Chapter 8-INTRODUCTION TO THERMODYNAMICS 



definition, 1 horsepower is equal to 33,000 foot- 

 pounds of work per minute or 550 foot-pounds of 

 work per second. Thus a machine that is capable 

 of doing 550 foot-pounds of work per second is 

 said to be a 1-horsepower machine. 



THERMAL ENERGY 



Energy associated primarily with systems 

 of molecules is called thermal energy. Like 

 other kinds of energy, thermal energy may exist 

 in stored form (in which case it is called in- 

 ternal energy) or as energy in transition (in 

 which case it is called heat) . 



In common usage, the term heat is often used 

 to include all forms of thermal energy. How- 

 ever, this lack of distinction between heat and the 

 stored forms of thermal energy can lead to 

 serious confusion. In this text, therefore, the 

 term internal energy is used to describe the 

 stored forms of thermal energy , and the term 

 heat is used only to describe thermal energy 

 in transition. 



Internal Energy 



Internal energy, like all stored forms of 

 energy, exists either as potential energy or as 

 kinetic energy. 



Internal potential energy is the energy asso- 

 ciated with the forces of attraction that exist 

 between molecules. The magnitude of internal 

 potential energy is dependent upon the mass of 

 the molecules and the average distance by which 

 they are separated, in much the same way that 

 mechanical potential energy depends upon the 

 mass of the bodies in the system and the dis- 

 tance by which they are separated. The force of 

 attraction between molecules is greatest in 

 solids, less in liquids and yielding substances, 

 and least of all in gases and vapors. Whenever 

 something happens to change the average dis- 

 tance between the molecules of a substance, 

 there is a corresponding change in the internal 

 potential energy of the substance. 



Internal kinetic energy is the energy asso- 

 ciated primarily with the activity of molecules, 

 just as mechanical kinetic energy is the energy 

 associated with the velocities of relatively large 

 bodies. It is important to note that the tempera- 

 ture of a substance arises from and is propor- 

 tional to the molecular activity with which in- 

 ternal kinetic energy is associated. 



For most purposes, we will not need to 

 distinguish between the two stored forms of 



internal energy. Instead of referring to internal 

 potential energy and internal kinetic energy, 

 therefore, we may often simply use the term 

 internal energy . When used in this way, without 

 qualification, the term internal energy should 

 be understood to mean the sum total of all 

 internal energy stored in the substance or sys- 

 tem by virtue of the motion of molecules or by 

 virtue of the forces of attraction between 

 molecules. 



Heat 



Although the term heat is more familiar than 

 the term internal energy , it may be more diffi- 

 cult to arrive at an accurate definition of heat. 

 Heat is thermal energy in transition. Like work, 

 heat is a transitory energy form existing be- 

 tween two or more other forms of energy. 



Since the flow of thermal energy can occur 

 only when there is a temperature difference 

 between two objects or regions, it is apparent 

 that heat is not a property or attribute of any 

 one object or substance. If a person accidentally 

 touches a hot stove, he may understandably feel 

 that heat is a property of the stove. More ac- 

 curately, however, he might reflect that his hand 

 and the stove constitute an energy system and 

 that thermal energy flows from the stove to his 

 hand because the stove has a higher tempera- 

 ture than his hand. 



As another example of the difference between 

 heat and internal energy, consider two equal 

 lengths of piping, made of identical materials 

 and containing steam at the same pressure and 

 temperature. One pipe is well insulated, one is 

 not. From everyday experience, we expect more 

 heat to flow from the uninsulated section of pipe 

 than from the insulated section. When the two 

 pipes are first filled with steam, the steam in 

 one pipe contains exactly as much internal enetgy 

 as the steam in the other pipe. We know this is 

 true because the two pipes contain equal volumes 

 of steam at equal pressures and temperatures. 

 After a few minutes, the steam in the uninsu- 

 lated pipe will contain much less internal energy 

 than the steam in the insulated pipe, as we can 

 tell by reading the pressure and temperature 

 gages on each pipe. What has happened? Stored 

 thermal energy— internal energy— has moved 

 from one place to another, first from the 

 steam to the pipe, then from the uninsulated 

 pipe to the air. It ic this movement, or 

 this flow, of energy that should properly be 

 called heat. Temperature is a reflection of the 



161 



