Page:International Library of Technology, Volume 93.djvu/69

 work are done in moving the car a mile? (6) What is the equivalent of this work in British thermal units? Ans. (a) 660,000 ft.-lb. (b) 848.33 B. T. U. 4. To what height could a weight of 720 pounds be lifted by the work equivalent of 60 British thermal units? Ans. 64.83 ft.

71. Change of State of a Gas. — If a quantity of gas has a definite pressure, volume, and temperature, it is said to have a certain state represented by those conditions; in other words, the state of a gas is its condition as regards pressure, volume, and temperature. If any one of these conditions is changed, the gas is said to undergo a change of state. A gas may change from one state to another in a number of ways. In practice, however, changes of state are usually the results of expansion and compression.

72. Isothermal Expansion and Compression. — If a quantity of air is placed in a cylinder fitted with a piston, and the piston is forced down quickly so as to confine the air in a much smaller space, it will be found that the temperature of the air is increased. This is due to the fact that it requires work to compress the air, and the work thus done appears as heat, raising the temperature of the air. However, if the piston is moved very slowly, the heat will pass through the cylinder walls into the surrounding atmosphere as fast as it appears, and the enclosed air will have the same temperature throughout the change of state. A change of state like that just described is known as an isothermal compression — that is, compression without change of temperature. If a gas is allowed to expand, and its temperature is kept constant throughout the change of state, the operation is known as an isothermal expansion.

73. According to formula 1, Art. 40, if the temperature of a gas remains constant, the product of the pressure and volume is always the same; that is, $$p v = p_1 v_1 =$$ a constant. Hence, this formula must represent the law governing all isothermal changes of state.