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.
The joule is the standard unit of heat
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.
Molecular structure affects temperature change
Heat measurement units, as we have seen, are defined in terms of a temperature change brought about by heating. But the same amount of heat applied to different materials does not necessarily bring about the same temperature change—it is well known that some materials can be heated or cooled more quickly than others. These differences are the result of variations in the number of molecules in different substances and the way these building blocks of matter are put together. The variation in temperature change in different substances that accompanies the addition or subtraction of a given quantity of heat can be quite large. For example, the amount of heat needed to raise the temperature of a pound of water by 10° will increase the temperature of a pound of granite by 32° and that of a pound of iron by about 94°. Because of the variation in the amount of temperature rise when the same amount of heat is applied to different substances, a warm object may actually have less heat than a cooler one. Temperature indicates only the average molecular activity in a substance-its relative degree of hotness or coldness-and heat units must be used to measure the amount of heat it contains or can absorb.
The quantity of heat required to increase a unit weight of a substance by one degree is the specific heat of the substance, and is usually given as Btu per pound per degree Fahrenheit (Btu/lb/°F) or as calories per gram per degree Celsius (cal/g/°C). Materials with high specific heats require a large amount of heat to increase their temperature, and hence contain more heat at a given temperature than do materials with low specific heats. The specific heat of many materials varies considerably with temperature, so the temperature at which the specific heat was determined is usually specified. Water, with a specific heat of 1.0 Btu/lb/°F at ordinary atmospheric temperatures has one of the highest specific heats of common substances. The specific heat of most metals is low. Lead, for example, has a specific heat of only 0.031 Btu/lb/°F at 32°F, and requires very little heat to increase its temperature. Most wildland fuels have specific heats in the range of 0.45 to 0.65 Btu/lb/°F.
Often information on the amount of heat needed to produce a given temperature change in some volume of a material is needed. In wildland fire, for example, we are interested in the amount of heat needed to raise the temperature of a layer or volume of the fuels, because this helps determine the characteristics of a firebrand that can ignite a particular fuel. The amount of heat required to raise the temperature of a unit volume of a substance by one degree is its heat capacity. In English units, this is given in Btu per cubic foot per degree Fahrenheit (Btu/ft³/°F).
Heat capacity varies with density and specific heat
Heat capacity is calculated from the density and the specific heat of a substance. Density is the weight of a unit of volume whereas specific heat indicates how much heat is required to increase the temperature of each unit of weight by one degree. Both density and specific heat vary widely in different substances; hence the heat capacities of these substances also vary widely. And since specific heat often varies with temperature, so also does heat capacity.
Materials with high heat capacities can absorb and lose large quantities of heat without much temperature change. Conversely, relatively little heat is needed to change the temperature of materials with low heat capacity. At ordinary temperatures, the heat capacity of air is about 0.017 Btu/ ft³/°F—little heat is needed to change its temperature. Under similar conditions, the heat capacity of dry soil and rock is 19 to 20 Btu/ft³/°F, and that of water is about 62 Btu/ ft³/°F. The high heat capacity of water is one of the reasons why the climate near oceans and large lakes is often more moderate than that of the climate further inland. The water absorbs and stores large quantities of heat during the summer without much change in temperature, and this tends to keep the air over the adjacent land relatively cool. In the winter the stored heat is released and warms the air over nearby land areas.
Because the specific heat of most wildland fuels varies over a relatively narrow span, differences in heat capacity of the fuels depend chiefly on their density. The differences in density of wildland fuels are quite large, and hence variations in heat capacity are also. Solid oak wood, for example, has a density of about 48 lb/ft³, while that of punky and decayed wood may be only 6 or 7 lb/ft³. The oak wood, then, requires a considerable amount of heat to raise its temperature to the ignition point, but decayed wood requires a relatively small amount. Thus, heat capacity is important in the ignition of wildland fuels.
Much energy is involved in changes of state
