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E234  Thermodynamics
Assignment 3
Due Date: Tuesday, February 10, 2009
Study material for Exam 1 on Monday:
1. Chapter 1; Module 1
2. Chapter 2 up to and including section 3 and including the section on heat
transfer; Module 2 to slide 28
3. "A Really Nice Way to Solve Heat Transfer Problems" under "Additional
Notes" on the web site. There will be a problem in the exam similar to the
one studied in this text.
Learning Objectives
You should be able to:
a) Describe the various contributions to the internal energy
b) Define the three modes of heat transfer
c) Calculate rates of heat transfer by these mechanisms including situations when
more than one mechanism is important
d) Define the first law of thermodynamics, internal energy, heat, work
e) Determine various forms of work such as electrical, boundary, shaft and spring
Reading and Study Assignment:
4. Chapter 2 including the section on heat transfer. Read section 27 so that you can
explain the three equations therein.
5. Lecture notes – Module 2
6. Study "A Really Nice Way to Solve Heat Transfer Problems" under "Additional
Notes" on the web site.
Homework
1. The rolling resistance of a car depends on its weight as: F = 0.006 mg. How long
will a car of 1400 kg drive for a work input of 25 kJ?
Work is force times distance so assuming a constant force we get 2. The air drag force on a car is 0.225 A ρV2. Assume air at 290 K, 100 kPa and a
car frontal area of 4 m2 driving at 90 km/h. How much energy is used to
overcome the air drag driving for 30 minutes?
To get the density:
This expression is very useful to determine the density of a gas from its
temperature and pressure. Spring 2009 3. If the car above has a maximum power of 150 HP, what fraction of the power is
used to overcome rolling resistance and air drag at 90 km/h, at 180 km/hr? Any
comment? Fraction of power: 25.50/150=0.17 (17% of max power is used) Fraction of power: 186.82/150=1.25 (125% of max power is used)
Car will not run (not enough horse power) 4. A hydraulic cylinder has a piston of cross sectional area 25 cm2 and a fluid
pressure of 2 MPa. If the piston is moved 0.25 m how much work is done?
This work is a force with a displacement and force is constant : F=PA 5. A linear spring, F = ks(x − x0), with spring constant ks = 500 N/m, is stretched
until it is 100 mm longer. Find the required force and work input. 6. A triple pane (each 5 mm thick) glass window has air gaps of 0.8 cm. The
outside is at –15oC with a convection heat transfer coefficient of 175 W/m2K.
The inside glass surface temperature of the inner pane is at 293 K.
a. Find the rate of heat transfer and the lowest temperature in the air gap.
Spring 2009 b. Compare the rate of heat transfer in a) to that of a single pane window
with a thickness of 4 mm
c. Compare the rate of heat transfer in a) to that of a double pane window
with a thickness of 4 mm and an air gap of 1 cm
kglass = 1.4 W/m.K kair = 0.026 W/m.K Use the notes on heat transfer on WebCT to do these calculations easily and
quickly.
a. Triple pane Lowest temperature in the air gap: 258.6K(14.4
b. Single pane c. Double pane Spring 2009 ...
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This note was uploaded on 05/25/2010 for the course E234 E234 taught by Professor Gallois during the Spring '09 term at Stevens.
 Spring '09
 GALLOIS

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