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
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Rev. 0
