This preview shows pages 1–3. Sign up to view the full content.
Chapter 2: Heat, Work, Internal Energy, Enthalpy, and the
First Law of Thermodynamics
Problem numbers in italics indicate that the solution is included in the
Student’s Solutions Manual.
Questions on Concepts
Q2.1)
Electrical current is passed through a resistor immersed in a liquid in an adiabatic
container.
The temperature of the liquid is varied by 1ºC. The system consists solely of
the liquid.
Does heat or work flow across the boundary between the system and
surroundings?
Justify your answer.
Although work is done on the resistor, this work is done in the surroundings. Heat
flows across the boundary between the surroundings and the system because of
the temperature difference between them.
Q2.2)
Explain how a mass of water in the surroundings can be used to determine
q
for a
process.
Calculate
q
if the temperature of 1.00kg water bath in the surroundings
increases by 1.25ºC.
Assume that the surroundings are at a constant pressure.
If heat flows across the boundary between the system and the surroundings, it will
lead to a temperature change in the surroundings given by
P
q
T
C
∆
=
.
For the
case of interest,
1
1
3
1000 g 4.19 Jg K
1.25 K
5.24 10 J.
surroundings
P
q
q
mC
T


= 
= 
∆
= 
×
×
= 
×
Q2.3)
Explain the relationship between the terms
exact differential
and
state function
.
In order for a function
f
(
x,y
) to be a state function, it must be possible to write the
total differential
df
in the form
.
y
x
f
f
df
dx
dy
x
y
∂
∂
=
+
∂
∂
If the form
df
as
written exists, it is an exact differential.
Q2.4)
Why is it incorrect to speak of the heat or work associated with a system?
Heat and work are transients that exist only in the transition between equilibrium
states.
Therefore, a state at equilibrium is not associated with values of heat or
work.
Q2.5)
Two ideal gas systems undergo reversible expansion starting from the same
P
and
V
. At the end of the expansion, the two systems have the same volume.
The pressure in
21
This preview has intentionally blurred sections. Sign up to view the full version.
View Full DocumentChapter 2/ Heat, Work, Internal Energy, Enthalpy, and the First Law of Thermodynamics
the system that has undergone adiabatic expansion is lower than in the system that has
undergone isothermal expansion.
Explain this result without using equations.
In the system undergoing adiabatic expansion, all the work done must come
through the lowering of
∆
U
, and therefore of the temperature.
By contrast, some
of the work done in the isothermal expansion can come at the expense of the heat
that has flowed across the boundary between the system and surroundings.
Q2.6)
A cup of water at 278 K (the system) is placed in a microwave oven and the oven
is turned on for 1 minute during which it begins to boil. Which of
q
,
w
, and
∆
U
are
positive, negative, or zero?
The heat
This is the end of the preview. Sign up
to
access the rest of the document.
 Spring '10
 dr.P
 Enthalpy

Click to edit the document details