Q

Source edition 1965. Please read the Introduction to find out about this dictionary and our plans for it. Caution, many entries have not been updated since the 1965 edition.
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q = dynamic pressure,

as the vehicle encountered maximum q 40 seconds after lift-off.

Q-band

A frequency band used in radar extending approximately from 36 to 46 kilomegacycles. See frequency band.

quadrant

See sextant.

quadrature

1. An elongation of 90 degrees, usually specified as east or west in accordance with the direction of the body from the sun. The moon is at quadrature at first and last quarters. Compare conjunction.

2. The situation of two periodic quantities differing by a quarter of a cycle.

quality factor (symbol Q)

A measure of the sharpness of resonance or frequency selectivity of a resonant vibratory system having a single degree of freedom, either mechanical or electrical. See vibration.

In a mechanical system, this quantity is very nearly equal to one-half the reciprocal of the damping ratio. When used with reference to a lightly damped system, it is also approximately equal to the following: (1) transmissibility at resonance; (2) π/δ where δ is the logarithmic decrement; (3) 2πW/ΔW where W is the stored energy and ΔW the energy dissipation per cycle; and (4) f_r/Δf where f_r is the resonance frequency and Δf is the bandwidth between the half-power points. Historically the letter Q was an arbitrarily chosen symbol to designate the ratio of reactance to resistance of a circuit element. The name quality factor was introduced later.

quanta

See quantum theory.

quantity

See number.

quantization

The process of converting from continuous values of information to a finite number of discrete values.

quantum theory

The theory first stated by Max Planck (before the Physical Society of Berlin on December 14, 1900) that all electromagnetic radiation is emitted and absorbed in quanta , each of magnitude hv , h being the Planck constant and v the frequency of the radiation. See radiation laws.

quasi-geostrophic equilibrium

See equivalent barotropic model.

quasi-Lagrangian coordinates

A system of mixed Eulerian and Lagrangian coordinates. At least one coordinate of each fluid parcel must therefore by unvarying with time.

quiet sun

The sun when it is free from unusual radio wave or thermal radiation such as that associated with sun spots. See IQSY.

quintant

See sextant.