Modern Engineering Thermodynamics

Problems 243
amount of work W irr is dissipated within the system due to irreversibilities inside the system, then the
entropy produced by these irreversibilities is
W
ðS P Þ = irr
(7.68a)
W 1 T 2
T =constant
and the corresponding entropy production rate is
_S P
W = _W
T
irr
(7.58a)
14. But if we know the irreversibilities are due to a velocity gradient dV/dx in a liquid with viscosity μ at a
temperature T and that they occur in a system of volume V during a time interval 0–τ, then we can calculate
the entropy production and entropy production rates directly from
Z Z
μ
ðS dV 2dt
P Þ W
=
dV (7.71)
τ V T dx
vis
and
Z
μ
dV 2dV
_S P W
=
(7.72)
V T dx
vis
Alternatively, if the irreversibilities are due to the flow of a constant electrical current I flowing in an
isothermal system with uniform electrical resistance R e at temperature T, then we have
_S P
W
elect
ðspecialÞ
= I2 R e
T
(7.74)
Problems (* indicates problems in SI units)
The first ten problems are designed to review some basic thermodynamic
concepts of this and earlier chapters. They may
have more than one correct answer.
1. A closed system becomes an open system when
a. There is no heat transfer to energy.
b. There is no work transfer of energy.
c. There is no mass flow.
d. There is no entropy production.
e. There is no kinetic or potential energy.
f. None of the above.
2. Which of the following are intensive properties: (a) pressure,
(b) temperature, (c) volume, (d) mass, (e) quality, (f) power.
3. The entropy change of a closed system is zero for which of the
following processes: (a) adiabatic, (b) isothermal, (c) isentropic,
(d) isenthalpic, (e) aergonic, (f) reversible.
4.* An insulated, rigid container is divided into two
compartments separated by a partition. One compartment
contains air at 15°C and 0.101 MPa, the other compartment
contains air at 40.°C and 0.101 MPa. When the dividing
partition is removed, the total internal energy of the system
(a) increases, (b) decreases, (c) does not change, (d) is converted
into entropy, (e) is converted into temperature, (f) is converted
into heat.
5. A rigid container contains air (an ideal gas), at 70.0°F and
14.7 psia. If the air is heated to 510.°F, its pressure (a) increases,
(b) decreases, (c) does not change, (d) causes moving boundary
work to occur, (e) causes polytropic work to occur, (f) is
converted into thermal energy.
6. A constant velocity throttling process (a) is reversible, (b) is
isothermal, (c) is isentropic, (d) is isenthalpic, (e) is aergonic,
(f) does not exist in the real world.
7. Heat and work are both (a) intensive properties, (b) extensive
properties, (c) process path dependent, (d) zero for an ideal gas,
(e) zero for an adiabatic process, (f) zero for an aergonic process.
8. An ideal gas must satisfy (a) pv n = constant, (b) u = u(T),
(c) s = constant, (d) p 2 /p 1 = v 1 /v 2 ,(e)pv = nRT, (f)pv = mRT.
9. In a steady flow irreversible process, the total entropy of a system
a. Always increases.
b. Always decreases.
c. Always remains constant.
d. Can increase, decrease, or remain constant.
e. Cannot decrease.
f. Cannot remain constant.
10. To determine the maximum possible work that a heat engine
could produce, one must assume
a. No entropy is produced by the engine.
b. No heat energy is discharged to the environment.