Heat (physics) I INTRODUCTION Heat Loss from a House A thermograph shows the large amount of heat lost through a house's windows during winter.

Heat (physics) I INTRODUCTION Heat Loss from a House A thermograph shows the large amount of heat lost through a house's windows during winter. Replacing conventional windows with double- or triple-paned windows cuts down the amount of heat that can escape from the house; this conserves energy and reduces heating bills. NASA/Science Source/Photo Researchers, Inc.. Heat (physics), in physics, transfer of energy from one part of a substance to another, or from one body to another by virtue of a difference in temperature. Heat is energy in transit; it always flows from a substance at a higher temperature to the substance at a lower temperature, raising the temperature of the latter and lowering that of the former substance, provided the volume of the bodies remains constant. Heat does not flow from a lower to a higher temperature unless another form of energy transfer, work, is also present. See also Power. Until the beginning of the 19th century, the effect of heat on the temperature of a body was explained by postulating the existence of an invisible substance or form of matter termed caloric. According to the caloric theory of heat, a body at a high temperature contains more caloric than one at a low temperature; the former body loses some caloric to the latter body on contact, increasing that body's temperature while lowering its own. Although the caloric theory successfully explained some phenomena of heat transfer, experimental evidence was presented by the American-born British physicist Benjamin Thompson in 1798 and by the British chemist Sir Humphry Davy in 1799 suggesting that heat, like work, is a form of energy in transit. Between 1840 and 1849 the British physicist James Prescott Joule, in a series of highly accurate experiments, provided conclusive evidence that heat is a form of energy in transit and that it can cause the same changes in a body as work. II TEMPERATURE Heat Flow Between Two Gases Two identical gases at different temperatures are separated by a barrier. The hotter gas consists of molecules with a larger average kinetic energy than the molecules of the cooler gas. When the gases are combined, the temperature of the mixture settles at an equilibrium temperature that is between the temperature of the hot gas and the temperature of the cold gas. The heat flows from the warmer gas to the cooler gas until the average kinetic energy of the two gases are the same. © Microsoft Corporation. All Rights Reserved. The sensation of warmth or coldness of a substance on contact is determined by the property known as temperature. Although it is easy to compare the relative temperatures of two substances by the sense of touch, it is impossible to evaluate the absolute magnitude of the temperatures by subjective reactions. Adding heat to a substance, however, not only raises its temperature, causing it to impart a more acute sensation of warmth, but also produces alterations in several physical properties, which may be measured with precision. As the temperature varies, a substance expands or contracts, its electrical resistivity (see Resistance) changes, and in the gaseous form, it exerts varying pressure. The variation in a standard property usually serves as a basis for an accurate numerical temperature scale (see below). Temperature depends on the average kinetic energy of the molecules of a substance, and according to kinetic theory (see Gases; Thermodynamics), energy may exist in rotational, vibrational, and translational motions of the particles of a substance. Temperature, however, depends only on the translational molecular motion. Theoretically, the molecules of a substance would exhibit no activity at the temperature termed absolute zero. See Molecule. III TEMPERATURE SCALES Five different temperature scales are in use today: the Celsius scale, known also as the Centigrade scale, the Fahrenheit scale, the Kelvin scale, the Rankine scale, and the international thermodynamic temperature scale (see Thermometer). The Celsius scale, with a freezing point of 0° C and a boiling point of 100° C, is widely used throughout the world, particularly for scientific work, although it was superseded officially in 1950 by the international temperature scale. In the Fahrenheit scale, used in English-speaking countries for purposes other than scientific work and based on the mercury thermometer, the freezing point of water is defined as 32° F and the boiling point as 212° F (see Mercury). In the Kelvin scale, the most commonly used thermodynamic temperature scale, zero is defined as the absolute zero of temperature, that is, -273.15° C, or -459.67° F. Another scale employing absolute zero as its lowest point is the Rankine scale, in which each degree of temperature is equivalent to one degree on the Fahrenheit scale. The freezing point of water on the Rankine scale is 492° R, and the boiling point is 672° R. Phase Diagram for Water Phase diagrams, like this phase diagram for water, show whether a substance exists as a vapor, liquid, or solid at a given temperature and pressure. The point where the three lines intersect in a phase diagram shows the pressure and temperature where the solid, liquid, and vapor all exist in equlibrium. This point, which occurs for water at 0.01°C (32.02°F), is known as the triple point. © Microsoft Corporation. All Rights Reserved. In 1933 scientists of 31 nations adopted a new international temperature scale with additional fixed temperature points, based on the Kelvin scale and thermodynamic principles. The international scale is based on the property of electrical resistivity, with platinum wire as the standard for temperature between -190° and 660° C. Above 660° C, to the melting point of gold, 1063° C, a standard thermocouple, which is a device that measures temperature by the amount of voltage produced between two wires of different metals, is used; beyond this point temperatures are measured by the so-called optical pyrometer, which uses the intensity of light of a wavelength emitted by a hot body for the purpose. In 1954 the triple point of water--that is, the point at which the three phases of water (vapor, liquid, and ice) are in equilibrium--was adopted by international agreement as 273.16 K. The triple point can be determined with greater precision than the freezing point and thus provides a more satisfactory fixed point for the absolute thermodynamic scale. In cryogenics, or low-temperature research, temperatures as low as 0.003 K have been produced by the demagnetization of paramagnetic materials. Momentary high temperatures estimated to be greater than 100,000,000 K have been achieved by nuclear explosions (see Nuclear Weapons). IV HEAT UNITS Heat is measured in terms of the calorie, defined as the amount of heat necessary to raise the temperature of 1 g of water at a pressure of 1 atm from 15° to 16° C. This unit is sometimes called the small or gram calorie to distinguish it from the large calorie, or kilocalorie, equal to 1000 cal, which is used in nutrition studies. In mechanical engineering practice in the United States and the United Kingdom, heat is measured in British thermal units, or Btu (see British Thermal Unit). One Btu is the quantity of heat required to raise the temperature of 1 lb of water 1° F and is equal to 252 cal. Mechanical energy can be converted into heat by friction, and the mechanical work necessary to produce 1 cal is known as the mechanical equivalent of heat. It is equal to 4.1855 × 107 ergs/cal or 778 ft-lb Btu. According to the law of conservation of energy, all the mechanical energy expended to produce heat by friction appears as energy in the objects on which the work is performed. This fact was first conclusively proven in a classic experiment performed by Joule, who heated water in a closed vessel by means of rotating paddle wheels and found that the rise in water temperature was proportional to the work expended in turning the wheels. If heat is converted into mechanical energy, as in an internal-combustion engine, the law of conservation of energy also applies. In any engine, however, some energy is always lost or dissipated in the form of heat because no engine is perfectly efficient. See Horsepower. V LATENT HEAT A number of physical changes are associated with the change of temperature of a substance. Almost all substances expand in volume when heated and contract when cooled. The behavior of water between 0° and 4° C (32° and 39° F) constitutes an important exception to this rule. The phase of a substance refers to its occurrence as either a solid, liquid, or gas, and phase changes in pure substances occur at definite temperatures and pressures (see Phase Rule). The process of changing from solid to gas is referred to as sublimation, from solid to liquid as melting, and from liquid to vapor as vaporization. If the pressure is constant, these processes occur at constant temperature. The amount of heat required to produce a change of phase is called latent heat, and hence, latent heats of sublimation, melting, and vaporization exist (see Distillation; Evaporation). If water is boiled in an open vessel at a pressure of 1 atm, the temperature does not rise above 100° C (212° F), no matter how much heat is added. The heat that is absorbed without changing the temperature of the water is the latent heat; it is not lost but is expended in changing the water to steam and is then stored as energy in the steam; it is again released when the steam is condensed to form water (see Condensation). Similarly, if a mixture of water and ice in a glass is heated, its temperature will not change until all the ice is melted. The latent heat absorbed is used up in overcoming the forces holding the particles of ice together and is stored as energy in the water. To melt 1 g of ice, 79.7 cal are needed, and to convert 1 g of water to steam at 100° C, 541 cal are needed. VI SPECIFIC HEAT Temperature Change Versus Heat Added: Water The graph represents the temperature change that occurs when heat is added to water. At 0° C and at 100° C, you can add heat to water without changing its temperature. This "latent heat" breaks bonds that hold the molecules together but does not increase their kinetic energy. Note that approximately seven times more heat must be added to evaporate one gram of water than to melt it. This is represented by the relative lengths of the horizontal portions of the graph. The slopes of the inclined lines represent the number of degrees that the temperature changes for each calorie of heat that is added to one gram. The reciprocal of this number is the amount of heat that must be added to make the temperature of one gram change by one degree. This is called the specific heat. © Microsoft Corporation. All Rights Reserved. The heat capacity, or the measure of the amount of heat required to raise the temperature of a unit mass of a substance one degree is known as specific heat. If the heating process occurs while the substance is maintained at a constant volume or is subjected to a constant pressure the measure is referred to as a specific heat at constant volume or at constant pressure. The latter is always larger than, or at least equal to, the former for each substance. Because 1 cal causes a rise of 1° C in 1 g of water, the specific heat of water is 1 cal/g/° C. In the case of water and other approximately incompressible substances, it is not necessary to distinguish between the constant-volume and constant-pressure specific heats, as they are approximately equal. Generally, the two specific heats of a substance depend on the temperature. VII TRANSFER OF HEAT Heat Transfer Heat can be transferred by three processes: conduction, convection, and radiation. Conduction is the transfer of heat along a solid object; it is this process that makes the handle of a poker hot, even if only the tip is in the fireplace. Convection transfers heat through the exchange of hot and cold molecules; this is the process through which water in a kettle becomes uniformly hot even though only the bottom of the kettle contacts the flame. Radiation is the transfer of heat via electromagnetic (usually infrared) radiation; this is the principal mechanism through which a fireplace warms a room. © Microsoft Corporation. All Rights Reserved. The physical methods by which energy in the form of heat can be transferred between bodies are conduction and radiation. A third method, which also involves the motion of matter, is called convection. Conduction requires physical contact between the bodies or portions of bodies exchanging heat, but radiation does not require contact or the presence of any matter between the bodies. Convection occurs when a liquid or gas is in contact with a solid body at a different temperature and is always accompanied by the motion of the liquid or gas. The science dealing with the transfer of heat between bodies is called heat transfer. Contributed By: Richard S. Thorsen Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.