Heat (physics)

I

Introduction

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

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.

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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.

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.