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.

« 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, andthe 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C and a boiling point of 100° C, is widely used throughout the world, particularly for scientific work, although it was superseded officially in 1950 by the international temperature scale.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, usedin 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 theboiling 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 isequivalent 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 WaterPhase diagrams, like this phase diagram for water, show whether a substance exists as a vapor, liquid, or solid at a giventemperature and pressure.

The point where the three lines intersect in a phase diagram shows the pressure andtemperature 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 thermodynamicprinciples.

The international scale is based on the property of electrical resistivity, with platinum wire as the standard for temperature between -190° and 660° C.

Above660° 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 twowires 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 wavelengthemitted 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 internationalagreement 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 theabsolute thermodynamic scale.

In cryogenics, or low-temperature research, temperatures as low as 0.003 K have been produced by the demagnetization ofparamagnetic 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.

Inmechanical 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 themechanical work necessary to produce 1 cal is known as the mechanical equivalent of heat.

It is equal to 4.1855 × 10 7 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 wasfirst 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 inwater 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 energyis 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 whencooled.

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 aseither 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 atconstant temperature.

The amount of heat required to produce a change of phase is called latent heat, and hence, latent heats of sublimation, melting, and vaporizationexist ( 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 tosteam 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 particlesof 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. »