63. The specific heat of a gas at constant pressure is greater than that at constant volume, as will be apparent from the following: If a cylinder partly full of gas is closed by a piston, so that the gas sustains a constant pressure due to the weight of the atmosphere plus the weight of the piston, and heat is applied until the temperature of the contained gas has been raised, say, 10°, the piston will move out a certain amount owing to the expansion of the gas. Consequently, the gas has received a certain quantity of heat, and at the same time has performed work by moving the piston against the pressure of the atmosphere. The work done by moving the piston represents a portion of the energy imparted to the gas by heating. If this energy is restored to the gas by compressing it to its original volume, the gas will be heated to a certain extent by the compression. In other words, it is brought to the same condition by compression that it might have had if heated at constant volume, with the exception that its temperature is higher. This indicates that more energy has been expended on it than if it had been heated to the required temperature at constant volume; that is, the specific heat at constant pressure is greater than that at constant volume.
EXAMPLES FOR PRACTICE
1. How many British thermal units are required to change 5 pounds of ice at 15° F. into steam at 212° F.? Ans. 6,491.84 B. T. U.
2. How many pounds of ice at 32° F. can be melted by 3 pounds of steam at 212° F.? Ans. 23.87 lb.
3. How many British thermal units are required to change 13 pounds of water at 59° F. into steam at 212° F.? Ans. 14,544.4 B. T. U.
4. It is found that to raise the temperature of 20 pounds of iron from 62° F. to 63° F. requires 2.276 British thermal units; what is the specific heat of iron? Ans. .1138
5. The specific heat of silver is .057; how many British thermal units are required to raise 22 pounds of silver from 50° F. to 60° F.? Ans. 12.54 B. T. U.
6. How many B. T. U. are required to raise the temperature of 26 pounds of copper from 57° F. to 93° F. Ans. 89.01 B. T. U.
