The Nature and Sources of 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.
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.
Heat is a particular form of energy; it is that form which is transferred from regions of higher temperature to those of lower temperature until thermal equilibrium is reached. The general term "energy" was first used in 10 1807 by the Englishman, Thomas Young, from the Greek meaning "work." This was the first time the two concepts had been connected. Currently the accepted definition of energy is "capacity to do work." Energy is often termed either kinetic (energy of motion) or potential ("stored" energy). The symbol Q is normally used for all forms of energy, and its unit is the joule (J). English units of heat energy are British thermal units (BTU). Energy may be conveniently, although arbitrarily, subdivided into categories, including heat. One of the most common subdivisions results in the following six general categories:
- Mechanical Energy
- Heat Energy
- Electrical Energy
- Chemical Energy
- Radiant Energy
- Nuclear Energy
All these forms of energy are interrelated. A machine (mechanical) with friction (heat) can drive a generator (electrical) which powers a light (radiant) and which feeds back into a backup battery (chemical) all fueled by oil supplied by decomposed plants, originally grown in light from the sun (nuclear). Precise relationships define the conversion of energy from one form to another. Mechanical energy, for instance, of 778 foot pounds equals one BTU of heat energy, as demonstrated by the following tale from the highly recommended book, Sunspots, by Steve Baer.
"Jacking up a car to change a tire requires increasing the potential energy of the car one or two BTU (lifting 1000 pounds one foot or so). You are not able to store all the work you have done as potential energy, since some of the work goes into friction. But the stored potential energy is all you have to show for your work, since the warm parts of the jack won't do you much good.
Drink a pint of beer. The beer is largely water and, therefore, will take close to one BTU for each I Fahrenheit degree that it rises in temperature. You can drink the beer at 40 °F and it warms in your stomach to 98 °F or 99 °F. This takes 58-59 BTU, which is an enormous amount of energy compared to the raised automobile."
The lines between these somewhat arbitrary classifications of energy blur at times. Heat energy , for instance, can be transferred as radiant energy and some forms of radiant energy, upon absorption, will change to heat energy. Energy, regardless of how we divide it into categories, is still the "capacity to do work."
A number of physical changes are associated with the change of temperature of a substance. Almost all substances expanded 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. 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. 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. 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 11 forces holding the particles of ice together and is stored as energy in the water. To melt 1g of ice, 79.7 cal are needed, and to convert 1g of water to steam at 100°C, 541 cal are needed.
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.
Other Properties of Heat
In the English system of measurements, the British thermal unit (BTU) 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.
Old and New Concepts of Heat
Physicists now regard heat as being related somewhat to mechanical motion. The modern kinetic theory holds that heat as it exists in matter is simply "molecules in motion." This was not established, however, until after centuries of working with mistaken concepts.
The ancients thought that heat was an element. Empedocles proposed that the roots of all things are the four elements--fire (heat), air, water, and earth. Therefore heat could only be analyzed as something that flows into and out of substances. Even as brilliant a scientist as Lavoisier, toward the end of the 18th century, considered heat as something of this sort which he called caloric.
Experiments, however, constantly disproved this view. Weighing matter when it was hot, then cold, showed no change that would correspond to a flow of caloric in or out of the substance. Eventually a group of physicists reasoned that motion was transformed into heat (as when a bullet strikes through a piece of wood), and they developed the view that heat was not an element or a substance. It was produced rather by the motion of the invisible particles which compose matter and which we now call molecules. Many different experiments showed this to be true and thus established the kinetic theory that heat is energy associated with molecular movement.
This theory proved satisfactory for the heat contained in matter. At the same time physicists developed a theory that radiant heat passing through empty space (infrared radiation) was a type of "wave in ether." The 12 waves were emitted by a hot body such as the sun; and when they struck matter (as on the Earth) they stimulated the molecules in the matter to greater motion. This "heated" the matter.
According to this theory, heat that was emitted and absorbed could be any amount between the "hottest" and "coldest" levels of the bodies concerned. In 1900, however, Max Planck forced a change in this view. He had experimented with a black body radiator, a hollow object which absorbed heat energy sent into it through a hole, then reradiated the energy somewhat as iron would. As iron is heated more and more, it first gets hot, then glows with dull red light, and finally becomes white hot--meaning that it is emitting every wave length of light in the spectrum as well as radiant heat and ultraviolet radiation.
The view that heat energy is infinitely divisible required a certain distribution of energy between the wave lengths of reradiated energy. Planck found an entirely different distribution. He explained this by saying that radiant heat energy is not infinitely divisible. Instead, it travels in particles and not in waves; and there is a certain smallest particle which cannot be divided. Planck called this smallest particle a quantum.
HEAT vs. TEMPERATURE
We define heat as that form of energy which is transferred from regions of higher temperature to those of lower temperature, or transferred when an object changes temperature. It is a quantity of energy, measured in British thermal units (BTU) or calories (C). One BTU=252 calories. • The heat content of a system is the total internal energy of the molecules making it up. • A BTU is defined as the amount of energy needed to raise the temperature of one pound of water one Fahrenheit degree. One wooden kitchen match, burned entirely, gives off approximately one BTU of heat energy.
In the metric system 1 calorie (c) of heat energy is needed to raise 1 gram of water one degree Centigrade. One BTU = 252 calories.
Heat is a measurement of quantity.
Temperature is a measurement of degree.