PRINCIPLES OF NAVAL ENGINEERING 



In words, then, we may say that the mass 

 times the specific heat times the temperature 

 change of the first substance must equal the 

 mass times the specific heat times the tem- 

 perature change of the second substance. In this 

 equation and in this verbal statement, we are 

 ignoring the thermal energy absorbed by the 

 apparatus, by the stirring rods, and by the 

 thermometers. In actually determining specific 

 heats, it is often necessary to account for all 

 thermal energy, even that relatively minute 

 quantity which is absorbed by the equipment. 

 In such a case, the heat absorbed by the equip- 

 ment is merely added to the right-hand side of 

 the equation. 



Specific heat is primarily useful in that 

 it allows us to determine the quantity of heat 

 added to a substance merely by observing the 

 temperature rise, when we know the mass and 

 the specific heat of the substance. And this, in 

 fact, is precisely what we did in the thermal 

 conductivity problem, where we calculated the 

 amount of heat that had been absorbed by the 

 water in the water chest by using the equation 



Q = mass x temperature change x specific heat 



Specific heat varies, in greater or lesser 

 degree, according to pressure, volume, and 

 temperature. Specific heat values quoted for 

 solids and liquids are obtained through experi- 

 mental procedures in which the substance is 

 kept at constant pressure. The specific heat of 

 any gas may vary tremendously, having in fact 

 an almost infinite variety of values because of 

 the almost infinite variety of processes and 

 states during which energy is transferred to or 

 by a gas. For convenience, specific heats of 

 gases are given as specific heat at constant 

 volume (Cy) and specific heat at constant pres- 

 sure (Cp). 



RADIATION.— Thermal radiation is a mode 

 of heat transfer that does not involve any physi- 

 cal contact between the emitting region and the 

 receiving region. A person sitting near a hot 

 stove is warmed by thermal radiation from the 

 stove, even though the air in between remains 

 relatively cold. Thermal radiation from the 

 sun warms the earth without warming the space 

 through which it passes. Thermal radiation 

 passes through any transparent substance— air, 

 glass, ice— without warming it to any extent 

 because transparent materials are very poor 

 absorbers of radiant energy. 



All substances— solids, liquids, and gases- 

 emit radiant energy at all times. We tend to 

 think of radiant energy as something that is 

 emitted only by extremely hot objects such as 

 the sun, a stove, or a furnace, but this is a very 

 limited view of the nature of radiant energy. 

 The earth absorbs radiant energy emitted by the 

 sun, but the earth in turn radiates energy to the 

 stars. A stove radiates energy to everything 

 surrounding it, but at the same time all the sur- 

 rounding objects are radiating energy to the 

 stove. A child standing near a snowman may 

 well believe that the snowman is "radiating 

 cold" rather than emitting radiant energy; ac- 

 tually, however, both the child and the snowman 

 are emitting radiant energy. The child, of course, 

 is radiating far more energy than the snowman, 

 so the net effect of this energy exchange is that 

 the snowman grows warmer and the child gi-ows 

 colder. We are literally surrounded by— and a 

 part of— such energy exchanges at all times. As 

 we consider these energy exchanges, we may ar- 

 rive at a new view of thermal equilibrium: 

 when objects are radiating precisely as much 

 thermal energy as they are receiving, in any 

 given period of time, they are in thermal equi- 

 librium. 



Thermal radiation is an electromagnetic 

 wave phenomenon, differing from light, radio 

 waves, and other electromagnetic phenomenon 

 merely in the wavelengths involved. When the 

 wavelengths are in the infrared part of the elec- 

 tromagnetic spectrum— that is, when they are 

 just below the range of visible light waves— we 

 refer to the radiated energy as thermal radia- 

 tion . It should be noted, however, that all elec- 

 tromagnetic waves transport energy which can 

 be absorbed by matter and which can in many 

 cases result in observable thermal effects. For 

 example, one energy unit of light absorbed by 

 a substance produces the same temperature rise 

 in that substance as is produced by the absorp- 

 tion of an equal amount of thermal (infrared) 

 energy. 



When radiant energy falls upon a body that 

 can absorb it, some of the energy is absorbed 

 and some is reflected. The amount absorbed and 

 the amount reflected depend in large part upon 

 the surface of the receiving body. Dark, opaque 

 bodies absorb more thermal radiation than shiny, 

 bright, white, or polished bodies. Shiny, bright, 

 white, or polished bodies reflect more thermal 

 radiation than dark, opaque bodies. Good radia- 

 tors are also good absorbers and poor radiators 



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