecules in atmosphere.