Faculty of Engineering
Engineering Physics 1BE (121)
Fall 2012
Dr Mohamed Fahmy
Sheet (4)
Heat transfer techniques and thermodynamics
1. A 5 cm bar of gold is in thermal contact with a
10 cm bar of silver of the same area (12.5
cm2). One end of the compound bar is
maintained at 80°C while the opposite end is
at 30°C. When the energy transfer reaches
steady state, what are the rate of energy transfer by conduction and the
temperature at the junction? The thermal conductivities of both gold
and silver are kAu =314 W/mC, and kAg =427 W/mC, respectively.
2. The surface of the Sun has a temperature of about 5800 K. The radius
of the Sun is 6.96×108 m. Calculate the total energy radiated by the Sun
each second. Assume the emissivity is 0.986 , and Stefan’s constant
 = 5.6696 10-8 W/m2 K4.
3- A thermal window with an area of 6.00 m2 is constructed of two layers
of glass, each 4.00 mm thick, separated from each other by an air space
of 5.00 mm. If the inside surface is at 20.0°C and the outside is at 30.0°C, what is the rate of energy transfer by conduction through the
window? (use the table enclosed)
Thermodynamics
4- A 1.00-kg block of aluminum is heated at atmospheric pressure so that
its temperature increases from 22.0°C to 40.0°C. Find (a) the work done
on the aluminum, (b) the energy added to it by heat, and (c) the change
in its internal energy.
(SHC of aluminum = 900J/Kg K, ⍴𝐴𝑙 = 2700 𝐾𝑔/𝑚3 )
5- In a constant-volume process, 209 J of energy is transferred by heat to
1.00 mol of an ideal monatomic gas initially at 300 K. Find (a) the
increase in internal energy of the gas, (b) the work done on it, and (c) its
final temperature.
1
Faculty of Engineering
Engineering Physics 1BE (121)
Fall 2012
Dr Mohamed Fahmy
6- During the compression stroke of a certain gasoline engine, the pressure
increases from 1.00 atm to 20.0 atm. If the process is adiabatic and the
fuel–air mixture behaves as a diatomic ideal gas, (a) by what factor
does the volume change and (b) by what factor does the temperature
change? (c) Assuming that the compression starts with 0.016 0 mol of
gas at 27.0°C, find the values of Q, W, and ∆𝐸𝑖𝑛𝑡 that characterize the
process.
Assignment
1- A glass windowpane has an area of 3.00 m2 and a thickness of 0.600 cm. If
the temperature difference between its faces is 25.0°C and the thermal
conductivity of glass is 0.8W/moC, what is the rate of energy transfer
by conduction through the window?
2- (a) Determine the work done
on a fluid that expands from i
to f as indicated in Figure. (b)
What If? How much work is
performed on the fluid if it is
compressed from f to i along
the same path?
3-An ideal gas initially at Pi , Vi , and Ti is
taken through a cycle as in Figure. (a)
Find the net work done on the gas per
cycle. (b) What is the net energy added
by heat to the system per cycle? (c)
Obtain a numerical value for the net
work done per cycle for 1.00 mol of gas
initially at 0°C.
2
Faculty of Engineering
Engineering Physics 1BE (121)
Fall 2012
Dr Mohamed Fahmy
Table of Thermal Conductivities
3