Chapter 7 | 4057–51E A 1.2-ft^3 well-insulated rigid can initially contains
refrigerant-134a at 140 psia and 70°F. Now a crack develops
in the can, and the refrigerant starts to leak out slowly,
Assuming the refrigerant remaining in the can has undergone
a reversible, adiabatic process, determine the final mass in
the can when the pressure drops to 20 psia.
and the total entropy change for this process. Answers:
168.4°C, 0.169 kJ/K
7–56 Reconsider Prob. 7–55. Using EES (or other)
software, study the effect of the mass of the iron
block on the final equilibrium temperature and the total
entropy change for the process. Let the mass of the iron vary
from 1 to 10 kg. Plot the equilibrium temperature and the
total entropy change as a function of iron mass, and discuss
the results.
7–57 A 50-kg iron block and a 20-kg copper block, both
initially at 80°C, are dropped into a large lake at 15°C. Ther-
mal equilibrium is established after a while as a result of heat
transfer between the blocks and the lake water. Determine the
total entropy change for this process.R-134a
140 psia
70 °FFIGURE P7–51EEntropy Change of Incompressible Substances
7–52C Consider two solid blocks, one hot and the other
cold, brought into contact in an adiabatic container. After a
while, thermal equilibrium is established in the container as a
result of heat transfer. The first law requires that the amount
of energy lost by the hot solid be equal to the amount of
energy gained by the cold one. Does the second law require
that the decrease in entropy of the hot solid be equal to the
increase in entropy of the cold one?
7–53 A 50-kg copper block initially at 80°C is dropped into
an insulated tank that contains 120 L of water at 25°C. Deter-
mine the final equilibrium temperature and the total entropy
change for this process.
Water120 LCopper
50 kgFIGURE P7–537–54 A 25-kg iron block initially at 350°C is quenched in
an insulated tank that contains 100 kg of water at 18°C.
Assuming the water that vaporizes during the process con-
denses back in the tank, determine the total entropy change
during this process.
7–55 A 20-kg aluminum block initially at 200°C is brought
into contact with a 20-kg block of iron at 100°C in an insu-
lated enclosure. Determine the final equilibrium temperature
LAKE 15°CCOPPER
20 kgIRON
50 kgFIGURE P7–577–58 An adiabatic pump is to be used to compress saturated
liquid water at 10 kPa to a pressure to 15 MPa in a reversible
manner. Determine the work input using (a) entropy data from
the compressed liquid table, (b) inlet specific volume and
pressure values, (c) average specific volume and pressure val-
ues. Also, determine the errors involved in parts (b) and (c).Pump15 MPa10 kPaFIGURE P7–58Entropy Change of Ideal Gases
7–59C Prove that the two relations for entropy change of
ideal gases under the constant-specific-heat assumption (Eqs.
7–33 and 7–34) are equivalent.
7–60C Starting with the second T dsrelation (Eq. 7–26),
obtain Eq. 7–34 for the entropy change of ideal gases under
the constant-specific-heat assumption.
7–61C Some properties of ideal gases such as internal
energy and enthalpy vary with temperature only [that is,u
u(T) and hh(T)]. Is this also the case for entropy?