quadratic equation (mathematics). An algebraic equation that contains both the first and second powers of the unknown quantity, as well as a constant term. An example of a quadratic equation is:
ax2 + bx + c =0
quadrilateral. A plane (flat), closed polygon with four sides. Squares, rectangles, parallelograms, and trapezoids are all forms of quadrilaterals.
quadrivalent element (chemical element). A chemical element with four electrons in its valence shell. Carbon, silicon, germanium, tin, and lead are quadrivalent elements.
quality control. A system of control in a manufacturing company that is responsible for the quality of the product produced by the company. The inspection department is normally a part of the quality control department.
quantum theory. The theory which states that an atom or molecule does not absorb energy in a continuous and even fashion. Energy is absorbed in steps (quanta), with the amount of energy in each step being related to the frequency of the energy.
quart. A unit of volume measurement. One quart is equal to: two pints; one-fourth U.S. gallon; 57.75 cubic
inches; 9.46 x 10-4 cubic meters.
quartersawed wood. Wood sawed from a tree in such a way that the annual rings cross the plank at an angle of greater than 45°.
quarter-wave antenna. A radio antenna whose length is one quarter of the wavelength of the frequency for which the antenna is used.
quartz. A mineral (silicon dioxide) found in nature as a hexagonal (six-sided) crystal. Quartz has piezoelectric characteristics, as it produces a voltage when it is bent or pressed, and it changes its shape when a voltage difference exists across two of its faces.
quartz crystal. A thin slab of quartz, used to control the frequency of an electronic oscillator. A quartz crystal vibrates at its natural resonant frequency when it is excited with a pulse of electrical energy. As the crystal vibrates, it produces an alternating current whose frequency is determined by the physical dimensions of the crystal.
quartz glass. A type of glass made from pure quartz. Quartz glass does not filter out ultraviolet rays.
quartz-iodine lamp. An incandescent lamp with a tungsten filament and a quartz envelope. The envelope is filled with iodine vapor that reacts with the vaporized tungsten to prevent the tungsten vapors from darkening the quartz envelope.
quartz lamp. A mercury-vapor lamp enclosed in a transparent housing made of quartz, rather than glass. Quartz resists heat better than glass and allows ultraviolet rays to pass through, rather than absorbing them as glass does.
quartz oscillator. An electronic oscillator whose frequency is determined by the resonant frequency of a quartz crystal.
quartz pressure transducer. A precision instrument that senses pressure changes by the change in frequency of a quartz oscillator. When the quartz crystal controlling the frequency of the oscillator is acted on by the pressure being measured, the oscillator frequency changes. The amount of frequency change is proportional to the amount of pressure being measured.
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