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W PDV: Heat and Work in Thermodynamic Processes

1) A thermodynamic system can exchange energy with its surroundings through heat transfer or work. The work done by a system is calculated by integrating its pressure over the change in volume. 2) Both the heat transferred to a system and the work done depend on the initial and final states as well as the path between those states. 3) According to the first law of thermodynamics, the change in a system's internal energy equals the heat transferred to it minus the work done by it.

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129 views1 page

W PDV: Heat and Work in Thermodynamic Processes

1) A thermodynamic system can exchange energy with its surroundings through heat transfer or work. The work done by a system is calculated by integrating its pressure over the change in volume. 2) Both the heat transferred to a system and the work done depend on the initial and final states as well as the path between those states. 3) According to the first law of thermodynamics, the change in a system's internal energy equals the heat transferred to it minus the work done by it.

Uploaded by

Jemar Lim
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
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643

CHAPTER
19
SUMMARY
Heat and work in thermodynamic processes: Athermo-
dynamic system has the potential to exchange energy
with its surroundings by heat transfer or by mechanical
work. When a system at pressure p changes volume
from to it does an amount of work W given by the
integral of p with respect to volume. If the pressure is
constant, the work done is equal to p times the change in
volume. Anegative value of W means that work is done
on the system. (See Example 19.1.)
In any thermodynamic process, the heat added to the
system and the work done by the system depend not
only on the initial and nal states, but also on the path
(the series of intermediate states through which the sys-
tem passes).
V
2
, V
1
(19.2)
(19.3)
(constant pressure only)
W = p1V
2
- V
1
2
W =
L
V
2
V
1
p dV
The rst law of thermodynamics: The rst law of ther-
modynamics states that when heat Q is added to a sys-
tem while the system does work W, the internal energy
U changes by an amount equal to This law can
also be expressed for an innitesimal process. (See
Examples 19.2, 19.3, and 19.5.)
The internal energy of any thermodynamic system
depends only on its state. The change in internal energy
in any process depends only on the initial and nal
states, not on the path. The internal energy of an isolated
system is constant. (See Example 19.4.)
Q - W.
(19.4)
(19.6)
(innitesimal process)
dU = dQ - dW
U = Q - W
Important kinds of thermodynamic processes:
Adiabatic process: No heat transfer into or out of a system;
Isochoric process: Constant volume;
Isobaric process: Constant pressure;
Isothermal process: Constant temperature.
W = p1V
2
- V
1
2.
W = 0.
Q = 0.
Thermodynamics of ideal gases: The internal energy of
an ideal gas depends only on its temperature, not on its
pressure or volume. For other substances the internal
energy generally depends on both pressure and
temperature.
The molar heat capacities and of an ideal gas
differ by R, the ideal-gas constant. The dimensionless
ratio of heat capacities, is denoted by (See
Example 19.6.)
g. C
p
>C
V
,
C
p
C
V
(19.17)
(19.18) g =
C
p
C
V
C
p
= C
V
+ R
Adiabatic processes in ideal gases: For an adiabatic
process for an ideal gas, the quantities and
are constant. The work done by an ideal gas during an
adiabatic expansion can be expressed in terms of the
initial and nal values of temperature, or in terms of the
initial and nal values of pressure and volume. (See
Example 19.7.)
pV
g
TV
g-1
(19.25)
(19.26) =
1
g - 1
1p
1
V
1
- p
2
V
2
2
=
C
V
R
1p
1
V
1
- p
2
V
2
2
W = nC
V
1T
1
- T
2
2
Work 5 Area
1
2
p
1
p
2
V
1
V
2
V
O
p
5
V
1
p dV .0
V
2
Volume increases
(V
2
. V
1
):
work and area
are positive.
Q 5 150 J W 5 100 J
U 5 Q 2 W 5 1 50 J
Surroundings
(environment)
System
O
p
3
2
1
4
a
Isochoric
T
2
, T
a
Adiabatic
T
1
, T
a
p
a
V
a
Isobaric
T
3
. T
a
Isothermal
T
4
5 T
a
V
p
1
p
V
1
O
V
Q 5 DU
Q 5 DU 1 W
T
1
, U
1
T
2
, U
2
V
2
p
2
W
p
b
a p
a
V
a
O
V
p
b
V
b
T T 1 dT
W
Adiabatic process a S b:
Q 5 0, U 5 2W

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