Page:International Library of Technology, Volume 93.djvu/62

Rule II. — To find the weight, in pounds, of a given substance that can be changed from one temperature to another by the application or abstraction of a certain amount of heat, divide the number of British thermal units by the product of the specific heat of the substance and the temperature difference, in degrees Fahrenheit.

Or,$G={\frac {Q}{s\left(t_{1}-t\right)}}$(2)

in which the letters have the same meaning as in formula 1.

Example 2. — The specific heat of air at constant pressure being .23751, how many pounds of air can be raised 1° F. by 1 British thermal unit?

Solution. — Substituting values in formula 2,

$G={\frac {1}{.23751\times 1}}=4.21$ lb. Ans.

61. Although Table II would seem to indicate that the specific heats of various substances are constant, in the case of many solids there is a slight increase with increase of temperature. For example, the mean specific heat of iron between 32° F. and 212° F. has been found to be .1098; and between 212° F. and 570° F. it has been found to be .1218. . Different substances show a great variation in this respect, and there does not seem to be any common law governing them. The specific heat of a substance varies according to its state, whether solid, liquid, or gaseous. The specific heat of water is nearly twice as great as that of ice, and is more than twice as great as that of steam at constant pressure. In general, a substance in the solid state has a smaller specific heat than in the liquid.

62. The specific heat of a gas may be measured in two ways: first, with the gas at constant pressure, but varying volume; second, with the gas at constant volume, but varying pressure. Let $c_{p}$ and $c_{v}$ represent, respectively, the specific heat at constant pressure and at constant volume. Then the relation between these two specific heats is given by the ratio ${c_{p} \over c_{v}}$. This ratio is usually expressed by the letter $k$ that is, ${c_{p} \over c_{v}}=k$. For dry air the value of k is 1.405.