LECTURE VII.-QUESTIONS. 1. Give free-hand sketches with index of parts, and a description in your own words of Rumford's, Davy's, and Joule's experiments. 2. State in your own words what you consider heat to be, and give Joule's mechanical equivalent for one British thermal unit. 3. How has the work done in raising the temperature of a pound of water through one degree been ascertained? A pound of coal gives out during combustion 12,000 units of heat; how much work in foot-pounds could be done per pound of coal burned, if there were no waste? Ans. 9;264,000 ft.-lbs. 4. It is estimated that every pound of average steaming coal burned in the furnace of a boiler gives out 13,000 units of heat. It is found that a good compound engine and boiler require 2 lbs. of coal per hour per indicated horse-power. What is the efficiency of the combined boiler and engine? Ans. 9.8 per cent. 5. Define a unit of heat. A steam engine indicates 25 H.P.; how many units of heat does it convert into useful work per minute? (S. and A. Exam., 1888.) Ans. 1068.8. 6. Using Fahrenheit's thermometer, define the British standard unit of heat. To what amount of energy, expressed in British units of work, is this heat unit equivalent? How many units of work must be converted into heat in order to raise the temperature of 3 lbs. of water from 50° F. to 120° F. ? Ans. 772 ft.-lbs. ; 162,120 ft.-lbs. (S. and A. Exam., 1894.) LECTURE VIII. CONTENTS.-Sensible and Latent Heats of Water and Steam - Worked Examples Explanation of Sensible and Latent Heats by the Kinetic Theory of Heat. Sensible and Latent Heats of Water and Steam.-Hitherto we have dealt with heat when imparted to or abstracted from bodies as indicated by a rise or fall of temperature in the body. It has been customary to call this condition sensible heat; but there are exceptional cases in which temperature does not vary in a mass of matter when heat is communicated to it from, or taken from it to, external matter. For instance, when the body is ice at the melting point, heat communicated to it does not raise its temperature above 32° F., or, if the body be water at the boiling point in the open air, heat slowly communicated to it, in however great a quantity, does not raise its temperature above 212° F., at the normal pressure of the atmosphere. This heat is termed latent heat. A short account of Professor Black's well-known experiments, carried out about 1762, will serve to illustrate the difference between what is termed the "sensible" and the "latent" heat of a substance. 5 oz.ICE. 32°F. 5oz WATER 32 F. BLACK'S EXPERIMENT ON LATENT HEAT OF WATER. Black procured two glass flasks, in one of which he placed 5 ozs. of ice at 32° F.,* and in the other 5 ozs. of water at the same *The ice was beginning to melt, and his estimate of the temperature at the surface was 33° F. temperature. He suspended them within a short distance of each other in a room which remained at a uniform temperature of about 47° F. He observed that in one half-hour the water increased in temperature by 7° F., but that it took twenty half-hours for the whole of the ice in the other flask just to become melted, and he reasoned thus-that from the time required the amount of heat which had entered the ice must have been twenty times as much as that which entered the water. He, therefore, computed that the latent heat of water must be 7 x 20 (half-hours) = 140. Another experiment of Black's was that of placing a lump of ice in an equal weight of water at 176° F., with the result that when the whole of the ice had melted, the temperature was no greater than that of water just ready to freeze. Therefore, assuming the final temperature of the mixture to have been 33° F., we have 176-33=143, as the amount of heat required to melt the ice, or the latent heat of water. In this estimate he was very near the truth; for, even at the present day the mean result of some of the best experimenters appears to be, that 143 British thermal units of heat are absorbed, or become latent, in the conversion of 1 lb. of ice into water at the same temperature; and, consequently, 143 B.T.U., are given out or let free in the conversion of 1 lb. of water at 32° F., into ice at the same temperature.* * BLACK'S EXPERIMENT ON THE LATENT HEAT OF STEAM. Black's third experiment consisted in placing a flat tin dish on a hot plate over a fire; into this plate he put a small quantity of water at 50° F., and observed that after 4 minutes the water began to boil, and in 20 minutes more it had all evaporated. Now, since the water increased by (212° -50°) = 162° in 4 minutes, he = * The latent heat of water by the Centigrade scale is 79'4; for, 143 × 5 9 79'4, say 79 units of heat required to convert 1 lb. of ice at o° C. into 1 lb. of water at the same temperature. = reasoned that it must have been receiving heat at the same rate throughout the experiment, or that in 20 minutes it had absorbed five times as much as in the first 4 minutes without any apparent rise in temperature as indicated by the thermometer, or, 5 × 162 810-Black's estimate of the latent heat of steam. In this last estimate Black was incorrect, as might be expected, from the rough nature of his experiment. It has since been found that the latent heat of steam at atmospheric pressure is 966.6. In other words, it requires 966.6 British thermal units of heat to convert 1 lb. of water at 212° F., into steam at the same temperature, or 1 lb. of steam at 212° F., gives out 966·6 B.T.U., in being condensed into water at the same temperature.* The following definition of sensible and latent heat will now be quite clear : "Heat given to a substance, and warming it, is said to be sensible in the substance. Heat given to a substance, and not warming it, is said to become latent" (Sir Wm. Thomson). Latent heat is the quantity of heat which must be communicated to (unit mass of †) a body in a given state, in order to convert it into another state without changing its temperature (Maxwell). In order to impress on junior students the foregoing statements with regard to the latent heats of water and steam, we now give a few examples worked out in detail. The student should carefully study them, and then work them out for himself; or, these examples, as well as several of the questions at the end of this lecture, may with advantage be worked out on the blackboard by the teacher. EXAMPLE I.—How many pounds of ice at 32° F. will be converted into water at 40° F. by mixing it with 6 lbs. of water at 160° F. 6 lbs. of water gives up 6(160°-40')= 720 units. ..720 151 = 4*768 lbs. EXAMPLE II.-Into I cwt. of water at 45° F. are poured 20 lbs. of water at 160° F., and then 4 lbs. of ice at 32° F. are added. What is the final temperature when the ice has just melted? *The Latent Heat of Steam by the Centigrade scale, is, therefore 966.6 × 5 9 · 537; or, 537 times the quantity of heat absorbed in raising I lb. of water by 1° C. + I have added the words (unit mass of) to Maxwell's definition, because it appears deficient without them. When we speak of 143 as the latent heat of water, and 966 as the latent heat of steam, it is understood that 143 and 966 units of heat are required respectively for every 1 lb. (or unit of mass) to change the state from solid to liquid, and from liquid to gaseous.-4. J. .. 3444136 = 25'32 above 32° or 57° 32 F. EXAMPLE III.—If the heat which just melts 8 lbs. of ice at 32° F. were applied to 30 lbs. water at 60° F., to what temperature would the water rise? 8 × 143 = 1144 units of heat required to melt the ice. Now, 30 lbs. of water raised 1° F. = 30 units of heat, ..114430 38°13 F. of rise from 60° F. or 98°.13 F. EXAMPLE IV.-There are mixed together 200 lbs. of water at 100° F., 3 lbs. steam at atmospheric pressure, and 15 lbs. of ice at 32° F. What is the resulting temperature when all the ice is just melted. The 200 lbs. water have + 13,600 u, more than water at 32° F. steam ice mixture + 3,438 68° 3 F. above 32 ... 218 = = 100°.3 F. And 14,893 218 EXAMPLE V.—If one pound of Newcastle coal develops 12,000 units of heat by its complete combustion, how much water at 60° F. should be converted into steam at 212° F. by the consumption of I cwt. of such fuel, assuming that there is no loss of heat during the operation? (S. & A. Exam., 1887.) Every pound of water in being raised from 60° F. to 212° F. absorbs1 lb. (212° - 60°) = 152 units of heat. Every pound of water raised to 212° F., in being converted into steam at the same temperature (212° F.), absorbs 966 units of heat. .. Every pound of water at 60° F., in being converted into steam at 212° F., absorbs 152 units+966 units = 1118 units of heat. From the question we are informed that every pound of coal develops12,000 units of heat. 1118 units 1,344,000 units :: I lb. water: x lbs. water. |