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Student Study Guide for 5
th
edition of Thermodynamics by Y. A.
Ç
engel & M. A. Boles
4 1
Chapter 4 1
Chapter 4: Energy Analysis of Closed Systems
The first law of thermodynamics is an expression of the conservation of
energy principle.
Energy can cross the boundaries of a closed system in
the form of heat or work.
Energy transfer across a system boundary due
solely to the temperature difference between a system and its surroundings
is called heat.
Work energy can be thought of as the energy expended to lift a weight.
Closed System First Law
A closed system moving relative to a reference plane is shown below
where z is the elevation of the center of mass above the reference plane
and
G
V
is the velocity of the center of mass.
For the closed system shown above, the
conservation of energy principle
or
the first law of thermodynamics
is expressed as
Total energy
entering the system
Total energy
leaving the system
The change in total
energy of the system
F
H
G
I
K
J
−
F
H
G
I
K
J
=
F
H
G
I
K
J
or
Heat
Work
z
Closed
System
Reference Plane,
z
= 0
G
V
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View Full DocumentStudent Study Guide for 5
th
edition of Thermodynamics by Y. A.
Ç
engel & M. A. Boles
4 2
Chapter 4 2
EE
E
in
out
system
−=
∆
According to classical thermodynamics, we consider the energy added to
be net heat transfer to the closed system and the energy leaving the closed
system to be net work done by the closed system.
So
QW
E
net
net
system
∆
Where
2
1
()
net
in
out
net
out
in other
b
b
QQ
Q
WW
W
W
WP
d
V
=−
+
=
∫
Normally the stored energy, or total energy, of a system is expressed as the
sum of three separate energies.
The
total energy of the system
,
E
system
, is
given as
E
Internal energy
Kinetic energy
Potential energy
EU
K
E
P
E
=
+
=
Recall that
U
is the sum of the energy contained within the molecules of
the system other than the kinetic and potential energies of the system as a
whole and is called the internal energy.
The internal energy
U
is
dependent on the state of the system and the mass of the system.
Student Study Guide for 5
th
edition of Thermodynamics by Y. A.
Ç
engel & M. A. Boles
4 3
Chapter 4 3
For a system moving relative to a reference plane, the kinetic energy
KE
and the potential energy
PE
are given by
2
0
0
2
V
V
z
z
mV
KE
mV dV
PE
mg dz
mgz
=
=
==
∫
∫
G
G
G
GG
The change in stored energy for the system is
∆∆∆
∆
EUK
E
P
E
=+
+
Now the
conservation of energy principle,
or the
first law of
thermodynamics for closed systems,
is written as
QW
UK
EP
E
net
net
−=+
+
∆∆
∆
If the system does not move with a velocity and has no change in
elevation, the conservation of energy equation reduces to
U
net
net
−=
∆
We will find that this is the most commonly used form of the first law.
Closed System First Law for a Cycle
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View Full DocumentStudent Study Guide for 5
th
edition of Thermodynamics by Y. A.
Ç
engel & M. A. Boles
4 4
Chapter 4 4
Since a thermodynamic cycle is composed of processes that cause the
working fluid to undergo a series of state changes through a series of
processes such that the final and initial states are identical, the change in
internal energy of the working fluid is zero for whole numbers of cycles.
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This note was uploaded on 09/17/2009 for the course MAE 301 taught by Professor Hassan during the Fall '08 term at N.C. State.
 Fall '08
 Hassan

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