Читать книгу Heat-Its Role in Wildland Fire - Clive M. Countryman - Страница 9

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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).