Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGIn words, then, we may say that the masstimes the specific heat times the temperaturechange of the first substance must equal themass times the specific heat times the temperaturechange of the second substance. In thisequation and in this verbal statement, we areignoring the thermal energy absorbed by theapparatus, by the stirring rods, and by thethermometers. In actually determining specificheats, it is often necessary to account for allthermal energy, even that relatively minutequantity which is absorbed by the equipment.In such a case, the heat absorbed by the equipmentis merely added to the right-hand side ofthe equation.Specific heat is primarily useful in thatit allows us to determine the quantity of heatadded to a substance merely by observing thetemperature rise, when we know the mass andthe specific heat of the substance. And this, infact, is precisely what we did in the thermalconductivity problem, where we calculated theamount of heat that had been absorbed by thewater in the water chest by using the equationQ =mass x temperature change x specific heatSpecific heat varies, in greater or lesserdegree, according to pressure, volume, andtemperature. Specific heat values quoted forsolids and liquids are obtained through experimentalprocedures in which the substance iskept at constant pressure. The specific heat ofany gas may vary tremendously, having in factan almost infinite variety of values because ofthe almost infinite variety of processes andstates during which energy is transferred to orby a gas. For convenience, specific heats ofgases are given as specific heat at constantvolume (Cy) and specific heat at constant pressure(Cp).RADIATION.— Thermal radiation is a modeof heat transfer that does not involve any physicalcontact between the emitting region and thereceiving region. A person sitting near a hotstove is warmed by thermal radiation from thestove, even though the air in between remainsrelatively cold. Thermal radiation from thesun warms the earth without warming the spacethrough which it passes. Thermal radiationpasses through any transparent substance— air,glass, ice—without warming it to any extentbecause transparent materials are very poorabsorbers of radiant energy.All substances— solids, liquids, and gasesemitradiant energy at all times. We tend tothink of radiant energy as something that isemitted only by extremely hot objects such asthe sun, a stove, or a furnace, but this is a verylimited view of the nature of radiant energy.The earth absorbs radiant energy emitted by thesun, but the earth in turn radiates energy to thestars. A stove radiates energy to everythingsurrounding it, but at the same time all the surroundingobjects are radiating energy to thestove. A child standing near a snowman maywell believe that the snowman is "radiatingcold" rather than emitting radiant energy; actually,however, both the child and the snowmanare emitting radiant energy. The child, of course,is radiating far more energy than the snowman,so the net effect of this energy exchange is thatthe snowman grows warmer and the child gi-owscolder. We are literally surrounded by— and apart of— such energy exchanges at all times. Aswe consider these energy exchanges, we may arriveat a new view of thermal equilibrium:when objects are radiating precisely as muchthermal energy as they are receiving, in anygiven period of time, they are in thermal equilibrium.Thermal radiation is an electromagneticwave phenomenon, differing from light, radiowaves, and other electromagnetic phenomenonmerely in the wavelengths involved. When thewavelengths are in the infrared part of the electromagneticspectrum— that is, when they arejust below the range of visible light waves—werefer to the radiated energy as thermal radiation. It should be noted, however, that all electromagneticwaves transport energy which canbe absorbed by matter and which can in manycases result in observable thermal effects. Forexample, one energy unit of light absorbed bya substance produces the same temperature risein that substance as is produced by the absorptionof an equal amount of thermal (infrared)energy.When radiant energy falls upon a body thatcan absorb it, some of the energy is absorbedand some is reflected. The amount absorbed andthe amount reflected depend in large part uponthe surface of the receiving body. Dark, opaquebodies absorb more thermal radiation than shiny,bright, white, or polished bodies. Shiny, bright,white, or polished bodies reflect more thermalradiation than dark, opaque bodies. Good radiatorsare also good absorbers and poor radiators166