HEAT

All living things need heat. Heat is a form of energy that is transferred from one body to another because of a difference in temperature. It is the cause of certain natural changes which occur in an endless cycle.

Tropical areas receive more heat from the sun than do polar regions. The tropical atmosphere is hotter than the atmosphere in other areas. As a result the warm tropical air moves toward the poles. This starts a global movement, or circulation, of air. The global air movement has a great deal to do with what man does on Earth. Oceans too receive differing amounts of heat. This results in the flow of water masses which move continuously through the Earth's oceans.

The Earth not only receives heat as radiation from the sun, but it also radiates heat back into space. Therefore a mammoth heat balance is maintained between the sun and the Earth. This balance keeps the Earth from getting so hot or so cold that life could not exist.

The amount of heat from the sun that falls on a region determines the temperature range of the region. The temperature of the atmosphere, in turn, determines whether the moisture of the region will be in the form of water or ice. The amount and kinds of plant and animal life depend upon the temperature of the environment. Bacteria that cause disease grow more rapidly at one temperature than at another. Thus the temperature influences the problem of disease in some areas.

The amount of heat supplied to a location also affects man's actions directly. It is a factor in determining where he will live, work, and build his civilization. A dependable supply of heat energy is one of the very important factors in making our life and our world what they are.

The Nature and Sources of Heat

Heat is so well known from our earliest childhood that we hardly think about it. The effect of heat burning can be detected easily, but it is harder to understand what heat itself actually is. Heat cannot be weighed, nor can it be seen or heard. However, the scientific study of heat has given us many facts about what it is and how it acts.

The kinetic theory of matter provides a basis for a definition of heat. According to this theory all matter is made of atoms and molecules in constant motion. When energy is absorbed by matter, the random internal energy and the motion of these atoms and molecules are increased. The increase is of two kinds an increase in straight-line motion and in rotational motion of the atom about its own axis. This increase makes itself felt in the form of heat, and when it occurs the temperature of the matter rises.

Firing a bullet against a metal target is an example of converting (changing) one kind of energy to heat. The explosion of the shell imparts kinetic energy in the form of motion to the bullet. When the bullet strikes the target, it is stopped in a split second. Where has its tremendous energy gone? The answer to this question is that the energy of motion has been transferred to the random motion of the atoms that make up the bullet and the target. The motion of the atoms is speeded up and heat is produced. This can be verified by measuring the temperature of the target and the bullet. The temperature rise is enough to melt the metals temporarily.

Since heat is necessary for life, it is important to know where it comes from and how it can be used. The most important source of heat for our Earth is the radiation from the sun. A part of this radiation is absorbed by the Earth. This keeps the temperature of the Earth's surface and atmosphere at a level which permits life to continue.

Of the quantity of heat energy radiated, the largest amount is received directly below the sun at the equator. As one moves away from the equator, the amount of heat received from the sun decreases. For this reason tropical areas are warm and polar regions are cold.

The Heating Values of Fuels

The second most important source of heat is the store of natural fuels that are on and in the Earth. The most important of these fuels are coal, oil, gas, and wood. These substances, however, do not provide heat constantly and automatically as the sun does. They are composed of carbon, hydrogen, and other elements. When a fuel is raised to a certain temperature, it reacts chemically with oxygen. We call the reaction burning, or combustion. This reaction releases a large quantity of heat.

Other Useful Sources of Fuel

Electrical energy can be converted into heat also. This change of energy is made use of in many familiar appliances. The heat we get from electric heaters, toasters, flatirons, and electric dryers is produced by converting electrical energy to heat energy.

Many chemical reactions produce heat that is not due to oxidation. It is due to a conversion of chemical energy to heat as a reaction takes place. Some common reactions that are accompanied by heat are those that occur when acids or bases are mixed with water. For the future, the most important source of controllable heat may be the energy released when an atomic nucleus is split.

The Meaning of Temperature

Temperature is the property that gives physical meaning to the concept of heat. If an object is cold, we say it has a low temperature. If it is hot, we say it has a high temperature. It can also be observed that if a hot poker is plunged into cold water, the poker becomes cooler and the water becomes warmer. This means that the hot body gives up some of its heat to the cold body. The exchange of heat will continue until the water and the poker have the same temperature. Thus the temperature of a substance will determine whether heat flows from it or to it when the substance is in contact with another body at a different temperature.

For accuracy it is necessary to have a definition of temperature that is based on some value which does not change. There is such a value for temperature. It is called absolute zero. The idea of absolute zero first appeared in 1802. The chemist Joseph L. Gay-Lussac found that all gases, when heated through one degree, expand by 1/273 of the volume that they occupy at the freezing point of water. It was reasoned that if the gas were cooled, its volume would decrease by the same amount as the temperature decreased.

Therefore if we assume the freezing point of water is 0 and the gas is cooled, its volume will shrink to zero at 273 degrees below 0 degrees. Further study has supported the idea of an absolute zero. It is now defined as the temperature at which all molecular and atomic motion stops completely. In this sense, then, the temperature of a substance is a measure of the intensity of motion of all atoms and molecules in the substance. Absolute zero is also the temperature below which it is impossible to go.

The Measurement of Temperature

To measure temperature exactly it is necessary to design and construct a thermometer scale. First, two natural events, each of which always occurs at the same temperature, are selected. The freezing point of water and the boiling point of water are two such fixed points. They can be reproduced easily. Then a number which indicates a temperature is arbitrarily assigned to each of these fixed points (32 and 212 F or 0 and 100 C). Finally the interval between these points is divided into a fixed number of equal degrees. A temperature below zero is marked negative.

Two temperature scales, based on the fixed points of boiling water and freezing water, are in general use. The Fahrenheit scale is used for engineering and household purposes. The Celsius scale, formerly called the centigrade scale, is universally used for scientific measurement. Each of these scales has a corresponding absolute scale based on the absolute zero. The absolute Celsius scale is called the Kelvin scale. The absolute Fahrenheit scale is called the Rankine scale.

It is often necessary to convert a temperature on one scale to a corresponding temperature on another. Some useful conversion relations are:

Fahrenheit to Celsius C = 5/9 ( F - 32)

Celsius to Fahrenheit F = 9/5 C + 32

Celsius to Kelvin K = C + 273

Fahrenheit to Rankine R = F + 460

Heat and Work

There is a direct relationship between heat and work. For example, a body does work to bring a moving body to rest. The kinetic energy of the moving body is changed into energy of motion within the molecules of both bodies. This kind of work is called friction. The increase in molecular motion results in an increase in the amount of heat contained in these bodies, and their temperatures rise.

The exact relationship which exists between heat and work has been determined. It is called the mechanical equivalent of heat. One calorie of heat energy equals 4.1840 joules of mechanical energy.

Other Properties of Heat

The specific heat, or heat capacity, of a substance is the amount of heat that is required to raise the temperature of a unit weight of the substance by one degree. The value for specific heat varies widely for different substances. In the metric system the unit of specific heat is the calorie. It is defined as the amount of heat that is required to raise the temperature of one gram of water by one degree Celsius. The specific heat of water is set at 1.000 calorie per gram. All other values are based on this unit. The relationship between temperature and specific heat is shown.

In the English system of measurements the British thermal unit (B.T.U.) is the unit of specific heat. One British thermal unit (252 calories) is the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. In measuring the heat content of fuels the British thermal unit is the unit of specific heat used. The heat of reaction is the quantity of heat that is absorbed or lost by the surroundings when a chemical reaction takes place. Q is the symbol for the heat of reaction. If heat is lost, Q is a positive number and the reaction is called exothermic. If heat is absorbed, Q is a negative number and the reaction is called endothermic.

A measurement of the heat of reaction can be made with an instrument called a calorimeter, a vessel placed in a larger vessel filled with water. This reaction vessel is provided with a sensitive thermometer, and the larger vessel is insulated from the surroundings. A weighed amount of the substance under test is completely burned in the reaction vessel. The rise in temperature of the water is measured. Since the amount of water and the rise in temperature are known, the amount of heat produced can be calculated as the heat of reaction or combustion.

The latent heat of vaporization is the amount of heat absorbed on vaporization at boiling temperature. For water this value is 539.6 calories per gram. The latent heat of fusion is the amount of heat required to change a crystal into a liquid at the melting point. For water this value is 79.7 calories per gram.

Expansion and Contraction with Heat

All matter increases in volume when there is an increase in temperature. In the case of gases the increase is a large one. If the pressure and the weight of gas remain the same, the increase in volume will be in direct proportion to the increase in temperature. The application of heat to a solid causes it to expand also but to a much smaller degree than a gas. In a metal rod every unit length of the rod becomes longer when it expands. The increase in length for each unit of length per degree rise in temperature is called the coefficient of linear expansion. Liquids in general behave like solids and expand slightly when the temperature is raised.

The Transfer of Heat

Heat transfer helps to shape the world in which we live. Great loss is suffered by man when heat transfer is impossible. If a way could be found to transfer heat to the polar regions they could support large populations just as the temperate countries do. Fortunately man has been more successful in making use of the natural methods of heat transfer on a smaller scale. A quantity of heat is useless if it is where it is not needed. It may be useful if it can be moved to another place.

Heat, by its very nature, helps to make this possible. Heat always travels, or flows, from a high temperature to a low temperature. It can do this by three different methods. These are called conduction, convection, and radiation.

Conduction

Conduction is a point-by-point process of heat transfer. If one part of a body is heated by direct contact with a source of heat, the neighboring parts become heated successively. Thus, as shown in the diagram, if a metal rod is placed in a burner, heat travels along the rod by conduction. This may be explained by the kinetic theory of matter. The molecules of the rod increase their energy of motion. This violent motion is passed along the rod from molecule to molecule.

In considering the flow of heat by conduction, it is sometimes helpful to compare the flow of heat to the flow of electricity. The temperature difference can be thought of as the pressure, or voltage, in an electrical circuit. The ability of a substance to transfer heat (its thermal conductivity) can be compared to electrical conductivity. When the temperature difference (or voltage) between two points is great, the driving force to move heat (or current) is high. The quantity of heat (or current) transferred will depend upon the temperature difference (or voltage difference) and the resistance to the flow of heat (or current) offered by the conductor.

Convection

The method of heat transfer called convection depends upon the movement of the material which is heated. It applies to free-moving substances; that is, liquids and gases. The motion is a result of changes of density that accompany the heating process. Water in a tea kettle is heated by convection. A stove heats the air in a room by convection.

When a liquid or gas is heated, its density (mass per unit volume) decreases; that is, it becomes lighter in weight. A warmer volume of gas will rise while a colder, and thus heavier, volume of gas will descend. This process is described as natural convection. A familiar example of natural convection is the circulation of air from a hot-air furnace. When a liquid or gas is moved from one place to another by some mechanical force, the process is known as forced convection. The circulation of air by an electric fan is an example of forced convection.

Radiation

The third method of transferring heat energy from one place to another is called radiation. This process begins when the internal energy of a system is converted into radiant energy at a source such as a heater. This energy is transmitted by waves through space, just as the sun radiates heat outwards through the solar system. Finally the radiant energy strikes a body where it is absorbed and converted to internal energy. It then appears as heat. An electric heater produces radiant energy in this way. It may be absorbed, reflected, or transmitted by a body in its path. When the radiant energy is absorbed, the internal energy of the body increases and its temperature rises.

All bodies, whether hot or cold, radiate energy. The hotter a body is, the more energy it radiates. Furthermore all bodies receive radiation from other bodies. The exchange of radiant energy goes on continuously. Thus a body at constant temperature has not stopped radiating. It is receiving energy at the same rate that it is radiating energy. There is no change in internal energy or temperature.

Heat transfer by radiation is not proportional to the difference in temperature between the hot and cold objects as it is in the case of heat transfer by conduction and convection. It is proportional to the difference between the fourth powers of the absolute temperatures of the two objects. Thus heat transfer by radiation is enormously more effective at high temperatures than at low temperatures. Radiation transfer depends also upon the shape of the radiating object.

Thermodynamics and the Theory of Heat

The nature of heat has been a major subject for study since the beginnings of modern science. Early investigators, including Galileo, Boyle, and Newton, explained heat as the motion of tiny particles of which bodies are made. In the 18th century scientists advanced understanding by concentrating on the flow of heat. It was thought of as a fluid, and experiments were made on the heat conductivity of metals.

Antoine Lavoisier attempted to develop a quantitative theory of heat. He showed that the heat produced in chemical reactions could be studied quantitatively. He developed a system of thermodynamics that helped to explain the relations between heat and chemical reactions. Lavoisier's theories were still based on the idea that heat was a fluid.

Kinetic Theory

In 1798 the physicist Benjamin Thompson (Count Rumford) revived the kinetic theory of heat. He became interested in the subject by observing the vast quantities of heat produced by friction during the boring of a cannon. Thompson decided that heat was not a material fluid but the result of a conversion of energy. Forty years later, the British physicist James P. Joule also proved that heat was a form of energy. Joule also proved the equivalence of mechanical energy and heat. He concluded that the amount of work required to bring about any given energy exchange was independent of the kind of work done, the rate of work, or the method of doing it. Therefore, in an isolated system, work can be converted into heat at a ratio of one to one. This discovery later became known as the First Law of Thermodynamics.

Nicolas L.S. Carnot did research on heat that explained the mechanics of the flow of heat from a hot region to a cooler region. Lord Kelvin used Carnot's concepts to develop his absolute thermodynamic temperature scale. By applying mathematics, Willard Gibbs and Ludwig Boltzmann refined thermodynamics into an exact science.

Source: Compton's Interactive Encyclopedia