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Chapter 2
Energy, Energy Transfer,
and General Energy
Analysis
Thermodynamics: An Engineering
Approach, 10th Edition
Yunus A. Cengel | Michael A. Boles |
Mehmet Kanoglu McGraw-Hill, 2024
© McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC.
Forms of Energy
1
Energy can exist in numerous forms such as thermal, mechanical,
kinetic, potential, electric, magnetic, chemical, and nuclear, and
their sum constitutes the total energy, E of a system.
Thermodynamics deals only with the change of the total energy.
Macroscopic forms of energy: Those a system possesses as a
whole with respect to some outside reference frame, such as
kinetic and potential energies.
Microscopic forms of energy: Those related to the molecular
structure of a system and the degree of the molecular activity.
Internal energy, U: The sum of all the microscopic forms of energy.
© McGraw Hill
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Forms of Energy
3
Kinetic energy, KE: The energy that a system possesses as a result of
its motion relative to some reference frame.
Potential energy, PE: The energy that a system possesses as a result of
its elevation in a gravitational field.
Figure 2-4
The macroscopic energy of an object changes with velocity and
elevation.
© McGraw Hill
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Forms of Energy
V2
KE = m
2
Kinetic energy
( kJ )
V2
ke =
2
( kJ/kg )
PE = mgz
(kJ)
pe = gz
( kJ/kg )
Kinetic energy per unit mass
Potential energy
Potential energy per unit mass
V2
E = U + KE + PE = U + m
+ mgz
2
V2
e = u + ke + pe = u +
+ gz
2
E = me
© McGraw Hill
4
( kJ/s or kW )
( kJ )
Total energy of a system
( kJ/kg )
Energy of a system per unit
mass
Energy flow rate
8
Forms of Energy
5
Figure 2-5
Mass and energy flow rates associated with the flow of steam in a pipe
of inner diameter D with an average velocity of Vavg .
Mass flow rate:
m = V =  AcVavg
Energy flow rate:
E = me
© McGraw Hill
( kg/s )
( kJ/s or kw )
9
Forms of Energy
6
Some Physical Insight to Internal
Energy
Sensible energy: The portion of the
internal energy of a system associated
with the kinetic energies of the
molecules.
Latent energy: The internal energy
associated with the phase of a system.
Chemical energy: The internal energy
associated with the atomic bonds in a
molecule.
Nuclear energy: The tremendous
amount of energy associated with the
strong bonds within the nucleus of the
atom itself.
© McGraw Hill
Figure 2-6
The various forms of microscopic
energies that make up sensible
energy.
10
Forms of Energy
7
Thermal = Sensible + Latent
Internal = Sensible + Latent + Chemical + Nuclear
Figure 2-7
The internal energy of a system is the sum of all forms of the microscopic
energies.
© McGraw Hill
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Forms of Energy
8
The total energy of a system, can be contained or stored in a system, and
thus can be viewed as the static forms of energy.
The forms of energy not stored in a system can be viewed as the
dynamic forms of energy or as energy interactions.
The dynamic forms of energy are recognized at the system boundary as
they cross it, and they represent the energy gained or lost by a system
during a process.
The only two forms of energy interactions associated with a closed
system are
• heat transfer
• work
The difference between heat transfer and work: An energy interaction
is heat transfer if its driving force is a temperature difference. Otherwise it
is work.
© McGraw Hill
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Forms of Energy
12
Mechanical Energy
Mechanical energy: The form of energy that can be converted to mechanical
work completely and directly by an ideal mechanical device such as an ideal
turbine.
Kinetic and potential energies: The familiar forms of mechanical energy.
V2
emech = +
+ gz
 2
P
Mechanical energy of a flowing
fluid per unit mass
 P V2

Emech = memech = m  +
+ gz 
 2

Rate of mechanical energy
of a flowing fluid
Mechanical energy change of a fluid during incompressible flow per unit mass
emech =
P2 − P1

V22 − V12
+
+ g ( z2 − z1 )
2
( kJ/kg )
Rate of mechanical energy change of a fluid during incompressible flow
 P2 − P1 V22 − V12

Emech = memech = m 
+
+ g ( z2 − z1 ) 
2
 

© McGraw Hill
( kW )
16
2-3 Energy Transfer by Heat
1
Heat: The form of energy that is transferred between two systems (or a
system and its surroundings) by virtue of a temperature difference.
Figure 2-14
Figure 2-15
Energy can cross the
boundaries of a closed system
in the form of heat and work.
Temperature difference is the driving force for
heat transfer. The larger the temperature
difference, the higher is the rate of heat
transfer.
© McGraw Hill
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2-3 Energy Transfer by Heat
2
Heat transfer per unit mass
q=
Q
m
( kJ/kg )
Amount of heat transfer when
heat transfer rate is constant
Q = Q t
( kJ )
Amount of heat transfer when
heat transfer rate changes with
time
t2
Q =  Q dt
Figure 2-16
Energy is recognized as heat
transfer only as it crosses the
system boundary.
t1
© McGraw Hill
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2-3 Energy Transfer by Heat
3
Figure 2-17
Figure 2-18
During an adiabatic process, a
system exchanges no heat with its
surroundings.
The relationships among q, Q, and
© McGraw Hill
Q
21
2-3 Energy Transfer by Heat
4
Historical Background on Heat
Caloric theory: It asserts that heat
is a fluidlike substance called the
caloric that is a massless,
colorless, odorless, and tasteless
substance that can be poured from
one body into another
Kinetic theory: Treats molecules
as tiny balls that are in motion and
thus possess kinetic energy.
Figure 2-19
Heat: The energy associated with
the random motion of atoms and
molecules.
In the early 19th century, heat was
thought to be an invisible fluid
called the caloric that flowed from
warmer bodies to cooler ones.
© McGraw Hill
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2-3 Energy Transfer by Heat
5
Heat transfer mechanisms
Conduction: The transfer of energy from the more energetic
particles of a substance to the adjacent less energetic ones as a
result of interaction between particles.
Convection: The transfer of energy between a solid surface and
the adjacent fluid that is in motion, and it involves the combined
effects of conduction and fluid motion.
Radiation: The transfer of energy due to the emission of
electromagnetic waves (or photons).
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2-4 Energy Transfer by Work
1
Work: The energy transfer associated
with a force acting through a distance.
A rising piston, a rotating shaft, and an
electric wire crossing the system
boundaries are all associated with
work interactions
Formal sign convention: Heat transfer
to a system and work done by a
system are positive; heat transfer from
a system and work done on a system
are negative.
Alternative to sign convention is to use
the subscripts in and out to indicate
direction. This is the primary approach
in this text.
© McGraw Hill
Figure 2-21
Specifying the directions of heat
and work.
24
2-4 Energy Transfer by Work
w=
W
m
( kJ/kg )
2
Work done per unit mass
Figure 2-20 The relationships among w, W, and W .
© McGraw Hill
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2-4 Energy Transfer by Work
3
Heat v s. Work
ersu
Both are recognized at the boundaries of a system as they cross the
boundaries. That is, both heat and work are boundary phenomena.
Systems possess energy but not heat or work.
Both are associated with a process, not a state.
Unlike properties, heat or work has no meaning at a state.
Both are path functions (that is, their magnitudes depend on the path
followed during a process as well as the end states).
Properties are point functions
have exact differentials (d).
Path functions have
inexact differentials
© McGraw Hill
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 dV = V − V = V
2
1
1
2
( )
  W = W ( not W )
12
1
26
2-4 Energy Transfer by Work
4
Figure 2-22
Properties are point functions; but heat and work are path functions (their
magnitudes depend on the path followed).
© McGraw Hill
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2-4 Energy Transfer by Work
5
Electrical Work
Electrical work
We = VI ( W )
Electrical power
When potential difference and
current change with time
2
We =  VI dt
( kJ )
1
When potential difference and
current remain constant
We = VI t
© McGraw Hill
( kJ )
Figure 2-27
Electrical power in terms of
resistance R, current I, and
potential difference V.
28
2-5 Mechanical Forms of Work
1
There are two requirements for a work interaction between a system and
its surroundings to exist:
• there must be a force acting on the boundary.
• the boundary must move.
Work = Force × Distance
W = Fs
( kJ )
When force is not constant:
2
W =  F ds
1
© McGraw Hill
( kJ )
Figure 2-28
The work done is proportional to the force
applied (F) and the distance traveled (s).
29
2-5 Mechanical Forms of Work
2
Shaft Work
T = Fr
→
s = ( 2 r ) n
F=
T
r
A force F acting through a moment arm
r generates a torque T
This force acts through a distance s
T 
Wsh = Fs =   ( 2 rn ) = 2 nT ( kJ )
r
Shaft work
The power transmitted through
the shaft is the shaft work done
per unit time:
Wsh = 2 nT
© McGraw Hill
( kW )
Figure 2-30
Shaft work is proportional to the
torque applied and the number of
revolutions of the shaft.
30
2-5 Mechanical Forms of Work
3
Figure 2-29
Energy transmission through rotating shafts is commonly encountered in practice.
© McGraw Hill
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2-5 Mechanical Forms of Work
4
Spring Work
When the length of the spring changes
by a differential amount dx under the
influence of a force F, the work done is
 Wspring = F dx
( kJ )
For linear elastic springs, the displacement
x is proportional to the force applied
F = kx
( kN )
k: spring constant (kN/m)
Spring work:
Wspring =
1
k ( x2 2 − x12 )
2
( kJ )
x1 and x 2 :the initial and the final
displacements
© McGraw Hill
Figure 2-32
Elongation of a spring under the
influence of a force.
32
2-5 Mechanical Forms of Work
5
Figure 2-33
The displacement of a linear spring doubles when the force is doubled.
© McGraw Hill
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2-5 Mechanical Forms of Work
Work Done on Elastic Solid Bars
2
2
1
1
Welastic =  F dx =   n Adx
6
( kJ )
Figure 2-34
Solid bars behave as springs under the influence of a force.
© McGraw Hill
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2-6 The First Law of Thermodynamics
1
The first law of thermodynamics (the
conservation of energy principle)
provides a sound basis for studying the
relationships among the various forms
of energy and energy interactions.
The first law states that energy can be
neither created nor destroyed during a
process; it can only change forms.
First Law: For all adiabatic processes
between two specified states of a
closed system, the net work done is the
same regardless of the nature of the
closed system and the details of the
process.
© McGraw Hill
Figure 2-39
Energy cannot be created
or destroyed; it can only
change forms.
38
2-6 The First Law of Thermodynamics
2
Figure 2-40
The increase in the energy of a potato in an oven is equal to the amount of heat
transferred to it.
© McGraw Hill
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2-6 The First Law of Thermodynamics
Figure 2-41
Figure 2-42
In the absence of any work
interactions, the energy change
of a system is equal to the net
heat transfer.
The work (electrical) done on an
adiabatic system is equal to the
increase in the energy of the
system.
© McGraw Hill
3
40
2-6 The First Law of Thermodynamics
Figure 2-43
Figure 2-44
The work (shaft) done on an
adiabatic system is equal to the
increase in the energy of the
system
The work (boundary) done on an
adiabatic system is equal to the
increase in the energy of the
system.
© McGraw Hill
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41
2-6 The First Law of Thermodynamics
5
Energy Balance
Total energy 
 Change in the total 
 Total energy 

=
−






 energy of the system 
 leaving the system 
 entering the system 
Ein − Eout = Esystem
The net change (increase or
decrease) in the total energy of
the system during a process is
equal to the difference between
the total energy entering and the
total energy leaving the system
during that process.
© McGraw Hill
Figure 2-45
The energy change of a system during a
process is equal to the net work and
heat transfer between the system and its
surroundings.
42
2-6 The First Law of Thermodynamics
6
Energy Change of a System, ΔEsystem
Energy change = Energy at final state − Energy at initial state
Esystem = Efinal − Einitial = E2 − E1
E = U + KE + PE
K E = PE = 0; thus E = U .
Internal, kinetic, and potential
energy changes:
U = m ( u2 − u1 )
KE =
1
m (V2 2 − V12 )
2
PE = mg ( z2 − z1 )
© McGraw Hill
Figure 2-46
For stationary systems,
43
2-6 The First Law of Thermodynamics
7
Mechanisms of Energy Transfer, Ein and Eout
Energy balance for any system undergoing any kind of process can be
expressed more compactly as
E −E
in
out
Net energy transfer
=
E
system
Change in internal, kinetic
by heat, work, and mass
( kJ )
(2-35)
( kW )
(2-36)
potential, etc., energies
In the rate form,
Ein − Eout
=
Rate of net energy transfer
by heat, work, and mass
Esystem
Rate of change in internal,
kinetic, potential, etc., energies
For constant rates, the total quantities during a time interval t are related
related to the quantities per unit time as
Q = Qt
© McGraw Hill
W = W t
 dE 
E = 
 t
dt


( kJ )
(2-37)
44
2-6 The First Law of Thermodynamics
8
The energy balance can be expressed on a per unit mass basis as
ein − eout = esystem
( kJ/kg )
(2-38)
which is obtained by dividing all the quantities in Eq. 2–35 by the mass m
of the system. Energy balance can also be expressed in the differential
form as
 Ein −  Eout = dEsystem
© McGraw Hill
or
 ein −  eout = desystem
(2-39)
45
2-6 The First Law of Thermodynamics
9
Mechanisms of energy transfer:
• Heat transfer
• Work
• Mass flow
(
)
Ein − Eout = ( Qin − Qout ) + (Win − Wout ) + Emass,in − Emass,out = Esystem
A closed system with no mass flow involves only heat transfer and work.
Wnet,out = Qnet,in
© McGraw Hill
or
Wnet,out = Qnet,in
( for a cycle )
46
2-6 The First Law of Thermodynamics
Figure 2-47
The energy content of a control
volume can be changed by mass
flow as well as by heat and work
interactions.
© McGraw Hill
Figure 2-48
For a cycle
10
E = 0, thus Q = W .
47
2-7 Energy Conversion Efficiencies
1
Efficiency is one of the most frequently used terms in
thermodynamics, and it indicates how well an energy
conversion or transfer process is accomplished.
Efficiency =
Desired output
Required input
Efficiency of a water heater: The ratio of the
energy delivered to the house by hot water to the
energy supplied to the water heater.
Type
Efficiency
Gas, conventional
55%
Gas, high-efficiency
62%
Electric, conventional
90%
Electric, highefficiency
94%
© McGraw Hill
Figure 2-53
Typical efficiencies of conventional
and high-efficiency electric and
natural gas water heaters.
48
Summary
• Forms of energy.
• Energy transfer by heat.
• Energy transfer by work.
• Mechanical forms of work.
• The first law of thermodynamics.
• Energy conversion efficiencies.
© McGraw Hill
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