Modern Engineering Thermodynamics

Problems 247
51. Develop a program that outputs the electrical work mode
entropy production rate ð_S P Þ w due to resistive dissipation in an
isothermal system with uniform properties and a constant
current density. Input the appropriate variables in proper units.
Allow the choice of working in either the Engineering English or
the SI units system.
52. Since Figure 7.22 is a semi-logarithmic plot, the straight line for
copper has an equation of the form ρ e = Ae ð
BT Þ. Estimate the
coefficients A and B for copper from this figure and develop a
program that outputs the electrical work mode entropy
production rate ð_S p Þ w
due to the temperature-dependent resistive
dissipation in an isothermal copper wire. Input the appropriate
variables in proper units. Allow the choice of working in either
the Engineering English or the SI units system.
Create and Solve Problems
Engineering education tends to focus on the process of solving
problems. It ignores teaching the process of formulating solvable
problems. However, working engineers are never given a wellphrasedproblemstatementtosolve.Instead,theyneedtoreactto
situational information and organize it into a structure that can be
solved using the methods learned in college.
These “Create and Solve” problems are designed to help you learn how
to formulate solvable thermodynamics problems from engineering
data. Since you provide the numerical values for some of the variables,
these problems have no unique solutions. Their solutions depend on
the assumptions you need to make and how you set them up to create
a solvable problem.
53.* You are an engineer at a company that manufactures domestic
cookware. You are working on a new electric pot to be used to
heat water to make spaghetti. The pot holds 2.5 kg of liquid
water. You need to know the heat transfer rate and the electrical
power required to heat the water from 15.0°C to 100.°C in
10.0 min. Tests have shown that 30% of the heat from the
electrical heater is lost to the environment during the 10.0 min
heating process. You also need to know the entropy production
rate for this process (assume the average pot surface temperature
is 55°C). Write and solve a thermodynamics problem that
answers these questions.
54. You are an engineer at a diesel truck manufacturing company.
Your work involves designing suitable braking systems for the
trucks. To properly size the brakes, you need to know the work
mode entropy transport rate and entropy production rate as
450. hp is absorbed in the brakes during an emergency stop.
Tests show that the average brake temperature during this test is
350.°F. Write and solve a thermodynamics problem that
provides the answers to these questions.
55. You have been transferred to the transmission section of the diesel
truck manufacturing company. The transmission gearbox has a
uniform surface temperature of 145°F whenitreceives400.hpat
the input shaft and transmits 375 hp out the output shaft. To
improve this design of the transmission, you need to know the
work mode entropy transport rate and entropy production rate of
the existing transmission. Write and solve a thermodynamics
problem that provides the answers to these questions.
56.* You are a newly employed engineer at a factory that produces
scuba diving gear. The apparatus that fills the scuba tanks
adiabatically compresses 1.50 m 3 of air from 100. kPa, 20.0°C
to 0.067 m 3 in a piston-cylinder apparatus with a mechanical
efficiency of 85.0%. You need to know the work mode entropy
production for this process. Write and solve a thermodynamics
problem that provides the answer you need.