ENERGY, WORK, AND HEAT ThermodynamicsExample:How much heat is required to raise the temperature of 5 lbm of water from 50°F to150°F? (Assume the specific heat (cp) for water is constant at 1.0 Btu/lbm-°F.)Solution:cp=QmDTQ = cpmDTQ = (1.0 Btu/lbm-°F)(5 lbm)(150°F - 50°F)Q = (1.0 Btu/lbm-°F)(5 lbm)(100°F)Q = 500 BtuFrom the previous discussions on heat and work, it is evident that there are many similaritiesbetween them. Heat and work are both transient phenomena. Systems never possess heat orwork, but either or both may occur when a system undergoes a change of energy state. Both heatand work are boundary phenomena in that both are observed at the boundary of the system. Bothrepresent energy crossing the system boundary.EntropyEntropy (S) is a property of a substance, as are pressure, temperature, volume, and enthalpy.Because entropy is a property, changes in it can be determined by knowing the initial and finalconditions of a substance. Entropy quantifies the energy of a substance that is no longeravailable to perform useful work. Because entropy tells so much about the usefulness of anamount of heat transferred in performing work, the steam tables include values of specificentropy (s = S/m) as part of the information tabulated. Entropy is sometimes referred to as ameasure of the inability to do work for a given heat transferred. Entropy is represented by theletter S and can be defined as DS in the following relationships.(1-18)DSDQTabs(1-19)DsDqTabswhere:S = the change in entropy of a system during some process (Btu/°R)DHT-01 Page 22 Rev. 0
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