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Sunday, April 6, 2014

Physics Dictionary (Contd...)

Accuracy
The degree of closeness of measurement value of a quantity to the expected value is called accuracy. In other words, the extent to which a given measurement agrees with the standard value for that measurement. 

Achromatic Combination of Lenses
A lens combination, for which the image of an object illuminated with white light is distinctly obtained at the same place for all colors of light, is said to be an achromatic combination of lenses.

Achromatic Lens
A lens combination, for which the image of an object illuminated with white light is distinctly obtained at same place for all colors of light, is said to be an achromatic combination of lenses.

Achromatic Prism
A Prism composed usually of two prisms which have unequal dispersive powers. They are combined in such a fashion that a ray of light passed through them is refracted or bent into a new position, but it is free from color.

Acoustic Impedance
It is the ratio of acoustic pressure to acoustic volume flow. Acoustic impedance varies strongly when the frequency of sound is changed. The acoustic impedance at a particular frequency indicates how much sound pressure is generated by air molecules vibration at that frequency.

Acoustics
The branch of physics that deals with the process of generation, reception and propagation of sound.

Actinides
Any of the series of 15 metallic elements from Actinium (atomic number=89) to Lawrencium (atomic no. 103) in the periodic table. They are all radioactive. The heavier elements are extremely unstable and not of naturally occurring.

Actinium Series
Set of unstable heavy nuclei which decay radioactively beginning with Actino-Uranium, isotope of Uranium U-235 and end with Actinium D, constituting non radioactive isotope Lead, of mass number 207. 

Actinometer
It is radiometric instrument used for measuring heating capability of radiation. They are used chiefly for meteorological measurement of terrestrial and solar radiation. 

Activator
The name assigned to the impurity atoms occurring in relatively small concentrations in host material or a small stoichometric excess of host material, which leads host material to exhibit phenomenon of Luminescence.

Active Device
It is a device that requires a source of energy for its operation and has an output that is a function of present and past input signals. Active devices are components that are capable of controlling voltages or currents and create switching action in the circuit.


Activity
The number of nuclei of a given radioactive sample disintegrating per second is referred to as activity of radioactive substance.

Acute Angle
Angle with a measure between 0o & 90o.

Adhesion
It is the force of attraction between molecules of different substances. 

Adiabatic Demagnetization
It is the process of cooling certain materials to extremely low temperatures down to milli Kelvin. The process was first proposed by chemists Peter Debye (1926) and William Francis Giauque. The process capitalizes on paramagnetic properties of some materials (usually in gaseous form) to reduce their temperatures to very low range. The process involves reducing temperature of sample (usually gas) by keeping in contact with cold reservoir; with magnetic field induced in it till thermal equilibrium is attained. After thermal equilibrium is attained, magnetic field intensity is increased thus reducing entropy due to orderly alignment of particles. After the isolation of sample from reservoir, the magnetic field is reduced which results in still further reduction of temperature by many fold. 

Admittance
It is the term coined by Oliver Heaviside. It is inverse of impedance. In fact, it is vector sum of Conductance and Susceptance. It determines with how much ease an alternating current flows through a complex circuit.

Adsorbate
The gas or liquid that is accumulated over the surface of a liquid or solid is referred to as adsorbate.

Adsorbent
The solid or liquid on whose surface, molecules of other substance are adsorbed.  

Adsorption

It is the process of attracting foreign atoms or molecules on to the host liquid or solid substance. Any surface of a solid or liquid exhibit residual forces of attraction normal to surface; hence molecules or atoms impinging on surface are adsorbed.