Definitions

# heat

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heat, nonmechanical energy in transit, associated with differences in temperature between a system and its surroundings or between parts of the same system.

## Measures of Heat

Temperature is a measure of the average translational kinetic energy of the molecules of a system. Heat is commonly expressed in either of two units: the calorie, an older metric unit, and the British thermal unit (Btu), an English unit commonly used in the United States. Scientists express heat in terms of the joule, a unit used for all forms of energy.

Specific Heat

As heat is added to a substance in the solid state, the molecules of the substance gain kinetic energy and the temperature of the substance rises. The amount of heat needed to raise a unit of mass of the substance one degree of temperature is called the specific heat of the substance. Because of the way in which the calorie and the Btu are defined, the specific heat of any substance is the same in either system of measurement. For example, the specific heat of water is 1 calorie per gram per degree Celsius; i.e., 1 calorie of heat is needed to raise the temperature of 1 gram of water by 1 degree Celsius; it is also 1 Btu per pound per degree Fahrenheit.

Heat of Fusion

When a solid reaches a certain temperature, it changes to a liquid. This temperature is a particular property of the substance and is called its melting point. While the solid-liquid transition is taking place, there is no change in temperature. All of the heat being added is being converted to the internal potential energy associated with the liquid state. The amount of heat needed to convert one unit of mass of a substance from a solid to liquid is called the heat of fusion, or latent heat of fusion, of the substance. Like specific heat, latent heat is also a property of the particular substance. The latent heat of fusion for the ice-to-water transition is 80 calories per gram.

Heat of Vaporization

After a substance is completely changed from a solid to a liquid, further addition of heat again causes the temperature to rise until it reaches the boiling point, the particular temperature at which the given substance changes from a liquid to a gas. During the liquid-gas transition, the temperature remains constant until the change is completed. The heat of vaporization, or latent heat of vaporization, is the heat that must be added to convert one unit of mass of the substance from a liquid to a gas.

## Transfer of Heat

Heat may be transferred from one substance to another by three means—conduction, convection, and radiation. Conduction involves the transfer of energy from one molecule to adjacent molecules without the substance as a whole moving. Convection involves the movement of warmer parts of a substance away from the source of heat and takes place only in fluids, i.e., liquids and gases. Radiation is the transfer of heat energy in the form of electromagnetic radiation, principally in the infrared radiation portion of the spectrum.

## Study and Analysis of Heat

The study of heat and its relationship to useful work is called thermodynamics and involves macroscopic quantities such as pressure, temperature, and volume without regard for the molecular basis of these quantities. Low-temperature physics is concerned with phenomena that occur at extremely low temperatures. The analysis of heat on the basis of the structure of matter is considered in the kinetic-molecular theory of gases and provides an explanation for the various gas laws. The gas laws in turn serve to define an absolute temperature scale based on theoretical considerations (see Kelvin temperature scale).

## Bibliography

See M. C. Mott-Smith, Heat and Its Workings (1933, repr. 1962); R. Becker, Theory of Heat (tr. 1967).

Ratio of the quantity of heat required to raise the temperature of a body one degree to that required to raise the temperature of an equal mass of water one degree. The term is also used to mean the amount of heat, in calories, required to raise the temperature of one gram of a substance by one Celsius degree.

Amount of heat that must be added or removed during a chemical reaction to keep all substances involved at the same temperature. If it is positive (heat must be added), the reaction is endothermic; if it is negative (heat is given off), the reaction is exothermic. Accurate heat of reaction values are needed for proper design of equipment used in chemical processes; they are usually estimated from compiled tables of thermodynamics data (heats of formation and heats of combustion of many known materials). The activation energy is unrelated to the heat of reaction.

Characteristic amount of energy absorbed or released by a substance during a change in physical state that occurs without a change in temperature. Heat of fusion is the latent heat associated with melting a solid or freezing a liquid. Heat of vaporization is the latent heat associated with vapourizing a liquid or condensing (see condensation) a vapour. For example, when water reaches its boiling point and is kept boiling, it remains at that temperature until it has all evaporated; all the heat added to the water is absorbed as latent heat of vaporization and is carried away by the escaping vapour molecules.

Device for transferring heat from a substance or space at one temperature to another at a higher temperature. It consists of a compressor, a condenser, a throttle or expansion valve, an evaporator, and a working fluid (refrigerant). The compressor delivers vapourized refrigerant to the condenser in the space to be heated. There, cooler air condenses the refrigerant and becomes heated during the process. The liquid refrigerant then enters the throttle valve and expands, coming out as a liquid-vapour mixture at a lower temperature and pressure. It then enters the evaporator, where the liquid is evaporated by contact with the warmer space. The vapour then passes to the compressor and the cycle is repeated. A heat pump is a reversible system and is commonly used both to heat and to cool buildings. It operates on the same thermodynamic principles as refrigeration.

Any of several devices that transfer heat from a hot to a cold fluid. In many engineering applications, one fluid needs to be heated and another cooled, a requirement economically accomplished by a heat exchanger. In double-pipe exchangers, one fluid flows inside the inner pipe, and the other in the annular space between the two pipes. In shell-and-tube exchangers, many tubes are mounted inside a shell; one fluid flows in the tubes and the other flows in the shell, outside the tubes. Special-purpose devices such as boilers, evaporators, superheaters, condensers, and coolers are all heat exchangers. Heat exchangers are used extensively in fossil-fuel and nuclear power plants, gas turbines, heating and air conditioning, refrigeration, and the chemical industry. Seealso cooling system.

Sum of the internal energy math.E and the product of the pressure math.P and volume math.V of a thermodynamic system (see thermodynamics). So, enthalpy math.H = math.E + math.Pmath.V. Its value is determined by the temperature, pressure, and composition of the system at any given time. According to the law of conservation of energy (see conservation law), the change in internal energy is equal to the heat transferred to the system minus the work done by the system. If the only work done is a change of volume at constant pressure, the enthalpy change is exactly equal to the heat transferred to the system.

Transfer of heat energy resulting from differences in temperature between adjacent bodies or adjacent parts of a body. In the absence of a heat pump, the energy will flow from a region of higher temperature to a region of lower temperature. The transfer of energy occurs as a result of collision among the particles of the matter involved. The rate of transfer of energy is proportional to the cross-sectional area of contact and to the difference in temperature between the two regions. A substance of high thermal conductivity, such as copper, is a good thermal conductor; one with low thermal conductivity, such as wood, is a poor thermal conductor. Seealso convection, radiation.

Ratio of heat absorbed by a material to the change in temperature. It is usually expressed as calories per degree in terms of the amount of the material being considered. Heat capacity and its temperature variation depend on differences in energy levels for atoms. Heat capacities are measured with a calorimeter and are important as a means of determining the entropies of materials. Seealso specific heat.

Energy transferred from one body to another as the result of a difference in temperature. Heat flows from a hotter body to a colder body when the two bodies are brought together. This transfer of energy usually results in an increase in the temperature of the colder body and a decrease in that of the hotter body. A substance may absorb heat without an increase in temperature as it changes from one phase to another—that is, when it melts or boils. The distinction between heat (a form of energy) and temperature (a measure of the amount of energy) was clarified in the 19th century by such scientists as J.-B. Fourier, Gustav Kirchhoff, and Ludwig Boltzmann.

The EADS Mako/High Energy Advanced Trainer (Mako/HEAT) is a high-performance jet training aircraft, intended for service with several European air forces. EADS are proposing Mako for the Eurotrainer program. The program is the final result of the AT-2000 project.

Subcontractors will include Diehl Aerospace, Finmeccanica-Aermacchi, Saab, EAB and Dassault Aviation. Customers will likely include Germany, France, Spain, Italy, Finland, Greece, Sweden, and the United Arab Emirates.

The engine will be the General Electric F414M, which is a slightly derated version (at 75 kN) of the standard F414. The engine may have final assembly conducted by Volvo, as that company already assembles a version of the related GE F404/RM12 for Saab's Gripen fighter aircraft. EADS originally intended to use the Eurojet EJ200, but chose not to do so.

The Mako/HEAT will be deployed at three shared bases around the continent of Europe, for use by all partner nations. There are nine candidate bases, in seven countries, but there has been no final selection yet.

The Aermacchi M-346 trainer, also being proposed for Eurotrainer, made its maiden flight on July 15, 2004, and has performed a series of test flights since then. As of 2005, EADS has yet to announce a date for the maiden flight.

## Design and development

The Mako design features a single aft-mounted jet engine, fed by 2 air intakes at the roots of the mid-fuselage-wing. The wing is trapezoidal with a sharp taper. The horizontal tail is an all-flying unit mounted close behind the wing and at nearly the same height. The retractable landing gear is a tricycle unit. The two occupants share a highly streamlined bubble canopy.