Thermodynamics
ENERGY, WORK, AND HEAT
Q
=
the amount of heat transferred to or from the system during the process
D
(Btu)
Tabs
=
the absolute temperature at which the heat was transferred (°R)
Ds
=
the change in specific entropy of a system during some process
(Btu/lbm -oR)
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 a
substance is given with respect to some reference value. For example, the specific entropy of
water 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 the
change in specific entropy (Ds) and not the absolute value that is important in practical problems.
Energy and Power Equivalences
The 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 basic
units. In general, three types of units are used to measure energy: (1) mechanical units, such
as 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 mechanical
units of energy are the joule (j) and the erg, the thermal units are the kilocalorie (kcal) and the
calorie (cal), and the electrical units are the watt-second (W-sec) and the erg. Although the units
of 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 and
thermal energy. These experiments showed that one kilocalorie equals 4,186 joules. These same
experiments, 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 and
thermal energy. Other experiments established the equivalence of electrical energy with both
mechanical and thermal energy. For engineering applications, these equivalences are expressed
by the following relationships.
1 ft-lbf = 1.286 x 10-3 Btu = 3.766 x 10-7 kW-hr
1 Btu = 778.3 ft-lbf = 2.928 x 10-4 kW-hr
1 kW-hr = 3.413 x 103 Btu = 2.655 x 106 ft-lbf
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