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.
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
, 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
See M. C. Mott-Smith, Heat and Its Workings (1933, repr. 1962); R. Becker, Theory of Heat (tr. 1967).
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For the purposes of thermodynamics, it is convenient to define all energy while in transit, but unassociated with matter, as either heat or work. Heat is that form of energy in transit due to a temperature difference between the source from which the energy is coming and the sink toward which the energy is going. The energy is not called heat before it starts to flow or after it has ceased to flow. A hot object does contain energy, but calling this energy heat as it resides in the hot object can lead to widespread confusion. See Energy, Internal energy, Temperature, Thermodynamic principles
McGraw-Hill Concise Encyclopedia of Physics. © 2002 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.
a sensation of excessive warmth, usually confirmed by elevation of body temperature. Heat may be felt by some persons without a rise of body temperature; this can be caused by functional disturbances of the nervous system or by the introduction of certain medications (for example, nicotinic acid, magnesium sulfate, and calcium chloride) that cause dilatation of the blood vessels. Local sensation of heat can result from arterial hyperemia, a change in blood circulation, and increased metabolism in inflamed tissues.
The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
a. the energy transferred as a result of a difference in temperature
b. the random kinetic energy of the atoms, molecules, or ions in a substance or body
2. the sensation caused in the body by heat energy; warmth
3. a period or condition of sexual excitement in female mammals that occurs at oestrus
a. a preliminary eliminating contest in a competition
b. a single section of a contest
5. on or in heat
a. (of some female mammals) sexually receptive
b. in a state of sexual excitement
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005