Thermodynamics

60 | ThermodynamicsSystem boundaryCLOSEDSYSTEM(m = constant)HeatWorkFIGURE 2–11Energy can cross the boundaries of aclosed system in the form of heat andwork.No heattransferINTERACTIVETUTORIALSEE TUTORIAL CH. 2, SEC. 3 ON THE DVD.Room air25°CHeat Heat8 J/s 16 J/s25°C 15°C 5°CFIGURE 2–12Temperature difference is the drivingforce for heat transfer. The larger thetemperature difference, the higher isthe rate of heat transfer.2–3 ■ ENERGY TRANSFER BY HEATEnergy can cross the boundary of a closed system in two distinct forms:heat and work (Fig. 2–11). It is important to distinguish between these twoforms of energy. Therefore, they will be discussed first, to form a soundbasis for the development of the laws of thermodynamics.We know from experience that a can of cold soda left on a table eventuallywarms up and that a hot baked potato on the same table cools down.When a body is left in a medium that is at a different temperature, energytransfer takes place between the body and the surrounding medium untilthermal equilibrium is established, that is, the body and the medium reachthe same temperature. The direction of energy transfer is always from thehigher temperature body to the lower temperature one. Once the temperatureequality is established, energy transfer stops. In the processes describedabove, energy is said to be transferred in the form of heat.Heat is defined as the form of energy that is transferred between twosystems (or a system and its surroundings) by virtue of a temperaturedifference (Fig. 2–12). That is, an energy interaction is heat only if ittakes place because of a temperature difference. Then it follows that therecannot be any heat transfer between two systems that are at the sametemperature.Several phrases in common use today—such as heat flow, heat addition,heat rejection, heat absorption, heat removal, heat gain, heat loss, heatstorage, heat generation, electrical heating, resistance heating, frictionalheating, gas heating, heat of reaction, liberation of heat, specific heat, sensibleheat, latent heat, waste heat, body heat, process heat, heat sink, and heatsource—are not consistent with the strict thermodynamic meaning of theterm heat, which limits its use to the transfer of thermal energy during aprocess. However, these phrases are deeply rooted in our vocabulary, andthey are used by both ordinary people and scientists without causing anymisunderstanding since they are usually interpreted properly instead ofbeing taken literally. (Besides, no acceptable alternatives exist for some ofthese phrases.) For example, the phrase body heat is understood to meanthe thermal energy content of a body. Likewise, heat flow is understoodto mean the transfer of thermal energy, not the flow of a fluidlike substancecalled heat, although the latter incorrect interpretation, which is based onthe caloric theory, is the origin of this phrase. Also, the transfer of heatinto a system is frequently referred to as heat addition and the transfer ofheat out of a system as heat rejection. Perhaps there are thermodynamic reasonsfor being so reluctant to replace heat by thermal energy: It takes lesstime and energy to say, write, and comprehend heat than it does thermalenergy.Heat is energy in transition. It is recognized only as it crosses the boundaryof a system. Consider the hot baked potato one more time. The potatocontains energy, but this energy is heat transfer only as it passes throughthe skin of the potato (the system boundary) to reach the air, as shown inFig. 2–13. Once in the surroundings, the transferred heat becomes part ofthe internal energy of the surroundings. Thus, in thermodynamics, the termheat simply means heat transfer.