Principles of naval engineering - Historic Naval Ships Association

Chapter 8-INTRODUCTION TO THERMODYNAMICSdefinition, 1 horsepower is equal to 33,000 footpoundsof work per minute or 550 foot-pounds ofwork per second. Thus a machine that is capableof doing 550 foot-pounds of work per second issaid to be a 1-horsepower machine.THERMAL ENERGYEnergy associated primarily with systemsof molecules is called thermal energy. Likeother kinds of energy, thermal energy may existin stored form (in which case it is called internalenergy) or as energy in transition (inwhich case it is called heat) .In common usage, the term heat is often usedto include all forms of thermal energy. However,this lack of distinction between heat and thestored forms of thermal energy can lead toserious confusion. In this text, therefore, theterm internal energy is used to describe thestored forms of thermal energy , and the termheat is used only to describe thermal energyin transition.Internal EnergyInternal energy, like all stored forms ofenergy, exists either as potential energy or askinetic energy.Internal potential energy is the energy associatedwith the forces of attraction that existbetween molecules. The magnitude of internalpotential energy is dependent upon the mass ofthe molecules and the average distance by whichthey are separated, in much the same way thatmechanical potential energy depends upon themass of the bodies in the system and the distanceby which they are separated. The force ofattraction between molecules is greatest insolids, less in liquids and yielding substances,and least of all in gases and vapors. Wheneversomething happens to change the average distancebetween the molecules of a substance,there is a corresponding change in the internalpotential energy of the substance.Internal kinetic energy is the energy associatedprimarily with the activity of molecules,just as mechanical kinetic energy is the energyassociated with the velocities of relatively largebodies. It is important to note that the temperatureof a substance arises from and is proportionalto the molecular activity with which internalkinetic energy is associated.For most purposes, we will not need todistinguish between the two stored forms ofinternal energy. Instead of referring to internalpotential energy and internal kinetic energy,therefore, we may often simply use the terminternal energy . When used in this way, withoutqualification, the term internal energy shouldbe understood to mean the sum total of allinternal energy stored in the substance or systemby virtue of the motion of molecules or byvirtue of the forces of attraction betweenmolecules.HeatAlthough the term heat is more familiar thanthe term internal energy ,it may be more difficultto arrive at an accurate definition of heat.Heat is thermal energy in transition. Like work,heat is a transitory energy form existing betweentwo or more other forms of energy.Since the flow of thermal energy can occuronly when there is a temperature differencebetween two objects or regions, it is apparentthat heat is not a property or attribute of anyone object or substance. If a person accidentallytouches a hot stove, he may understandably feelthat heat is a property of the stove. More accurately,however, he might reflect that his handand the stove constitute an energy system andthat thermal energy flows from the stove to hishand because the stove has a higher temperaturethan his hand.As another example of the difference betweenheat and internal energy, consider two equallengths of piping, made of identical materialsand containing steam at the same pressure andtemperature. One pipe is well insulated, one isnot. From everyday experience, we expect moreheat to flow from the uninsulated section of pipethan from the insulated section. When the twopipes are first filled with steam, the steam inone pipe contains exactly as much internal enetgyas the steam in the other pipe. We know this istrue because the two pipes contain equal volumesof steam at equal pressures and temperatures.After a few minutes, the steam in the uninsulatedpipe will contain much less internal energythan the steam in the insulated pipe, as we cantell by reading the pressure and temperaturegages on each pipe. What has happened? Storedthermal energy— internal energy— has movedfrom one place to another, first from thesteam to the pipe, then from the uninsulatedpipe to the air. It ic this movement, orthis flow, of energy that should properly becalled heat. Temperature is a reflection of the161