Thermodynamics

Now if asked to name the energy transformations associated with theoperation of a refrigerator, we may still have a hard time answering becauseall we see is electrical energy entering the refrigerator and heat dissipatedfrom the refrigerator to the room air. Obviously there is need to study thevarious forms of energy first, and this is exactly what we do next, followedby a study of the mechanisms of energy transfer.2–2 ■ FORMS OF ENERGYe E m 1kJ>kg2 (2–1)Chapter 2 | 53INTERACTIVETUTORIALSEE TUTORIAL CH. 2, SEC. 2 ON THE DVD.Energy can exist in numerous forms such as thermal, mechanical, kinetic,potential, electric, magnetic, chemical, and nuclear, and their sum constitutesthe total energy E of a system. The total energy of a system on a unitmass basis is denoted by e and is expressed asThermodynamics provides no information about the absolute value of thetotal energy. It deals only with the change of the total energy, which is whatmatters in engineering problems. Thus the total energy of a system can beassigned a value of zero (E 0) at some convenient reference point. Thechange in total energy of a system is independent of the reference pointselected. The decrease in the potential energy of a falling rock, for example,depends on only the elevation difference and not the reference levelselected.In thermodynamic analysis, it is often helpful to consider the variousforms of energy that make up the total energy of a system in two groups:macroscopic and microscopic. The macroscopic forms of energy are those asystem possesses as a whole with respect to some outside reference frame,such as kinetic and potential energies (Fig. 2–3). The microscopic forms ofenergy are those related to the molecular structure of a system and thedegree of the molecular activity, and they are independent of outside referenceframes. The sum of all the microscopic forms of energy is called theinternal energy of a system and is denoted by U.The term energy was coined in 1807 by Thomas Young, and its use inthermodynamics was proposed in 1852 by Lord Kelvin. The term internalenergy and its symbol U first appeared in the works of Rudolph Clausiusand William Rankine in the second half of the nineteenth century, and iteventually replaced the alternative terms inner work, internal work, andintrinsic energy commonly used at the time.The macroscopic energy of a system is related to motion and the influenceof some external effects such as gravity, magnetism, electricity, andsurface tension. The energy that a system possesses as a result of its motionrelative to some reference frame is called kinetic energy (KE). When allparts of a system move with the same velocity, the kinetic energy isexpressed asKE m V 22 1kJ2(2–2)FIGURE 2–3The macroscopic energy of an objectchanges with velocity and elevation.