heat

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Concept

Heat is a form of energy—specifically, the energy that flows between two bodies because of differences in temperature. Therefore, the scientific definition of heat is different from, and more precise than, the everyday meaning. Physicists working in the area of thermodynamics study heat from a number of perspectives, including specific heat, or the amount of energy required to change the temperature of a substance, and calorimetry, the measurement of changes in heat as a result of physical or chemical changes. Thermodynamics helps us to understand such phenomena as the operation of engines and the gradual breakdown of complexity in physical systems—a phenomenon known as entropy.

How It Works

Heat, Work, and Energy

Thermodynamics is the study of the relationships between heat, work, and energy. Work is the exertion of force over a given distance to displace or move an object, and is, thus, the product of force and distance exerted in the same direction. Energy, the ability to accomplish work, appears in numerous manifestations—including thermal energy, or the energy associated with heat.

Thermal and other types of energy, including electromagnetic, sound, chemical, and nuclear energy, can be described in terms of two extremes: kinetic energy, or the energy associated with movement, and potential energy, or the energy associated with position. If a spring is pulled back to its maximum point of tension, its potential energy is also at a maximum; once it is released and begins springing through the air to return to its original position, it begins gaining kinetic energy and losing potential energy.

All manifestations of energy appear in both kinetic and potential forms, somewhat like the way football teams are organized to play both offense or defense. Just as a football team takes an offensive role when it has the ball, and a defensive role when the other team has it, a physical system typically undergoes regular transformations between kinetic and potential energy, and may have more of one or the other, depending on what is taking place in the system.

What Heat Is and Is Not

Thermal energy is actually a form of kinetic energy generated by the movement of particles at the atomic or molecular level: the greater the movement of these particles, the greater the thermal energy. Heat is internal thermal energy that flows from one body of matter to another—or, more specifically, from a system at a higher temperature to one at a lower temperature. Thus, temperature, like heat, requires a scientific definition quite different from its common meaning: temperature measures the average molecular kinetic energy of a system, and governs the direction of internal energy flow between them.

Two systems at the same temperature are said to be in a state of thermal equilibrium. When this occurs, there is no exchange of heat. Though in common usage, "heat" is an expression of relative warmth or coldness, in physical terms, heat exists only in transfer between two systems. What people really mean by "heat" is the internal energy of a system—energy that is a property of that system rather than a property of transferred internal energy.

No Such Thing As "cold."

Though the term "cold" has plenty of meaning in the everyday world, in physics terminology, it does not. Cold and heat are analogous to darkness and light: again, darkness means something in our daily experience, but in physical terms, darkness is simply the absence of light. To speak of cold or darkness as entities unto themselves is rather like saying, after spending 20 dollars, "I have 20 non-dollars in my pocket."

If you grasp a snowball in your hand, of course, your hand gets cold. The human mind perceives this as a transfer of cold from the snowball, but, in fact, exactly the opposite happens: heat moves from your hand to the snow, and if enough heat enters the snowball, it will melt. At the same time, the departure of heat from your hand results in a loss of internal energy near the surface of your hand, which you experience as a sensation of coldness.

Transfers of Heat

In holding the snowball, heat passes from the surface of the hand by one means, conduction, then passes through the snowball by another means, convection. In fact, there are three methods heat is transferred: conduction, involving successive molecular collisions and the transfer of heat between two bodies in contact; convection, which requires the motion of fluid from one place to another; or radiation, which takes place through electromagnetic waves and requires no physical medium, such as water or air, for the transfer.

Conduction

Solids, particularly metals, whose molecules are packed relatively close together, are the best materials for conduction. Molecules of liquid or nonmetallic solids vary in their ability to conduct heat, but gas is a poor conductor, because of the loose attractions between its molecules.

The qualities that make metallic solids good conductors of heat, as a matter of fact, also make them good conductors of electricity. In the conduction of heat, kinetic energy is passed from molecule to molecule, like a long line of people standing shoulder to shoulder, passing a secret. (And, just as the original phrasing of the secret becomes garbled, some kinetic energy is inevitably lost in the series of transfers.)

As for electrical conduction, which takes place in a field of electric potential, electrons are freed from their atoms; as a result, they are able to move along the line of molecules. Because plastic is much less conductive than metal, an electrician uses a screwdriver with a plastic handle; similarly, a metal cooking pan typically has a wooden or plastic handle.

Convection

Wherever fluids are involved—and in physics, "fluid" refers both to liquids and gases—convection is a common form of heat transfer. Convection involves the movement of heated material—whether it is air, water, or some other fluid.

Convection is of two types: natural convection and forced convection, in which a pump or other mechanism moves the heated fluid. When heated air rises, this is an example of natural convection. Hot air has a lower density than that of the cooler air in the atmosphere above it, and, therefore, is buoyant; as it rises, however, it loses energy and cools. This cooled air, now denser than the air around it, sinks again, creating a repeating cycle that generates wind.

Examples of forced convection include some types of ovens and even a refrigerator or air conditioner. These two machines both move warm air from an interior to an exterior place. Thus, the refrigerator pulls hot air from the compartment and expels it to the surrounding room, while an air conditioner pulls heat from a building and releases it to the outside.

But forced convection does not necessarily involve humanmade machines: the human heart is a pump, and blood carries excess heat generated by the body to the skin. The heat passes through the skin by means of conduction, and at the surface of the skin, it is removed from the body in a number of ways, primarily by the cooling evaporation of perspiration.

Radiation

Outer space, of course, is cold, yet the Sun's rays warm the Earth, an apparent paradox. Because there is no atmosphere in space, convection is impossible. In fact, heat from the Sun is not dependant on any fluid medium for its transfer: it comes to Earth by means of radiation. This is a form of heat transfer significantly different from the other two, because it involves electromagnetic energy, instead of ordinary thermal energy generated by the action of molecules. Heat from the Sun comes through a relatively narrow area of the light spectrum, including infrared, visible light, and ultraviolet rays.

Every form of matter emits electromagnetic waves, though their presence may not be readily perceived. Thus, when a metal rod is heated, it experiences conduction, but part of its heat is radiated, manifested by its glow—visible light. Even when the heat in an object is not visible, however, it may be radiating electromagnetic energy, for instance, in the form of infrared light. And, of course, different types of matter radiate better than others: in general, the better an object is at receiving radiation, the better it is at emitting it.

Measuring Heat

The measurement of temperature by degrees in the Fahrenheit or Celsius scales is a part of everyday life, but measurements of heat are not as familiar to the average person. Because heat is a form of energy, and energy is the ability to perform work, heat is, therefore, measured by the same units as work.

The principal unit of work or energy in the metric system (known within the scientific community as SI, or the SI system) is the joule. Abbreviated "J," a joule is equal to 1 newton-meter (N · m). The newton is the SI unit of force, and since work is equal to force multiplied by distance, measures of work can also be separated into these components. For instance, the British measure of work brings together a unit of distance, the foot, and a unit of force, the pound. A foot-pound (ft · lb) is equal to 1.356 J, and 1 joule is equal to 0.7376 ft · lb.

In the British system, Btu, or British thermal unit, is another measure of energy used for machines such as air conditioners. One Btu is equal to 778 ft · lb or 1,054 J. The kilocalorie in addition to the joule, is an important SI measure of heat. The amount of energy required to change the temperature of 1 gram of water by 1°C is called a calorie, and a kilocalorie is equal to 1,000 calories. Somewhat confusing is the fact that the dietary Calorie (capital C), with which most people are familiar, is not the same as a calorie (lowercase C)—rather, a dietary Calorie is the equivalent of a kilocalorie.

Real-Life Applications

Specific Heat

Specific heat is the amount of heat that must be added to, or removed from, a unit of mass for a given substance to change its temperature by 1°C. Thus, a kilocalorie, because it measures the amount of heat necessary to effect that change precisely for a kilogram of water, is identical to the specific heat for that particular substance in that particular unit of mass.

The higher the specific heat, the more resistant the substance is to changes in temperature. Many metals, in fact, have a low specific heat, making them easy to heat up and cool down. This contributes to the tendency of metals to expand when heated (a phenomenon also discussed in the Thermal Expansion essay), and, thus, to their malleability.

Measuring and Calculating Specific Heat

The specific heat of any object is a function of its mass, its composition, and the desired change in temperature. The values of the initial and final temperature are not important—only the difference between them, which is the temperature change.

The components of specific heat are related to one another in the formula Q = mcδT. Here Q is the quantity of heat, measured in joules, which must be added. The mass of the object is designated by m, and the specific heat of the particular substance in question is represented with c. The Greek letter delta (δ) designates change, and δT stands for "change in temperature."

Specific heat is measured in units of J/kg · °C (joules per kilogram-degree Centigrade), though for the sake of convenience, this is usually rendered in terms of kilojoules (kJ), or 1,000 joules—that is, kJ/kg · °C. The specific heat of water is easily derived from the value of a kilo-calorie: it is 4.185, the same number of joules required to equal a kilocalorie.

Calorimetry

The measurement of heat gain or loss as a result of physical or chemical change is called calorimetry (pronounced kal-IM-uh-tree). Like the word "calorie," the term is derived from a Latin root meaning "heat."

The foundations of calorimetry go back to the mid-nineteenth century, but the field owes much to scientists' work that took place over a period of about 75 years prior to that time. In 1780, French chemist Antoine Lavoisier (1743-1794) and French astronomer and mathematician Pierre Simon Laplace (1749-1827) had used a rudimentary ice calorimeter for measuring the heats in formations of compounds. Around the same time, Scottish chemist Joseph Black (1728-1799) became the first scientist to make a clear distinction between heat and temperature.

By the mid-1800s, a number of thinkers had come to the realization that—contrary to prevailing theories of the day—heat was a form of energy, not a type of material substance. Among these were American-British physicist Benjamin Thompson, Count Rumford (1753-1814) and English chemist James Joule (1818-1889)—for whom, of course, the joule is named.

Calorimetry as a scientific field of study actually had its beginnings with the work of French chemist Pierre-Eugene Marcelin Berthelot (1827-1907). During the mid-1860s, Berthelot became intrigued with the idea of measuring heat, and by 1880, he had constructed the first real calorimeter.

Calorimeters

Essential to calorimetry is the calorimeter, which can be any device for accurately measuring the temperature of a substance before and after a change occurs. A calorimeter can be as simple as a styrofoam cup. Its quality as an insulator, which makes styrofoam ideal for holding in the warmth of coffee and protecting the hand from scalding as well, also makes styrofoam an excellent material for calorimetric testing. With a styrofoam calorimeter, the temperature of the substance inside the cup is measured, a reaction is allowed to take place, and afterward, the temperature is measured a second time.

The most common type of calorimeter used is the bomb calorimeter, designed to measure the heat of combustion. Typically, a bomb calorimeter consists of a large container filled with water, into which is placed a smaller container, the combustion crucible. The crucible is made of metal, having thick walls with an opening through which oxygen can be introduced. In addition, the combustion crucible is designed to be connected to a source of electricity.

In conducting a calorimetric test using a bomb calorimeter, the substance or object to be studied is placed inside the combustion crucible and ignited. The resulting reaction usually occurs so quickly that it resembles the explosion of a bomb—hence, the name "bomb calorimeter." Once the "bomb" goes off, the resulting transfer of heat creates a temperature change in the water, which can be readily gauged with a thermometer.

To study heat changes at temperatures higher than the boiling point of water (212°F or 100°C), physicists use substances with higher boiling points. For experiments involving extremely large temperature ranges, an aneroid (without liquid) calorimeter may be used. In this case, the lining of the combustion crucible must be of a metal, such as copper, with a high coefficient or factor of thermal conductivity.

Heat Engines

The bomb calorimeter that Berthelot designed in 1880 measured the caloric value of fuels, and was applied to determining the thermal efficiency of a heat engine. A heat engine is a machine that absorbs heat at a high temperature, performs mechanical work, and as a result, gives off heat at a lower temperature.

The desire to create efficient heat engines spurred scientists to a greater understanding of thermodynamics, and this resulted in the laws of thermodynamics, discussed at the conclusion of this essay. Their efforts were intimately connected with one of the greatest heat engines ever created, a machine that literally powered the industrialized world during the nineteenth century: the steam engine.

How a Steam Engine Works

Like all heat engines (except reverse heat engines such as the refrigerator, discussed below), a steam engine pulls heat from a high-temperature reservoir to a low-temperature reservoir, and in the process, work is accomplished. The hot steam from the high-temperature reservoir makes possible the accomplishment of work, and when the energy is extracted from the steam, the steam condenses in the low-temperature reservoir, becoming relatively cool water.

A steam engine is an external-combustion engine, as opposed to the internal-combustion engine that took its place at the forefront of industrial technology at the beginning of the twentieth century. Unlike an internal-combustion engine, a steam engine burns its fuel outside the engine. That fuel may be simply firewood, which is used to heat water and create steam. The thermal energy of the steam is then used to power a piston moving inside a cylinder, thus, converting thermal energy to mechanical energy for purposes such as moving a train.

Evolution of Steam Power

As with a number of advanced concepts in science and technology, the historical roots of the steam engine can be traced to the Greeks, who—just as they did with ideas such as the atom or the Sun-centered model of the universe—thought about it, but failed to develop it. The great inventor Hero of Alexandria (c. 65-125) actually created several steam-powered devices, but he perceived these as mere novelties, hardly worthy of scientific attention. Though Europeans adopted water power, as, for instance, in waterwheels, during the late ancient and medieval periods, further progress in steam power did not occur for some 1,500 years.

Following the work of French physicist Denis Papin (1647-1712), who invented the pressure cooker and conducted the first experiments with the use of steam to move a piston, English engineer Thomas Savery (c. 1650-1715) built the first steam engine. Savery had abandoned the use of the piston in his machine, but another English engineer, Thomas Newcomen (1663-1729), reintroduced the piston for his own steam-engine design.

Then in 1763, a young Scottish engineer named James Watt (1736-1819) was repairing a Newcomen engine and became convinced he could build a more efficient model. His steam engine, introduced in 1769, kept the heating and cooling processes separate, eliminating the need for the engine to pause in order to reheat. These and other innovations that followed—including the introduction of a high-pressure steam engine by English inventor Richard Trevithick (1771-1833)—transformed the world.

Carnot Provides Theoretical Understanding

The men who developed the steam engine were mostly practical-minded figures who wanted only to build a better machine; they were not particularly concerned with the theoretical explanation for its workings. Then in 1824, a French physicist and engineer by the name of Sadi Carnot (1796-1832) published his sole work, the highly influential Reflections on the Motive Power of Fire (1824), in which he discussed heat engines scientifically.

In Reflections, Carnot offered the first definition of work in terms of physics, describing it as "weight lifted through a height." Analyzing Watt's steam engine, he also conducted groundbreaking studies in the nascent science of thermodynamics. Every heat engine, he explained, has a theoretical limit of efficiency related to the temperature difference in the engine: the greater the difference between the lowest and highest temperature, the more efficient the engine.

Carnot's work influenced the development of more efficient steam engines, and also had an impact on the studies of other physicists investigating the relationship between work, heat, and energy. Among these was William Thomson, Lord Kelvin (1824-1907). In addition to coining the term "thermodynamics," Kelvin developed the Kelvin scale of absolute temperature and established the value of absolute zero, equal to −273.15°C or −459.67°F.

According to Carnot's theory, maximum effectiveness was achieved by a machine that could reach absolute zero. However, later developments in the understanding of thermodynamics, as discussed below, proved that both maximum efficiency and absolute zero are impossible to attain.

Reverse Heat Engines

It is easy to understand that a steam engine is a heat engine: after all, it produces heat. But how is it that a refrigerator, an air conditioner, and other cooling machines are also heat engines? Moreover, given the fact that cold is the absence of heat and heat is energy, one might ask how a refrigerator or air conditioner can possibly use energy to produce cold, which is the same as the absence of energy. In fact, cooling machines simply reverse the usual process by which heat engines operate, and for this reason, they are called "reverse heat engines." Furthermore, they use energy to extract heat.

A steam engine takes heat from a high-temperature reservoir—the place where the water is turned into steam—and uses that energy to produce work. In the process, energy is lost and the heat moves to a low-temperature reservoir, where it condenses to form relatively cool water. A refrigerator, on the other hand, pulls heat from a low-temperature reservoir called the evaporator, into which flows heat from the refrigerated compartment—the place where food and other perishables are kept. The coolant from the evaporator take this heat to the condenser, a high-temperature reservoir at the back of the refrigerator, and in the process it becomes a gas. Heat is released into the surrounding air; this is why the back of a refrigerator is hot.

Instead of producing a work output, as a steam engine does, a refrigerator requires a work input—the energy supplied via the wall outlet. The principles of thermodynamics show that heat always flows from a high-temperature to a low-temperature reservoir, and reverse heat engines do not defy these laws. Rather, they require an external power source in order to effect the transfer of heat from a low-temperature reservoir, through the gases in the evaporator, to a high-temperature reservoir.

The Laws of Thermodynamics

The First Law of Thermodynamics

There are three laws of thermodynamics, which provide parameters as to the operation of thermal systems in general, and heat engines in particular. The history behind the derivation of these laws is discussed in the essay on Thermodynamics; here, the laws themselves will be examined in brief form.

The physical law known as conservation of energy shows that within a system isolated from all outside factors, the total amount of energy remains the same, though transformations of energy from one form to another take place. The first law of thermodynamics states the same fact in a somewhat different manner.

According to the first law of thermodynamics, because the amount of energy in a system remains constant, it is impossible to perform work that results in an energy output greater than the energy input. Thus, it could be said that the conservation of energy law shows that "the glass is half full": energy is never lost. On the hand, the first law of thermodynamics shows that "the glass is half empty": no machine can ever produce more energy than was put into it. Hence, a perpetual motion machine is impossible, because in order to keep a machine running continually, there must be a continual input of energy.

The Second Law of Thermodynamics

The second law of thermodynamics begins from the fact that the natural flow of heat is always from a high-temperature to a low-temperature reservoir. As a result, no engine can be constructed that simply takes heat from a source and performs an equivalent amount of work: some of the heat will always be lost. In other words, it is impossible to build a perfectly efficient engine.

In effect, the second law of thermodynamics compounds the "bad news" delivered by the first law with some even worse news: though it is true that energy is never lost, the energy available for work output will never be as great as the energy put into a system. Linked to the second law is the concept of entropy, the tendency of natural systems toward breakdown, and specifically, the tendency for the energy in a system to be dissipated. "Dissipated" in this context means that the high-and low-temperature reservoirs approach equal temperatures, and as this occurs, entropy increases.

The Third Law of Thermodynamics

Entropy also plays a part in the third law of thermodynamics, which states that at the temperature of absolute zero, entropy also approaches zero. This might seem to counteract the "worse news" of the second law, but in fact, what the third law shows is that absolute zero is impossible to reach.

As stated earlier, Carnot's engine would achieve perfect efficiency if its lowest temperature were the same as absolute zero; but the second law of thermodynamics shows that a perfectly efficient machine is impossible. Relativity theory (which first appeared in 1905, the same year as the third law of thermodynamics) showed that matter can never exceed the speed of light. In the same way, the collective effect of the second and third laws is to prove that absolute zero—the temperature at which molecular motion in all forms of matter theoretically ceases—can never be reached.

Where to Learn More

Beiser, Arthur. Physics, 5th ed. Reading, MA: Addison-Wesley, 1991.

Bonnet, Robert L and Dan Keen. Science Fair Projects: Physics. Illustrated by Frances Zweifel. New York: Sterling, 1999.

Encyclopedia of Thermodynamics (Web site). <http://therion.minpet.unibas.ch/minpet/groups/thermodict/> (April 12, 2001).

Friedhoffer, Robert. Physics Lab in the Home. Illustrated by Joe Hosking. New York: Franklin Watts, 1997.

Manning, Mick and Brita Granström. Science School. New York: Kingfisher, 1998.

Macaulay, David. The New Way Things Work. Boston: Houghton Mifflin, 1998.

Moran, Jeffrey B. How Do We Know the Laws of Thermodynamics? New York: Rosen Publishing Group, 2001.

Santrey, Laurence. Heat. Illustrated by Lloyd Birmingham. Mahwah, NJ: Troll Associates, 1985.

Suplee, Curt. Everyday Science Explained. Washington, D.C.: National Geographic Society, 1996.

"Temperature and Thermodynamics" PhysLINK.com (Web site). <http://www.physlink.com/ae_thermo.cfm> (April 12, 2001).

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 also Energy; Internal energy.

noun

1. Intense warmth: fervor, hotness, torridity, torridness. See hot/cold/lukewarm.
2. Intensity of feeling or reaction: excitation, excitement, warmth. See excite/bore/interest, feelings, hot/cold/lukewarm.
3. A regular period of sexual excitement in female mammals: estrus, rut², season. See sex/asexual.
4. A member of a law-enforcement agency: bluecoat, finest, officer, patrolman, patrolwoman, peace officer, police, policeman, police officer, policewoman. Informal cop, law. Slang bull¹, copper, flatfoot, fuzz, gendarme, man (often uppercase). Chiefly British bobby, constable, peeler. See law.

Idioms beginning with heat:
heat up

See also dead heat; in heat; in the heat of the moment; turn up the heat.

Definition: anger, passion
Antonyms: apathy, coolness, disinterest, frigidity

n

Definition: warmth
Antonyms: cold, cool, coolness

v

Definition: make or become hot
Antonyms: cool, freeze

In Aristotle's physics, heat is one of the active qualities of a body. Hotness, as well as dryness, and their opposites, coldness and moistness, were the qualities that defined an element. Later, heat and temperature came to be viewed as lesser fundamental properties of matter, but opinions about the nature of heat varied sharply. Francis Bacon defined heat as local motion, and René Descartes and Robert Hooke described heat as the ceaseless motion of particles. Others, such as the Dutch physician Hermann Boerhaave, described heat as a subtle fluid, later called caloric, that could pass from one object to another.

During the beginning of the 19th century, the theory that heat was a form of motion won over the liquid, or caloric, theory. Even earlier, at the end of the 18th century, Count Rumford (Benjamin Thompson) had observed the large amount of heat produced during the boring of cannons and reached the conclusion that heat was added to the cannon by motion.

The German physician Julius Robert Mayer was one of the first to relate heat directly to mechanical work, writing of his discovery in 1842. That heat and mechanical work are both forms of energy was a very important discovery. It became the basis of a new science that gained quickly in importance during the 19th century: thermodynamics.

Sadi Carnot, intrigued by why British steam engines were more efficient than French ones, started investigating the physical processes that take place in a steam engine. Although he believed that heat was an indestructible fluid, he was the first to apply physics to a technical problem. He published the results in 1824 as Réflexions sur la puissance motrice du feu ("reflections on the motive power of fire"). However, this paper remained little known for ten years, until Emile Clapeyron discussed it in the Journal de l'Ecole Polytechnique

Carnot had found that the efficiency of a machine depends on the difference in temperature between the heat source that supplies the work (hot reservoir) to the machine and the temperature reservoir into which the machine discharges excess heat (cold reservoir). The mechanical equivalent of heat is the amount of heat that can produce a given amount of motion. Although Carnot had found an approximate value for the mechanical equivalent of heat, he died too early, at the age of 36, to publish this result.

Contrary to Carnot, James Prescott Joule believed that heat was a quantity that was destroyed in a machine to produce mechanical work. In a series of famous experiments, he measured the increase of temperature of water stirred by a determined amount of mechanical work. As a result, Joule also determined the mechanical equivalent of heat, five years after the similar work of Julius Mayer. His work was more quickly recognized than Mayer's, however, since Joule was a physicist and Mayer a physician.

The form of energy that is transferred by virtue of a temperature difference between two bodies, the transfer being from the warmer to the cooler body.

Energy possessed by a substance in the form of atomic or molecular kinetic energy. Heat contained within a body is the product of the body's mass, temperature, and specific heat capacity. Heat is transmitted by radiation, conduction, and convection. Changes in the heat content of a body may result in changes of state of matter within the body (e.g. liquid water converted to vapour; see latent heat of vaporization) or changes of temperature. The SI unit for heat is the joule.

A model that applies forward rates to an existing term structure of interest rates to determine appropriate prices for securities that are sensitive to changes in interest rates.

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n.

    Heat, says Professor Tyndall, is a mode
        Of motion, but I know now how he's proving
    His point; but this I know -- hot words bestowed
        With skill will set the human fist a-moving,
    And where it stops the stars burn free and wild.
    Crede expertum -- I have seen them, child.
                                                          Gorton Swope

LearnThatWord.com is a free vocabulary and spelling program where you only pay for results!

In physics, a form of energy associated with the movement of atoms and molecules in any material. The higher the temperature of a material, the faster the atoms are moving, and hence the greater the amount of energy present as heat. (See infrared radiation.)

1. The temperature of a food or flavor.
   
2. The trigeminal burn of capsaicin or capsicum pepper extract.
   
3. The latent energy of a food product that when oxidized provides an amount of heat energy measured in calories. See Trigeminal Nerves, Calorie (c), Calorie (C).
   

The state of a body or of matter that is perceived as being opposite of cold and is characterized by elevation of temperature.

• Reproduction and Development - heat: estrus
• Animal Types and Parts - heat: estrus condition
• Branches and Disciplines - heat: study of production, properties, and effects of thermal energy
• Principles of Mechanics, Waves, and Measurement - heat: energy resulting from disordered motion of molecules
• Heat - heat: quantity of thermal energy being added to an object, or moved from one object to another, because of a temperature difference
• Physical Properties and Changes - heat: energy associated with the motion of particles
• Movie People and Show Business - heat: Slang. good word of mouth about a new movie; buzz
• Baseball - heat: fast pitch(es)
• Swimming and Diving
• Track and Field
• Sexuality and Libido - heat: period of sexual arousal and interest in mating; estrus
• Police Procedures and Detection - heat: Slang. police

For other uses, see Heat (disambiguation).

Heat generated from the nuclear fusion in the Sun, and transported to Earth as electromagnetic radiation, is one of the driving forces of life on Earth.

Heat is energy transferred from one system to another by thermal interaction.^[1][2] In contrast to work, heat is always accompanied by a transfer of entropy. Heat flow is characteristic of macroscopic objects and systems, but its origin and properties can be understood in terms of their microscopic constituents.

Heat flow from a high to a low temperature body occurs spontaneously. This flow of energy can be harnessed and converted into useful work by means of a heat engine. The second law of thermodynamics prohibits heat flow from a low to a high temperature body, but with the aid of a heat pump external work can be used to transport energy from low to the high temperature.

The SI unit of heat is the joule. Heat can be measured with a calorimeter, or determined indirectly by calculations based on other quantities, relying for instance on the first law of thermodynamics.

Heat transfer can occur in a variety of ways: by conduction^[3], radiation,^[4] convection, net mass transfer, friction or viscosity,^[5] and by chemical dissipation.^[6][7][8]

In physics, especially in calorimetry, and in meteorology, the concepts of latent heat and of sensible heat are used. Latent heat is associated with phase changes, while sensible heat is associated with temperature change.

Because in physics it is by definition a transfer of energy, heat is always associated with a process of some kind, and "heat" is used interchangeably with "heat flow" and "heat transfer". In ordinary language, heat has a diversity of meanings, including temperature.^[9]

Contents 1 Overview 2 Definitions 2.1 Mechanisms of heat transfer 3 Notation and units 4 Estimation of quantity of heat 5 Internal energy and enthalpy 5.1 Path-independent examples for an ideal gas 6 Latent and sensible heat 7 Specific heat 8 Microscopic origin of heat 9 Entropy 10 Heat transfer in engineering 11 Practical applications 12 See also 13 Notes 14 References 14.1 Bibliography 15 External links

Overview

Heat may flow across the boundary of the system and thus change its internal energy.

Heat is defined as energy transferred to the system by thermal interactions. Heat flows spontaneously from systems of higher temperature to systems of lower temperature. When two systems come into thermal contact, they exchange thermal energy due to the microscopic interactions of their particles. When the systems are at different temperatures, the net flow of thermal energy is not zero and is directed from the hotter region to the cooler region, until their temperatures are equal and the net flow of energy is zero. Spontaneous heat transfer is an irreversible process, which leads to the systems coming closer to mutual thermodynamic equilibrium.

The first law of thermodynamics requires that the energy of an isolated system is conserved. To change the energy of a system, energy must be transferred to or from the system. For a closed system, heat and work are the mechanisms by which energy can be transferred. For an open system, total energy can be changed also by transfer of matter.^[10] Work performed on a system is, by definition ^[11], an energy transfer to the system that is due to a change to external or mechanical parameters of the system, such as the volume, magnetization, center of mass in a gravitational field.

In a thermodynamic sense, heat is never regarded as being stored within a system. Like work, it exists only as energy in transit from one system to another or between a system and its surroundings. When energy in the form of heat is added to a system, it is stored as kinetic and potential energy of the atoms and molecules in the system.^[12]

Definitions

Scottish physicist James Clerk Maxwell, in his 1871 classic Theory of Heat, was one of many who began to build on the already established idea that heat was something to do with matter in motion. This was the same idea put forwards by Sir Benjamin Thompson in 1798, who said he was only following on from the work of many others. One of Maxwell's recommended books was by John Tyndall Heat as a Mode of Motion. Maxwell outlined four stipulations for the definition of heat:

• It is something which may be transferred from one body to another, according to the second law of thermodynamics.
• It is a measurable quantity, and thus treated mathematically.
• It cannot be treated as a substance, because it may be transformed into something that is not a substance, e.g., mechanical work.
• Heat is one of the forms of energy.

Mechanisms of heat transfer

Referring to conduction, Partington writes: "If a hot body is brought in conducting contact with a cold body, the temperature of the hot body falls and that of the cold body rises, and it is said that a quantity of heat has passed from the hot body to the cold body."^[13]

Referring to radiation, Maxwell writes: "In Radiation, the hotter body loses heat, and the colder body receives heat by means of a process occurring in some intervening medium which does not itself thereby become hot."^[14]

From these empirically based ideas of heat and from other empirical observations, the notions of internal energy and of entropy can be derived, so as to lead to the recognition of the first and second laws of thermodynamics.^[15] This was the way of the historical pioneers of thermodynamics.^[16][17]

Notation and units

As a form of energy heat has the unit joule (J) in the International System of Units (SI). However, in many applied fields in engineering the British Thermal Unit (BTU) and the calorie are often used. The standard unit for the rate of heat transferred is the watt (W), defined as joules per second.

The total amount of energy transferred as heat is conventionally written as Q for algebraic purposes. Heat released by a system into its surroundings is by convention a negative quantity (Q < 0); when a system absorbs heat from its surroundings, it is positive (Q > 0). Heat transfer rate, or heat flow per unit time, is denoted by . This should not be confused with a time derivative of a function of state (which can also be written with the dot notation) since heat is not a function of state. Heat flux is defined as rate of heat transfer per unit cross-sectional area, resulting in the unit watts per square metre.

Estimation of quantity of heat

The quantity of heat transferred by some process can either be directly measured, or determined indirectly through calculations based on other quantities.

Direct measurement is by calorimetry and is the primary empirical basis of the idea of quantity of heat. The transferred heat is measured by changes in a body of known properties, for example, temperature rise, change in volume or length, or phase change, such as melting of ice.^[18][19]

Indirect estimations heat rely on the law of conservation of energy, and in particular cases on the first law of thermodynamics. Indirect estimation is the primary approach of many theoretical studies of heat.^[20][21][22]

Internal energy and enthalpy

In the case where the number of particles in the system is constant (closed systems), the first law of thermodynamics states that the differential change in internal energy dU of a system is given by an infinitesimal amount of heat δQ supplied to the system minus the infinitesimal amount of work δW exerted by the system:^{[note 1]}

This can also be interpreted as that δQ makes contributions to the internal energy and to the work done by the system:

The work, done by the system, includes boundary work, caused the expanding boundaries of the system (usually a piston), in addition to other work (e.g. shaft work performed by a compressor fan):

In this Section we will neglect the "other-work" contribution. The internal energy, U, is a state function. In cyclical processes, such as the operation of a heat engine, state functions return to their initial values after completing one cycle. Thus, the differential for the internal energy is an exact differential dU. The symbol for exact differentials is the lowercase letter d.

In contrast, neither Q nor W represents the state of the system. Thus, infinitesimal amounts of heat and work are inexact differentials, denoted by δQ and δW, respectively. The lowercase Greek letter delta, δ, is the symbol for inexact differentials. The integral of any inexact differential over the time it takes for a system to leave and return to the same thermodynamic state does not necessarily equal zero. However, if heat is supplied to a system in which no irreversible processes take place and which has a well-defined temperature T, the heat δQ and the temperature T form the exact differential

with S the entropy of the system. Likewise, with a well-defined pressure p behind the moving boundary, the work δW and the pressure p form the exact differential

with V the volume of the system. In general, for homogeneous systems,

If V is constant

and if p is constant

with H the enthalpy given by

Path-independent examples for an ideal gas

For a simple compressible system such as an ideal gas inside a piston, the internal energy change ΔU at constant volume and the enthalpy change ΔH at constant pressure are modeled by separate heat capacity values, which are C_V and C_p, respectively.

Constrained to have constant volume, the heat, Q, required to change its temperature from an initial temperature, T₀, to a final temperature, T_f, is given by

Allowing the system to expand or contract at constant pressure, the heat, Q, required to change its temperature from an initial temperature, T₀, to a final temperature, T_f, is given by

Here we used the definition of the enthalpy and the fact that p is constant. When integrating an exact differential (e.g. dU), the lowercase letter d is substituted for Δ (e.g. ΔU). Note that the symbol Δ is convenient since it is compact, but it can lead to sign errors. So it may be better to write U_f - U₀ instead of ΔU.

When integrating an inexact differential (e.g. δQ), the lowercase Greek letter δ is removed with no replacement (e.g. Q).

Latent and sensible heat

Joseph Black

In an 1847 lecture entitled On Matter, Living Force, and Heat, James Prescott Joule characterized the terms latent heat and sensible heat as components of heat each affecting distinct physical phenomena, namely the potential and kinetic energy of particles, respectively.^[23] He described latent energy as the energy possessed via a distancing of particles where attraction was over a greater distance , i.e. a form of potential energy, and the sensible heat as an energy involving the motion of particles or what was known as a living force. At the time of Joule kinetic energy either held 'invisibly' internally or held 'visibly' externally was known as a living force.

Latent heat is the heat released or absorbed by a chemical substance or a thermodynamic system during a change of state that occurs without a change in temperature. Such a process may be a phase transition, such as the melting of ice or the boiling of water.^[24][25] The term was introduced around 1750 by Joseph Black as derived from the Latin latere (to lie hidden), characterizing its effect as not being directly measurable with a thermometer.

Sensible heat, in contrast to latent heat, is the heat exchanged by a thermodynamic system that has as its sole effect a change of temperature.^[26] Sensible heat therefore only increases the thermal energy of a system.

Consequences of Black's distinction between sensible and latent heat are examined in the Wikipedia article on calorimetry.

Specific heat

Specific heat, also called specific heat capacity, is defined as the amount of energy that has to be transferred to or from one unit of mass (kilogram) or amount of substance (mole) to change the system temperature by one degree. Specific heat is a physical property, which means that it depends on the substance under consideration and its state as specified by its properties.

The specific heats of monatomic gases (e.g., helium) are nearly constant with temperature. Diatomic gases such as hydrogen display some temperature dependence, and triatomic gases (e.g., carbon dioxide) still more.

Microscopic origin of heat

Heat is a macroscopic characteristic of systems, but like other thermodynamic quantities it has a microscopic explanation given by statistical mechanics. Temperature in many systems is the kinetic energy of motion of microscopic particles, and heat is the exchange of such energy. An early and vague expression of this was by Francis Bacon.^[27][28] Precise and detailed versions of it were developed in the nineteenth century.^[29]

Entropy

Main article: Entropy

Rudolf Clausius

In 1856, German physicist Rudolf Clausius defined the second fundamental theorem (the second law of thermodynamics) in the mechanical theory of heat (thermodynamics): "if two transformations which, without necessitating any other permanent change, can mutually replace one another, be called equivalent, then the generations of the quantity of heat Q from work at the temperature T, has the equivalence-value:"^[30][31]

In 1865, he came to define the entropy symbolized by S, such that, due to the supply of the amount of heat Q at temperature T the entropy of the system is increased by

and thus, for small changes, quantities of heat δQ (an inexact differential) are defined as quantities of TdS, with dS an exact differential:

This equality is only valid for a closed system and if no irreversible processes take place inside the system while the heat δQ is applied. If, in contrast, irreversible processes are involved, e.g. some sort of friction, then there is entropy production and, instead of the above equation, one has

This is the second law of thermodynamics for closed systems.

Heat transfer in engineering

A red-hot iron rod from which heat transfer to the surrounding environment will be primarily through radiation.

The discipline of heat transfer, typically considered an aspect of mechanical engineering and chemical engineering, deals with specific applied methods by which thermal energy in a system is generated, or converted, or transferred to another system. Although the definition of heat implicitly means the transfer of energy, the term heat transfer encompasses this traditional usage in many engineering disciplines and laymen language.

Heat transfer includes the mechanisms of heat conduction, thermal radiation, and mass transfer. In engineering, the term convective heat transfer is used to describe the combined effects of conduction and fluid flow and is often regarded as an additional mechanism of heat transfer. Although distinct physical laws may describe the behavior of each of these methods, real systems often exhibit a complicated combination which are often described by a variety of complex mathematical methods.

Practical applications

In accordance with the first law, heat may be converted to or from work.

Heat engines operate by converting heat flow from a high temperature reservoir to a low temperature reservoir into work. One example are steam engines, where the high temperature reservoir is steam generated by boiling water. The flow of heat from the hot steam to water is converted into mechanical work via a turbine or piston. Heat engines achieve high efficiency when the difference between initial and final temperature is high.

Heat pumps, by contrast, use work to cause thermal energy to flow from low to high temperature, the opposite direction heat would flow spontaneously. An example is a refrigerator or air conditioner, where electric power is used to cool a low temperature system (the interior of the refrigerator) while heating a higher temperature environment (the exterior). High efficiency is achieved when the temperature difference is small.

See also

Energy portal

• Effect of sun angle on climate
• Heat death of the Universe
• Heat diffusion
• Heat equation
• Heat exchanger
• Heat flux sensor
• Heat transfer coefficient
• History of heat
• Sigma heat
• Shock heating
• Thermal management of electronic devices and systems
• Thermometer
• Relativistic heat conduction
• Waste heat

Notes

1. ^ An alternate convention is to consider the work performed on the system by its surroundings. This leads to a change in sign of the work. This is the convention adopted by many modern textbooks of physical chemistry, such as those by Peter Atkins and Ira Levine, but many textbooks on physics define work as work done by the system.

References

1. ^ Reif, F. (1965), pp. 67, 73.
2. ^ Kittel, C. Kroemer, H. (1980). Thermal Physics, second edition, W.H. Freeman, San Francisco, ISBN 0-7167-1088-9, p. 227.
3. ^ Guggenheim, E.A. (1949/1967).
4. ^ Planck. M. (1914). The Theory of Heat Radiation, a translation by Masius, M. of the second German edition, P. Blakiston's Son & Co., Philadelphia.
5. ^ Lebon, G., Jou, D., Casas-Vásquez, J. (2008). Understanding Non-equilibrium Thermodynamics. Foundations, Applications, Frontiers, Springer, ISBN 978–3–540–74252–4, pages 120 and 62.
6. ^ Planck 1927.
7. ^ Partington, J.R. (1949)
8. ^ Guggenheim, E.A. (1949/1967)
9. ^ Oxford English Dictionary, second edition, Oxford University Press, Oxford, UK.
10. ^ Kondepudi, D. (2008)
11. ^ Reif, F. (1965), p. 73.
12. ^ Smith, J.M., Van Ness, H.C., Abbot, M.M. (2005). Introduction to Chemical Engineering Thermodynamics. McGraw-Hill. ISBN 0073104450. 
13. ^ Partington, J.R. (1949), p. 118.
14. ^ Maxwell, J.C. (1871), p. 10.
15. ^ Planck, M. (1903).
16. ^ Partington, J.R. (1949).
17. ^ Truesdell, C. (1980), page 15.
18. ^ Maxwell J.C. (1872), p. 54.
19. ^ Planck (1927), Chapter 3.
20. ^ Bryan, G.H. (1907), p. 47.
21. ^ C. Carathéodory (1909). "Untersuchungen über die Grundlagen der Thermodynamik". Mathematische Annalen 67: 355–386. A partly reliable translation is to be found at Kestin, J. (1976). The Second Law of Thermodynamics, Dowden, Hutchinson & Ross, Stroudsburg PA.. 
22. ^ Callen, H.B. (1960/1985). Thermodynamics and an Introduction to Thermostatistics, second edition, John Wiley & Sons, New York, ISBN 0-471-86256-8, Section 1–8.
23. ^ J. P. Joule (1884), The Scientific Paper of James Prescott Joule, The Physical Society of London, p. 274, "Heat must therefore consist of either living force or of attraction through space. In the former case we can conceive the constituent particles of heated bodies to be, either in whole or in part, in a state of motion. In the latter we may suppose the particles to be removed by the process of heating, so as to exert attraction through greater space. I am inclined to believe that both of these hypotheses will be found to hold good,—that in some instances, particularly in the case of sensible heat, or such as is indicated by the thermometer, heat will be found to consist in the living force of the particles of the bodies in which it is induced; whilst in others, particularly in the case of latent heat, the phenomena are produced by the separation of particle from particle, so as to cause them to attract one another through a greater space." , Lecture on Matter, Living Force, and Heat. May 5 and 12, 1847
24. ^ Perrot, Pierre (1998). A to Z of Thermodynamics. Oxford University Press. ISBN 0-19-856552-6. 
25. ^ Clark, John, O.E. (2004). The Essential Dictionary of Science. Barnes & Noble Books. ISBN 0-7607-4616-8. 
26. ^ Ritter, Michael E. (2006). "The Physical Environment: an Introduction to Physical Geography". http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/energy/energy_balance.html. 
27. ^ Bacon, F. (1620). Novum Organum Scientiarum, translated by Devey, J., P.F. Collier & Son, New York, 1902.
28. ^ Partington, J.R. (1949), page 131.
29. ^ Partington, J.R. (1949), pages 132–136.
30. ^ Published in Poggendoff’s Annalen, Dec. 1854, vol. xciii. p. 481; translated in the Journal de Mathematiques, vol. xx. Paris, 1855, and in the Philosophical Magazine, August 1856, s. 4. vol. xii, p. 81
31. ^ Clausius, R. (1865). The Mechanical Theory of Heat] –with its Applications to the Steam Engine and to Physical Properties of Bodies. London: John van Voorst, 1 Paternoster Row. MDCCCLXVII.

Bibliography

• Beattie, J.A., Oppenheim, I. (1979). Principles of Thermodynamics, Elsevier, Amsterdam, ISBN 0–444–41806–7.
• Bryan, G.H. (1907). Thermodynamics. An Introductory Treatise dealing mainly with First Principles and their Direct Applications, B.G. Teubner, Leipzig.
• Guggenheim, E.A. (1967) [1949], Thermodynamics. An Advanced Treatment for Chemists and Physicists (fifth ed.), Amsterdam: North-Holland Publishing Company. 
• Haase, R. (1971). Survey of Fundamental Laws, chapter 1 of Thermodynamics, pages 1–97 of volume 1, ed. W. Jost, of Physical Chemistry. An Advanced Treatise, ed. H. Eyring, D. Henderson, W. Jost, Academic Press, New York, lcn 73–117081.
• Kondepudi, D. (2008), Introduction to Modern Thermodynamics, Chichester UK: Wiley, ISBN 978–0–470–01598–8. 
• Kondepudi, D., Prigogine, I. (1998). Modern Thermodynamics: From Heat Engines to Dissipative Structures, John Wiley & Sons, Chichester, ISBN 0–471–97393–9.
• Maxwell, J.C. (1871), Theory of Heat (first ed.), London: Longmans, Green and Co. 
• Partington, J.R. (1949), An Advanced Treatise on Physical Chemistry., volume 1, Fundamental Principles. The Properties of Gases, London: Longmans, Green and Co. 
• Planck, M. (1903) [1897] (in English), Treatise on Thermodynamics (first ed.), London: Longmans, Green and Co., http://www.onread.com/reader/145819 
• Planck, M. (1927) [1923] (in English), Treatise on Thermodynamics (third ed.), London: Longmans, Green and Co. 
• Reif, F. (1965). Fundamentals of Statistical and Thermal Physics. New York: McGraw-Hll, Inc.. 
• Truesdell, C. (1980). The Tragicomical History of Thermodynamics 1822-1854, Springer, New York, ISBN 0–387–90403–4.

External links

• Heat on In Our Time at the BBC. (listen now)
• Plasma heat at 2 gigakelvins - Article about extremely high temperature generated by scientists (Foxnews.com)
• Correlations for Convective Heat Transfer - ChE Online Resources
• An Introduction to the Quantitative Definition and Analysis of Heat written for High School Students

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

Dansk (Danish)
n. - varme, hede, temperatur, ophidselse, brunst, heat, løb
v. tr. - varme, opvarme, gøre hed
v. intr. - blive varm, blive hed

idioms:

• heat barrier    varmemur
• heat capacity    varmekapacitet, opvarmningsevne
• heat of the moment    et øjebliks ophidselse
• heat shield    varmeskjold
• heat stroke    hedeslag
• heat up    opvarme
• heat wave    hedebølge
• in heat    brunstig, i løbetid
• the heat of something    varmen fra noget, højdepunktet for et eller andet
• the heat of the day    dagens højdepunkt

Nederlands (Dutch)
hitte, vuur, kookplaat, voorronde (sport), loopsheid, drift, druk, temperatuur, verhitten, verwarmen, stoken, opwinden, aan de kook brengen

Français (French)
n. - (gén, Phys, Météo) chaleur, (Culin) feu, température, chauffage, (Sport) épreuve éliminatoire, série, (Zool) en chaleur, (fig) véhémence
v. tr. - (gén) chauffer, (Culin) faire chauffer, (Méd) échauffer (le sang)
v. intr. - chauffer

idioms:

• heat barrier    mur thermique, mur de chaleur
• heat capacity    capacité calorifique
• heat shield    (Aérosp) bouclier thermique
• heat stroke    coup de chaleur (avec collapsus)
• heat up    chauffer, se réchauffer (l'air), faire chauffer, faire réchauffer
• heat wave    vague de chaleur
• in heat    en chaleur
• in the heat of the moment    dans le feu de l'action, au moment le plus chaud
• on heat    en chaleur
• take the heat    soulager, apaiser
• the heat of    la chaleur de
• the heat of the day    au moment le plus chaud de la journée

Deutsch (German)
v. - erhitzen, heizen, aufwärmen
n. - Hitze, Heizung, Druck, (Zool.) Brunst, Vorlauf, Wärme

idioms:

• heat barrier    (aer.) Hitzemauer
• heat capacity    Wärmekapazität
• heat shield    Wärmeschutz, (Astronaut) Hitzeschild
• heat stroke    Hitzschlag
• heat up    erwärmen
• heat wave    Hitzewelle
• in heat    läufig, brünstig
• in the heat of the moment    die Hitze des Gefechts
• on heat    brünstig werden/sein, rossig werden/sein, läufig werden/sein, rollig werden/sein
• take the heat    eine Situation entschärfen
• the heat of    die Hitze des
• the heat of the day    heißester Teil des Tages

Ελληνική (Greek)
v. - θερμαίνω/-ομαι, ζεσταίνω/-ομαι, εξάπτω, ανάβω
n. - θερμότητα, ζέστη, κάψα, λάβρα, καύσωνας, έξαψη, πάθος, φλόγα, ενθουσιασμός, οργή, οργασμός, σεξουαλική διέγερση, (αθλοπ.) αγώνας δρόμου, κούρσα, προκριματικός αγώνας, (αστυνομικοί) μπάτσοι, μπασκιναρία

idioms:

• heat barrier    θερμό κύμα
• heat capacity    (φυσ.) θερμοχωρητικότητα
• heat of the moment    η έξαψη της στιγμής
• heat shield    θερμανθεκτική ασπίδα
• heat stroke    (παθολ.) θερμοπληξία
• heat up    ανάβω, υπερθερμαίνω/-ομαι
• heat wave    (μετεωρ.) κύμα καύσωνα
• in heat    σε οργασμό
• the heat of something    στη φούντωση, στο φόρτε
• the heat of the day    στη λάβρα της ημέρας

Italiano (Italian)
scaldare, riscaldare, calore, gara, calura

idioms:

• heat barrier    barriera del calore
• heat capacity    capacità termica
• heat of the moment    nella foga del momento
• heat shield    schermo anti-calore
• heat stroke    colpo di sole
• heat up    riscaldarsi
• heat wave    ondata di caldo
• in heat    in calore
• the heat of    il centro di
• the heat of the day    il momento più caldo della giornata

Português (Portuguese)
v. - esquentar
n. - calor (m), ardor (m), cio (m), inflamação (f), coerção (f)

idioms:

• heat barrier    barreira (f) térmica (Téc.)
• heat capacity    capacidade (f) térmica (Téc.)
• heat of the moment    moda (f)
• heat shield    isolamento (m) térmico (Téc.)
• heat stroke    ataque (m) de insolação (Med.)
• heat up    aquecer
• heat wave    onda (f) de calor
• in heat    no cio
• the heat of    o calor de
• the heat of the day    a notícia quente do dia

Русский (Russian)
зной, повышенная температура, нагрев, горячность, что-л. сделанное за один прием, пыл, нагревать, накаливать, возбуждать

idioms:

• heat barrier    тепловой барьер
• heat capacity    теплоемкость
• heat of the moment    сгоряча
• heat shield    теплозащитный экран
• heat stroke    тепловой удар
• heat up    подогревать
• heat wave    тепловая волна
• in heat    в разгаре
• the heat of    разгар чего-л.
• the heat of the day    разгар дня, самые жаркие часы дня

Español (Spanish)
n. - calor, calefacción, eliminatoria, serie, temperatura, fiebre, picante, pasión, acción intensa, esfuerzo único e intenso, (jer) presión policiaca, (jer) la policía, (zool) celo
v. tr. - calentar, recalentar, calor, calefacción, eliminatoria, serie
v. intr. - calentarse, excitarse

idioms:

• heat barrier    barrera del calor
• heat capacity    capacidad térmica
• heat shield    escudo contra el calor
• heat stroke    insolación
• heat up    calentar, acalorar
• heat wave    ola de calor
• in heat    en celo, encelado
• in the heat of the moment    mientras tiene la sangre caliente, en el calor del momento
• on heat    en celo
• take the heat    llevarse la culpa
• the heat of    momento de máxima acción
• the heat of the day    cuando el sol aprieta, las horas de más calor

Svenska (Swedish)
v. - upphetta (äv. bildl.), elda, hetsa
n. - hetta, värme, iver, lopp, brunst, tryck, smälta (bergsvet.), polis, skjutvapen, påtryckningar

中文（简体）(Chinese (Simplified))
热, 高潮, 热度, 把...加热, 使激动, 加热, 发热, 激昂

idioms:

• heat barrier    热障, 速度极限
• heat capacity    热容量
• heat of the moment    一时的激动, 盛怒之下
• heat shield    热屏蔽, 挡热板
• heat stroke    中暑
• heat up    加热
• heat wave    热浪, 热波
• in heat    发情, 发情期
• the heat of something    ...的白热阶段
• the heat of the day    在大太阳下

中文（繁體）(Chinese (Traditional))
n. - 熱, 高潮, 熱度
v. tr. - 把...加熱, 使激動
v. intr. - 加熱, 發熱, 激昂

idioms:

• heat barrier    熱障, 速度極限
• heat capacity    熱容量
• heat of the moment    一時的激動, 盛怒之下
• heat shield    熱遮罩, 擋熱板
• heat stroke    中暑
• heat up    加熱
• heat wave    熱浪, 熱波
• in heat    發情, 發情期
• the heat of something    ...的白熱階段
• the heat of the day    在大太陽下

한국어 (Korean)
n. - 더위, 열, 온도, 열렬함
v. tr. - 뜨겁게 하다, 흥분 시키다
v. intr. - 뜨거워지다, 흥분하다

idioms:

• heat up    데우다
• in heat    암내가 나서
• the heat of something    ~의 열
• the heat of the day    한낮 더위

日本語 (Japanese)
n. - 熱さ, 熱, 温度, 暑さ, 暖気, 熱烈さ, 興奮, 紅潮, 盛り, 一回, 熱情
v. - 熱くする, 熱くなる, 暖める, 暖まる, 興奮させる, 憤激させる, 熱する, 激させる

idioms:

• heat barrier    熱障壁, 熱の障壁
• heat capacity    熱容量
• heat of the moment    勢い, 時のはずみ
• heat shield    熱遮蔽, 熱シールド
• heat stroke    熱中症
• heat up    暖める, 加熱する
• heat wave    長期にわたる酷暑, 熱波
• in heat    盛りがついて, 激怒して, 温床に
• the heat of    温度, 真っ最中
• the heat of the day    炎天下で

العربيه (Arabic)
‏(فعل) يسخن, يحمي, يثير (الاسم) احترار, حماوة, حرارة, طقس حار, توقد, تأجج, اهتياج, انفعال, حدة في المذاق, تشدد في تطبيق القانون, ضغط للتأثير في مجرى الأحداث, ذروة, معمعان, وطيس‏

עברית (Hebrew)
n. - ‮חום, להט, תחרות מוקדמת, שיזוף, טעם חריף, התקופה הקולטנית של המעגל המיני אצל יונקים ממין נקבה‬
v. tr. - ‮חימם, הלהיט, ריגש‬
v. intr. - ‮התחמם‬