The temperature at which matter reaches a lowest energy state and all thermal activity theoretically ceases. Absolute zero is popularly conceived as the coldest possible temperature that a given system of matter, such as an atom, may reach. According to classical physics, all particle motion stops at absolute zero. However, according to the uncertainty principle in quantum mechanics, which states that the velocity and position of a particle cannot be known with total precision simultaneously, a small amount of residual particle motion—known as zero-point motion—would continue even at absolute zero. See also: Classical mechanics; Kinetic theory of matter; Physics; Quantum mechanics; Temperature; Uncertainty principle

A specific system or procedure for determining accurate manufacturing costs in order to achieve profitability goals. Traditional accounting systems allocate overhead costs by using a volume-oriented base, such as direct labor hours or direct material dollars. The cost allocation bases of direct labor or production quantity were designed primarily for inventory valuation. As a consequence, traditional cost accounting methods are fully effective only when direct labor (or direct materials) is the dominant cause of cost.

The activity of a substance is a thermodynamic property that is related to the chemical potential of that substance. Activities are closely related to measures of concentration, such as partial pressures and mole fractions, and are more convenient to use than chemical potentials. The conditions that hold in chemical reaction equilibrium and in phase equilibrium can be expressed in terms of activities of the species involved.

A combination of two sciences, fluid mechanics and thermodynamics. Theodore von Kármán defined aerothermodynamics in this way in the 1950s, following G. Arturo Crocco, who introduced the term in 1931. Taken literally, this term includes classical compressible flow. (There is no problem in compressible flow that can be solved without invoking thermodynamics). In more recent times, aerothermodynamics has been associated with high-temperature, chemically reacting flows; however, such flows are simply part of the overall subject of aerothermodynamics. Aerothermodynamics is a broad and important discipline that cuts across all aspects of high-speed flight, combustion processes, high-speed flow in ducts, and any other application that requires the combination of aerodynamics with thermodynamics. See also: Aerodynamics; Compressible flow; Fluid mechanics; Thermodynamic principles

The uniform and unique direction associated with the apparent inevitable flow of time into the future. There appears to be a fundamental asymmetry in the universe. Yet herein lies a paradox, for all the laws of physics, whether they are the equations of classical mechanics, classical electromagnetism, general relativity, or quantum mechanics, are time-reversible in the sense that they admit solutions in either direction of time. This reversibility raises the question of how these fundamentally time-symmetrical equations can result in the perceived asymmetry of temporally ordered events.

An ideal energy radiator, which at any specified temperature emits in each part of the electromagnetic spectrum the maximum energy obtainable per unit time from any radiator due to its temperature alone. A blackbody also absorbs all the energy which falls upon it. The radiation properties of real radiators are limited by two extreme cases—a radiator which reflects all incident radiation, and a radiator which absorbs all incident radiation. Neither case is completely realized in nature. Carbon and soot are examples of radiators which, for practical purposes, absorb all radiation. Both appear black to the eye at room temperature, hence the name blackbody. Often a blackbody is also referred to as a total absorber. Such a total absorber constitutes a standard for the radiation of nonblackbodies, since Kirchhoff's law demands that the blackbody radiate the maximum amount of energy possible in any wavelength interval. For an extended discussion of blackbody radiation and Kirchhoff's law See also: Graybody; Heat radiation

A process in which a liquid phase is converted into a vapor phase. The energy for phase change is generally supplied by the surface on which boiling occurs. Boiling differs from evaporation at predetermined vapor/gas–liquid interfaces because it also involves creation of these interfaces at discrete sites on the heated surface. Boiling is an extremely efficient process for heat removal and is utilized in various energy-conversion and heat-exchange systems and in the cooling of high-energy density components. See also: Boiler; Evaporation; Heat exchanger; Heat transfer

A law of gases which states that at constant temperature the volume of a gas varies inversely with its pressure. This law, formulated by Robert Boyle (1627–1691), can also be stated thus: The product of the volume of a gas times the pressure exerted on it is a constant at a fixed temperature; that is; PV = k, where P is the pressure, V the volume, and k is a constant.. The relation is approximately true for most gases, but is not followed at high pressures. The phenomenon was discovered independently by Edme Mariotte about 1650 and is also known as the Boyle–Mariotte law, or Mariotte's law. See also: Gas; Kinetic theory of matter

A thermodynamic cycle (also variously called the Joule or complete expansion diesel cycle) consisting of two constant-pressure (isobaric) processes interspersed with two reversible adiabatic (isentropic) processes (Fig. 1). The ideal cycle performance, predicated on the use of perfect gases, is given by relationships (1) and (2). Thermal

(1)

(2)

The measurement of the quantity of heat transferred during chemical reactions, changes of state, mixing of substances, as well as other chemical and physical processes. Devices known as calorimeters (Fig. 1) are used to conduct calorimetry. Even though the unit of energy in the International System of Units is the joule, symbolized J, another unit still in use is the calorie, symbolized cal and defined as 4.184 J, and which accordingly serves as the name basis for calorimetry. Most calorimetric measurements are made at constant pressure and the measured change is called the enthalpy change. See also: Energy; Enthalpy; Heat transfer; Pressure; Thermochemistry; Units of measurement