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Heat - h1012v1_47
Energy and Power Equivalences - h1012v1_49

Thermodynamics Heat Transfer and Fluid Flow Volume 1 of 3
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ENERGY, WORK, AND HEAT Thermodynamics Example: How  much  heat  is  required  to  raise  the  temperature  of  5  lbm  of  water  from  50°F  to 150°F?   (Assume the specific heat (cp) for water is constant at 1.0 Btu/lbm-°F.) Solution: cp = Q mDT Q     =   cpmDT Q     =   (1.0 Btu/lbm-°F)(5 lbm)(150°F - 50°F) Q     =   (1.0 Btu/lbm-°F)(5 lbm)(100°F) Q     =   500 Btu From  the  previous  discussions  on  heat  and  work,  it  is  evident  that  there  are  many  similarities between  them.   Heat  and  work  are  both  transient  phenomena.   Systems  never  possess  heat  or work, but either or both may occur when a system undergoes a change of energy state.  Both heat and work are boundary phenomena in that both are observed at the boundary of the system.  Both represent energy crossing the system boundary. Entropy Entropy  (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 final conditions  of  a  substance.    Entropy  quantifies  the  energy  of  a  substance  that  is  no  longer available  to  perform  useful  work.   Because  entropy  tells  so  much  about  the  usefulness  of  an amount  of  heat  transferred  in  performing  work,  the  steam  tables  include  values  of  specific entropy (s = S/m) as part of the information tabulated.   Entropy is sometimes referred to as a measure of the inability to do work for a given heat transferred.   Entropy is represented by the letter S and can be defined as DS in the following relationships. (1-18) DS DQ Tabs (1-19) Ds Dq Tabs where: S = the change in entropy of a system during some process (Btu/°R) D HT-01 Page 22 Rev. 0







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