Part 1: The Nature of Heat (1975)
Part 2: Heating Conduction (1975)
Part 3: Heat Conduction and Wildland Fire (1976)
Part 4: Radiation (1976)
Part 5: Radiation and Wildland Fire (1976)
Three ingredients are essential for a wildland fire to start and to burn. First, there must be burnable fuel available. Then enough heat must be applied to the fuel to raise its temperature to the ignition point. And finally, there must be enough air to supply oxygen needed to keep the combustion process going and thus maintain the heat supply for ignition of unburned fuel. These three indispensable ingredients—fuel, heat, and oxygen—make up the fire triangle. All must be present if there is to be fire. In the following discussion, we will examine some of the basic characteristics of the heat segment of the fire triangle—the nature of heat itself.
Everyone knows how heat feels, and is well aware of its many applications in daily life. Heat from the sun is the basic control of our weather, and this heat is also essential in the growing of food crops and other vegetation. Without the sun's heat, life could not exist on the earth.
Through the science of thermodynamics, heat has a place in many of the industrial processes that bring us the conveniences of modern life. But as recently as 200 years ago, the true nature of heat was not understood. In the early days of science the phenomena associated with heat were ascribed to a mystical and intangible fluid called "caloric." This fluid was believed to have the power of penetrating and expanding materials, sometimes melting or dissolving them, and converting some substances to vapor. Heat produced by friction or the compression of gases was attributed to stored caloric that was squeezed or ground out of the material. The caloric fluid was considered intangible, since even the most careful experiments in adding or subtracting heat by nondestructive heating or cooling of a substance failed to produce any changes in its weight.
We know now that heat is not a fluid, nor a substance at all, but is really one of the several forms of energy. Other common forms are electrical energy, radiant energy, chemical energy, and the mechanical or kinetic energy possessed by moving materials and objects, such as falling water or a rotating wheel. Atomic and nuclear energy are still other forms. Heat is often labeled thermal energy.
According to molecular theory, all substances are made up of molecules, and as long as the temperature of the substance is above absolute zero (-469°F), these molecules are in some degree of motion. When heat is applied to a substance, the molecular activity increases and the temperature rises. Conversely, if a substance loses heat, the molecular activity decreases and so does the temperature. We can think of heat, then, as the energy of molecules in motion.
Energy cannot be created or destroyed in any way that we know. But it can be changed from one form to another, and this transformation is constantly going on, both in nature and through the activities of man. For example, radiant energy from the sun is transformed by the process of photosynthesis to stored chemical energy in vegetation. When the vegetation is burned, the chemical energy is transformed to thermal energy, radiant energy, and the kinetic energy in the rising air in the convection column over the fire. The kinetic energy in falling water can be used to generate electrical energy, which in turn can be changed back to thermal and radiant energy by an electric heater, or to kinetic energy again through the use of an electric motor.
Because water is one of the most abundant compounds on earth and was believed to have constant characteristics, early scientists frequently used water as a standard for the measurement of characteristics of other materials. And the standard unit of heat was first based on the amount of heat needed to increase the temperature of water. In the metric system of measurement, the unit of heat was the calorie and was defined as the amount of heat required to increase the temperature of one gram of water by 1° C. The British Thermal Unit (Btu) was the heat unit in the English system, and was defined as the quantity of heat needed to raise the temperature of a pound of water (about a pint) by 1° F.
Unfortunately, water did not prove to be a good standard for heat measurement, for it was soon found that the amount of heat required to raise water temperature changes with the initial temperature of the water. The amount of heat needed to raise water temperature from 40° to 41° for example, is not the same as that needed to raise it from 90° to 91°. Because standards of electrical voltage and of resistance are maintained at national standardizing laboratories throughout the world, scientists agreed internationally to use the heat produced in one second by a current of one ampere through a resistance of one ohm as the standard unit of heat. This heat unit is called the joule, in honor of the early English physicist, James Joule.
Although the joule has been used as the standard unit of heat in most scientific work for a long time, the terms "calorie" and "Btu" have continued to be used in industrial and other practical applications, particularly in countries where the metric system of measurement has not yet been adopted. Because of the continued use of these terms, the calorie has now been arbitrarily defined as equal to 4.1840 joules, which is nearly equivalent to the quantity of heat needed to raise the temperature of a gram of water from 14.5° to 15.5° C. One Btu equals 1055 joules or 252 calories. But by definition and derivation, the calorie and Btu are no longer connected in any way with the properties of water.
