Heat

Heat is the energy that flows between two objects because of a difference in temperature. Heat always flows from a body at a higher temperature to one at a lower temperature.

Scientists use the term heat differently than do nonscientists. The average person may think of heat as the amount of energy contained in a body. The correct term for that property, however, is thermal energy.

Thermal energy and temperature

According to the kinetic theory of matter, all matter is composed of particles that are constantly in motion. Temperature is a measure of the motion of those particles. The more rapidly particles are in motion, the higher the temperature; the less rapidly they are moving, the lower the temperature.

In theory, it would be possible to reduce the motion of the particles in an object to zero. In that case, the object would contain no thermal energy. The temperature at which all particle motion ends is called absolute zero. Scientists have come within a few millionths of a degree of absolute zero but have never actually reached that point.

Heat Transfer

Another way to think of heat is as a transfer of thermal energy from one place to another. This process occurs in one of three ways: conduction, convection, and radiation.

Conduction. Conduction is the process of heat transfer. Rapidly moving molecules in a hot material collide with slower moving molecules in a cool material. The fast-moving molecules slow down and the slow moving molecules increase their speed. Conduction occurs, then, when two bodies of different temperatures are in contact with each other.

Convection. Convection is the process by which large masses of a fluid (a liquid or gas) move, carrying thermal energy. When water in a container is heated, for example, it expands. Cooler water around it pushes the lighter water upward. As the warm water rises, it begins to cool and starts to move downward in the liquid again. Eventually, a circular motion is produced within the liquid, forcing heat to be transferred throughout the liquid.

Radiation. Finally, thermal energy can be transferred by radiation. Hot bodies emit electromagnetic radiation that corresponds to their temperature. This radiation passes through space until it comes into contact with a body with less thermal energy. The cooler body then absorbs this radiation and becomes warmer.

Transfer of thermal energy. The transfer of thermal energy from one place to another occurs in one of three ways: conduction, convection, and radiation. In conduction, rapidly moving molecules in a hot material collide with slower moving molecules in a cool material. The fast-moving molecules slow down and the slow-moving molecules increase their speed. Conduction occurs, then, when two bodies of different temperatures are in contact with each other.

Convection is the process by which large masses of a fluid (a liquid or gas) move, carrying thermal energy. When water in a container is heated, for example, it expands. Cooler water around it pushes the lighter water upward. As the warm water rises, it begins to cool and starts to move downward in the liquid again. Eventually, a circular motion is produced within the liquid, forcing heat to be transferred throughout the liquid.

Finally, thermal energy can be transferred by radiation. Hot bodies emit electromagnetic radiation that corresponds to their temperature. This radiation passes through space until it comes into contact with a body with less thermal energy. The cooler body then absorbs this radiation and becomes warmer.

Heat units

Since heat is a form of energy, the units used to measure heat are the same as those used to measure energy. In the metric system, one of the earliest units used to measure heat was the calorie. The calorie is defined as the amount of heat energy needed to raise the temperature of one gram of water one degree Celsius. To be precise, the temperature change is specified as an increase from 14.5°C to 15.5°C.

In the International System of Units (the SI system), the unit of energy is the joule. A calorie is defined as 4.184 joules.

Specific heat

Materials differ from each other with regard to how easily they can be warmed. One could add a joule of heat to a gram of water, a gram of iron, a gram of mercury, and a gram of ethyl alcohol and notice very different results. The temperature of the mercury would rise the most, and the temperature of the water would rise the least.

The specific heat capacity (or just specific heat) of a material is defined as the amount of heat required to raise the temperature of one gram of the material one degree Celsius. It takes 4.18 joules to raise the temperature of 1 gram of water 1 degree Celsius (at a temperature of 25°C). In comparison, it takes only 0.14 joule to raise the temperature of the same amount of mercury by one degree Celsius and 0.45 joule to raise the temperature of the same amount of iron by one degree Celsius. It takes 2.46 joules to raise the temperature of the same amount of ethyl alcohol by one degree Celsius.

[See also Energy; Temperature; Thermodynamics ]

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).

200. Heat

See also 90. COLD ; 162. FIRE

actinism

the ability of light and heat and other forms of radiant energy to cause chemical changes, as hormonal changes in birds causing them to migrate or brood. —actinic , adj.

adiathermancy

imperviousness to radiant heat or infrared radiation. Also called athermancy .

athermancy

adiathermancy.

calorimetry

the science of measuring heat. —calorimeter , n. —calorimetric , adj.

calorist

Rare. one who believed the caloric theory, that heat is a material substance. —caloristic , adj.

galvanothermy

the process of generating heat by means of an electric current.

geothermometry

the branch of geology that measures temperatures deep below the surface of the earth; geologic thermometry.

pyrogenesis

the production or generation of heat. —pyrogenetic , adj.

pyrolysis

the chemical process of decomposition under the effect of heat. —pyrolitic , adj.

pyrophotometer

a type of pyrometer that measures temperature optically or photometrically.

tepidity

a moderate warmth; lukewarmness. —tepid , adj.

thermatology

Medicine. the study of heat as a medical remedy or therapy. Also called thermotherapy .

thermionics

the science or study of the emission of electrons from substances at high temperatures. —thermionic , adj.

thermochemistry

the branch of chemistry that studies the relationship of heat to chemical changes, including the production of energy. —thermochemist , n. —thermochemical , adj.

thermodynamics

the branch of physics that studies the relationship of heat and mechanical energy and the conversion, in various materials, of one into the other. —thermodynamicist , n. —thermodynamic, thermodynamical , adj.

thermogenesis, thermogeny

the production of heat, especially in an animal body by physiological processes. —thermogenic, thermogenous , adj.

thermography

1. Engineering, a method of measuring surf ace temperatures by using luminescent materials.

2. a printing or photocopying process using infrared rays and heat.

3. a process of photography using far-infrared radiation; thermal photography. —thermographer , n. —thermographic , adj.

thermokinematics

the study of the movement of heat. —thermokinematic , adj.

thermology

Archaic. the science and study of heat. Also called thermotics .

thermoluminescence

Atomic Physics. any luminescence appearing in materials upon application of heat, caused by electron movement which increases as the temperature rises. —thermoluminescent , adj.

thermolysis

Physiology. the dispersion of heat from the body. —thermolytic , adj.

thermometry

the branch of physics that deals with the measurement of temperature. —thermometric , adj.

thermophobia

an abnormal fear of heat.

thermoscope

a device for giving an approximation of the temperature change of a substance by noting the accompanying change in its volume. —thermoscopic , adj.

thermostatics

the science or study of the equilibrium of heat.

thermotaxis

1. Biology. the movement of an organism toward or away from a source of heat.

2. Physiology. the regulation of body temperature by various physiological processes. —thermotactic, thermotaxic , adj.

thermotherapy

thermatology.

thermotics

thermology.

transcalency

the property or quality by which matter permits the passage of heat. —transcalent , adj.

Heat

Heat is the transfer of energy that results from the difference in temperature between a system and its surroundings. At a molecular level, heat is the transfer of energy that makes use of or stimulates disorderly molecular motion in the surroundings. For instance, when a hydrocarbon fuel burns, the energy released in the reaction stimulates the surrounding atoms and molecules into more vigorous random motion, and we refer to this escape of energy as heat. Heat is not stored: Heat is energy in transit.

The measurement of quantities of energy transferred as heat is called calorimetry. Such a measurement is commonly made by observing the rise in temperature caused by the process being studied and interpreting that rise in terms of the heat produced. Calorimetry is used to measure the changes in internal energy and enthalpy that accompany chemical reactions. The field of study is called thermochemistry, and it is used to assess the efficacy of fuels, the energy flow in chemical plants, and the strengths of chemical bonds. Measurements of the heat produced or absorbed by chemical reactions are central to thermodynamics, and to assessments of whether or not a particular reaction will tend to occur.

In thermodynamics, the quantity of energy transferred as heat as a result of a chemical reaction is identified with the change in the internal energy of the system if the transfer takes place without change in the system's volume, and with the change in enthalpy of the system if the transfer takes place at constant pressure. The energy or enthalpy change accompanying a chemical reaction that is inaccessible to measurement may be determined by using Hess's law, which states that the enthalpy change accompanying a chemical reaction can be regarded as the sum of the enthalpy changes of the reactions into which the overall reaction may be divided. Hess's law is no more than a special application of the first law of thermodynamics.

The source of heat as a fuel burns is the energy released when the bonds characteristic of the reactants are replaced by the bonds characteristic of the products. Energy is released when hydrocarbons burn because of the great strengths of the oxygen–hydrogen and oxygen–carbon bonds that are formed in the products (water and carbon dioxide), replacing the relatively weak carbon–hydrogen and carbon–carbon bonds of the fuel. Ultimately, the energy of burning fuel is the energy released as the electrons and atomic nuclei settle into more favorable arrangements (just as nucleons do in the much more exothermic processes accompanying nuclear rearrangements).

Although the term "heat energy" is commonly encountered in casual conversation, strictly speaking there is no such entity. The term is commonly used in place of the more precise term "energy of thermal motion," where thermal motion is random molecular motion, as in the motion of molecules in a gas. Nor is heat stored: Only energy is stored, and heat is one of the modes by which it may be increased or extracted.

see also Chemistry and Energy; Energy; Explosions; Temperature; Thermochemistry; Thermodynamics.

Peter Atkins

Bibliography

Atkins, Peter, and de Paula, Julio (2002). Atkins' Physical Chemistry, 7th edition. New York: Oxford University Press.

Smith, Crosbie (1998). The Science of Energy: A Cultural History of Energy Physics in Victorian Britain. Chicago: University of Chicago Press.

heat / hēt/ • n. 1. the quality of being hot; high temperature: it is sensitive to both heat and cold. ∎  hot weather conditions: the oppressive heat was making both men sweat. ∎  a source or level of heat for cooking: remove from the heat and beat in the butter. ∎  a spicy quality in food that produces a burning sensation in the mouth: chili peppers add taste and heat to food. ∎  Physics heat seen as a form of energy arising from the random motion of the molecules of bodies, which may be transferred by conduction, convection, or radiation. ∎ technical the amount of heat that is needed to cause a specific process or is evolved in such a process: the heat of formation. ∎  technical a single operation of heating something, esp. metal in a furnace. 2. intensity of feeling, esp. of anger or excitement: words few men would dare use to another, even in the heat of anger. ∎  (the heat) inf. intensive and unwelcome pressure or criticism, esp. from the authorities: a flurry of legal proceedings turned up the heat in the dispute. 3. a preliminary round in a race or contest: the 200-meter heats. • v. make or become hot or warm: [tr.] the room faces north and is difficult to heat | [intr.] the pipes expand as they heat up. ∎  [intr.] (heat up) (of a person) become excited or impassioned. ∎  [intr.] (heat up) become more intense and exciting: the action really begins to heat up. ∎  [tr.] archaic inflame; excite: this discourse had heated them. PHRASES: in the heat of the moment while temporarily angry, excited, or engrossed, and without stopping for thought. in heat (of a female mammal) in the receptive period of the sexual cycle; in estrus.

heat (symbol Q) Form of energy that transfers as a consequence of a difference in temperature. The amount of heat gained or lost by a body equals the product of its heat capacity and the temperature through which it rises or falls. Heat transfers in three forms: convection (within fluids), conduction (within objects), and radiation. The total kinetic and potential energy of a body is its internal energy (U). If this body changes temperature, there is a corresponding change (ΔU) in its internal energy. According to the first law of thermodynamics, Δ = Q−W, where Q is the heat and W is the work. The SI unit of heat and energy is the joule(J).

heat if you don't like the heat get out of the kitchen proverbial saying, mid 20th century, meaning that if you choose to work in a particular sphere you must also deal with its pressures; particularly associated with President Truman (1884–1972).
in the heat of the moment while temporarily angry, excited, or engrossed, and without stopping for thought.

See also white heat.

heat sb. OE. hǣtu = MDu. hēte, OHG. heizi :- WGmc. *χaitīn. f. Gmc. *χaitaz HOT; also OE. hǣte (:- *χaitja).
So heat vb. OE hǣtan.

HEAT abbr.high explosive antitank (denoting a class of warheads).

heat •accrete, beat, beet, bittersweet, bleat, cheat, cleat, clubfeet, compete, compleat, complete, conceit, Crete, deceit, delete, deplete, discreet, discrete, eat, effete, élite, entreat, escheat, estreat, excrete, feat, feet, fleet, gîte, greet, heat, leat, leet, Magritte, maltreat, marguerite, meat, meet, mesquite, mete, mistreat, neat, outcompete, peat, Pete, petite, pleat, receipt, replete, seat, secrete, sheet, skeet, sleet, splay-feet, street, suite, sweet, teat, treat, tweet, wheat •backbeat • heartbeat • deadbeat •breakbeat • offbeat • browbeat •downbeat • drumbeat • upbeat •sugar beet • Blackfeet • flatfeet •forefeet • exegete • polychaete •lorikeet • parakeet •athlete, biathlete, decathlete, heptathlete, pentathlete, triathlete •kick-pleat • paraclete • obsolete •gamete • crabmeat • sweetmeat •mincemeat • forcemeat • backstreet •concrete • window seat