Thermodynamics ENERGY, WORK, AND HEATQ = the amount of heat transferred to or from the system during the processD(Btu)T_{abs}= the absolute temperature at which the heat was transferred (°R)Ds = the change in specific entropy of a system during some process(Btu/lbm -^{o}R)Dq = the amount of heat transferred to or from the system during the process(Btu/lbm)Like enthalpy, entropy cannot be measured directly. Also, like enthalpy, the entropy of asubstance is given with respect to some reference value. For example, the specific entropy ofwater or steam is given using the reference that the specific entropy of water is zero at 32°F.The fact that the absolute value of specific entropy is unknown is not a problem because it is thechange in specific entropy (Ds) and not the absolute value that is important in practical problems.EnergyandPowerEquivalencesThe various forms of energy involved in energy transfer systems (such as potential energy,kinetic energy, internal energy, P-V energy, work and heat) may be measured in numerous basicunits. In general, three types of units are used to measure energy: (1) mechanical units, suchas the foot-pound-force (ft-lbf); (2) thermal units, such as the British thermal unit (Btu); and (3)electrical units, such as the watt-second (W-sec). In the mks and cgs systems, the mechanicalunits of energy are the joule (j) and the erg, the thermal units are the kilocalorie (kcal) and thecalorie (cal), and the electrical units are the watt-second (W-sec) and the erg. Although the unitsof the various forms of energy are different, they are equivalent.Some of the most important experiments in science were those conducted by J. P. Joule in 1843,who showed quantitatively that there was a direct correspondence between mechanical andthermal energy. These experiments showed that one kilocalorie equals 4,186 joules. These sameexperiments, when performed using English system units, show that one British thermal unit(Btu) equals 778.3 ft-lbf. These experiments established the equivalence of mechanical andthermal energy. Other experiments established the equivalence of electrical energy with bothmechanical and thermal energy. For engineering applications, these equivalences are expressedby the following relationships.1 ft-lbf = 1.286 x 10^{-3} Btu = 3.766 x 10^{-7} kW-hr1 Btu = 778.3 ft-lbf = 2.928 x 10^{-4} kW-hr1 kW-hr = 3.413 x 10^{3} Btu = 2.655 x 10^{6} ft-lbfRev. 0 Page 23 HT-01