thermodynamics tutorial.pptxyjhhjnnbhhbhh

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Thermodynamics tutorial
For exit exam
By: Taz D.

THERMODYNAMICS AND ENERGY
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 Thermodynamics: The science of energy.
 Energy: The ability to cause changes.
 The most fundamental laws of nature
Conservation of energy principle:
During an interaction, energy can change
from one form to another but the total
amount of energy remains constant.
 Energy cannot be created or destroyed.
 The first law of thermodynamics: An
expression of the conservation of energy
principle.
 The first law asserts that energy is a
thermodynamic property.
 Example: A rock falling off a cliff

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 The second law of thermodynamics:
It asserts that energy has quality as
well as quantity, and actual processes
occur in the direction of decreasing
quality of energy.
 A pure crystalline substance at absolute
zero temperature is in perfect order,
and its entropy is zero (the third law of
thermodynamics)
 The zeroth law of thermodynamics:
If two bodies are in thermal equilibrium
with a third body, they are also in
thermal equilibrium with each other.
 By replacing the third body with a
thermometer, the zeroth law can be
restated as two bodies are in thermal
equilibrium if both have the same
temperature reading even if they are
not in contact.

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 Classical thermodynamics: A macroscopic approach
to the study of thermodynamics that does not require a
knowledge of the behavior of individual particles.
 It is based more upon experimental observations
 It provides a direct and easy way to the solution of
engineering problems and it is used in this text.
 Statistical thermodynamics: A microscopic approach,
based on the average behavior of large groups of
individual particles.
 It is used in this text only in the supporting role.
Classical and Statistical Thermodynamics

Application Areas of Thermodynamics
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All activities in nature involve some interaction between
energy and matter; thus, it is hard to imagine an area
that does not relate to thermodynamics in some manner.

Thermodynamics system
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 System: A quantity of matter or a region in space chosen for study.
 Surroundings: The mass or region outside the system
 Boundary: The real or imaginary surface that separates the system
from its surroundings.
 The boundary of a system can be fixed or movable.
 Systems may be considered to be closed or open.
 depending on whether a fixed mass or a fixed volume in space is
chosen for study
 Closed system (Control mass): A fixed amount of mass, and no mass
can cross its boundary
 no mass can enter or leave a closed system
 But energy, in the form of heat or work, can cross the boundary
 the volume of a closed system does not have to be fixed

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 Energy is not allowed to cross the
boundary, that system is called an
isolated system

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 Open system (control volume): A properly selected
region in space.
 It usually encloses a device that involves mass flow
such as a compressor, turbine, or nozzle.
 Both mass and energy can cross the boundary of a
control volume.
 Control surface: The boundaries of a control volume.
It can be real or imaginary.
A control volume can involve
fixed, moving, real, and imaginary
boundaries.
 A large number of engineering problems involve mass flow in and
out of a system and, therefore, are modeled as control volumes.

PROPERTIES OF A SYSTEM
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 Property: Any characteristic of a
system.
 Some familiar properties are
pressure P, temperature T, volume
V, and mass m.
 Properties are considered to be
either intensive or extensive.
 Intensive properties: Those that
are independent of the mass of a
system, such as temperature,
pressure, and density.
 Extensive properties: Those whose
values depend on the size—or
extent—of the system.
 Specific properties: Extensive
properties per unit mass.

DENSITY AND SPECIFIC GRAVITY
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Density is
mass per unit
volume;
specific volume
is volume per
unit mass.
Specific gravity: The ratio
of the density of a
substance to the density of
some standard substance
at a specified temperature
(usually water at 4°C).
Density
Specific weight: The
weight of a unit volume
of a substance.
Specific volume

STATE AND EQUILIBRIUM
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 Thermodynamics deals with equilibrium
states.
 Equilibrium: A state of balance.
 In an equilibrium state there are no
unbalanced potentials (or driving forces)
within the system.
 Thermal equilibrium: If the temperature is
the same throughout the entire system.
 Mechanical equilibrium: If there is no
change in pressure at any point of the
system with time.
 Phase equilibrium: If a system involves
two phases and when the mass of each
phase reaches an equilibrium level and
stays there.
 Chemical equilibrium: If the chemical
composition of a system does not change
with time, that is, no chemical reactions
occur.

The State Postulate
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 The number of properties
required to fix the state of a
system is given by the state
postulate:
 The state of a simple
compressible system is
completely specified by two
independent, intensive
properties.
 Simple compressible
system: If a system involves
no electrical, magnetic,
gravitational, motion, and
surface tension effects.

PROCESSES AND CYCLES
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Process: Any change that a system undergoes from one equilibrium state to
another.
Path: The series of states through which a system passes during a process.
To describe a process completely, one should specify the initial and final states, as
well as the path it follows, and the interactions with the surroundings.
Quasistatic or quasi-equilibrium process: When a process proceeds in such a
manner that the system remains infinitesimally close to an equilibrium state at all
times.

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 Process diagrams plotted by
employing thermodynamic properties
as coordinates are very useful in
visualizing the processes.
 Some common properties that are
used as coordinates are temperature
T, pressure P, and volume V (or
specific volume v).
 The prefix iso- is often used to
designate a process for which a
particularproperty remains constant.
 Isothermal process: A process during
which the temperature T remains
constant.
 Isobaric process: A process during
which the pressure P remains
constant.
 Isochoric (or isometric) process: A
process during which the specific
volume v remains constant.
 Cycle: A process during which the
initial and final states are identical.

The Steady-Flow Process
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 The term steady implies no change
with time. The opposite of steady is
unsteady, or transient.
 A large number of engineering
devices operate for long periods of
time under the same conditions, and
they are classified as steady-flow
devices.
 Steady-flow process: A process
during which a fluid flows through a
control volume steadily.
 Steady-flow conditions can be closely
approximated by devices that are
intended for continuous operation
such as turbines, pumps, boilers,
condensers, and heat exchangers or
power plants or refrigeration
systems.

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1. Which of the following occurs without a change in the
internal energy?
a) Isochoric process
b) Isenthalpic process
c) Steady-state process
d) Isenthalpic process
Answer: C
Question
2. Which of the following is not steady state device?
a) Turbines and Pumps
b) Boilers and Condensers
c) Power plants or Refrigeration systems
d) Operation of Irrigation and Power Canals
e) heat exchanger and heat sinks
Answer: D

Temperature Scales
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 All temperature scales are based on some easily reproducible
states such as the freezing and boiling points of water: the ice
point and the steam point.
 Ice point: A mixture of ice and water that is in equilibrium with air
saturated with vapor at 1 atm pressure (0°C or 32°F).
 Steam point: A mixture of liquid water and water vapor (with no
air) in equilibrium at 1 atm pressure (100°C or 212°F).
 Celsius scale: in SI unit system
 Fahrenheit scale: in English unit system
 Thermodynamic temperature scale: A temperature scale that is
independent of the properties of any substance.
 Kelvin scale (SI) Rankine scale (E)
 A temperature scale nearly identical to the Kelvin scale is the
ideal-gas temperature scale. The temperatures on this scale are
measured using a constant-volume gas thermometer.

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Comparison of
temperature
scales.
• The reference temperature in the original Kelvin scale was the ice point,
273.15 K, which is the temperature at which water freezes (or ice melts).
• The reference point was changed to a much more precisely reproducible
point, the triple point of water (the state at which all three phases of water
coexist in equilibrium), which is assigned the value 273.16 K.
Comparison of
magnitudes of
various
temperature
units.

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1. What is the value of the absolute thermodynamic temperature
scale?
a) 3K
b) 0K
c) 1K
d) 4K
Question
Answer: B
2. Which of the following is chosen as the standard thermometric
substance?
a) Liquid
b) Solid
c) Gas
d) None of the mentioned
Answer: C
Explanation: Smallest variation is observed among different gas
thermometers.

Chapter Two
Properties of Pure Substance
Simple System
A simple system is one in which the effects of motion, viscosity, fluid shear,
capillarity, anisotropic stress, and external force fields are absent.
Homogeneous Substance
A substance that has uniform thermodynamic properties throughout is said
to be homogeneous.
Pure Substance
A pure substance has a homogeneous and invariable chemical composition
and may exist in more than one phase.
Examples: 1. Water (solid, liquid, and vapor phases)
2. Mixture of liquid water and water vapor
3. Carbon dioxide, CO2
4. Nitrogen, N2
5. Mixtures of gases, such as air, as long as there is no change of phase.
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1. Which of the following is a property of a pure substance?
a) it has constant chemical composition throughout its mass
b) it is a one-component system
c) it may exist in one or more phases
d) all of the mentioned
Question
Answer: D
2. Which of the following is not a pure substance?
a) Mixture of liquid water and water vapor
b) Carbon dioxide, CO2
c) A mixture of liquid air and gaseous air,
Answer: C
 Explanation: the composition of liquid air is different from the
composition of gaseous air, and thus the mixture is no longer chemically
homogeneous.
 This is due to different components in air condensing at different
temperatures at a specified pressure.

Figure below shows the P-T diagram, often called the phase diagram, for
pure substances that contract and expand upon freezing.
 The states on the triple line of a substance have the same pressure and
temperature but different specific volumes
 The triple point of water is 0.01oC, 0.6117 kPa (See Table 3-3).
 The critical point of water is 373.95oC, 22.064 MPa (See Table A-1).
The critical point, is the
point at which the
saturated liquid and
saturated vapor states
are identical.
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The Triple point, is the
point at which the three
phases coexist in
equilibrium.

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1. Which of the following is a line on the P-V diagram, where all the
three phases, solid, liquid and gas exist.
a) Critical point c) Saturation point
b) Triple point d) sub-cooled point
2. At a pressure below the triple point line,
a) the substance cannot exist in the liquid phase
b) the substance when heated transforms from solid to vapour
c) both of the mentioned
d) none of the mentioned
Question
Answer: B
Answer: C

Let's consider the results of heating liquid water from 20C, 1 atm while
keeping the pressure constant. First place liquid water in a piston-cylinder
device where a fixed weight is placed on the piston to keep the pressure of
the water constant at all times. As liquid water is heated while the pressure
is held constant, the following events occur.
Process 1-2:
The temperature and specific volume
will increase from the compressed
liquid, or sub-cooled liquid, state 1, to
the saturated liquid state 2. In the
compressed liquid region, the properties
of the liquid are approximately equal to
the properties of the saturated liquid
state at the temperature.
 Compressed liquid, or sub-cooled liquid
is not about to vaporize
Constant pressure heating process of water
Compressed liquid,
or sub-cooled liquid
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Process 2-3:
At state 2 the liquid has reached the temperature at which it begins to boil,
called the saturation temperature, and is said to exist as a saturated liquid.
v
Properties at the saturated liquid state are noted by the subscript f and v2 = vf.
During the phase change both the temperature and pressure remain constant
(according to the International Temperature Scale of 1990, ITS-90, water
boils at 99.975C  100C when the pressure is 1 atm or 101.325 kPa).
v
At state 3 the liquid and vapor phase are in equilibrium and any point on the
line between states 2 and 3 has the same temperature and pressure.
 A liquid that is about to vaporize is called a saturated liquid.
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Process 3-4:
At state 4 a saturated vapor exists and vaporization is complete.
The subscript g will always denote a saturated vapor state.
Note v4 = vg.
A vapor that is about to condense is called a saturated vapor.
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1. A saturation state is a state from which a change of phase may occur
a) without a change of pressure or temperature
b) with a change of pressure or temperature
2. Which of the following statement is true?
a) saturation temperature is a function of pressure
b) saturation pressure is a function of temperature
Question
Answer: A
Answer: C

Process 4-5:
If the constant pressure heating is continued, the temperature will begin to
increase above the saturation temperature, 100 C in this example, and the
volume also increases. State 5 is called a superheated state because T5 is
greater than the saturation temperature for the pressure.
v
Thermodynamic properties for water in the superheated region are found in
the superheated steam tables, Table A-6.
A vapor that is not about to condense (i.e., not a saturated vapor) is
called a superheated vapor.
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99.975 
The amount of energy absorbed or released during a phase-change
process is called the latent heat.
The amount of energy absorbed during melting is called the latent heat
of fusion and is equivalent to the amount of energy released during
freezing.
Similarly, the amount of energy absorbed during vaporization is called
the latent heat of vaporization and is equivalent to the energy released
during condensation.
The magnitudes of the latent heats depend on the temperature or
pressure at which the phase change occurs.
This constant pressure heating process is illustrated in the following figure.
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If all of the saturated liquid states are connected, the saturated liquid line is
established. If all of the saturated vapor states are connected, the saturated
vapor line is established.
These two lines intersect at the critical point and form what is often called
the “steam dome.”
Consider repeating this process for other constant pressure lines.

P2 = 1000 kPa
P1 = 100 kPa
99.61oC
179.88oC
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2. In which of the following state does water
exist?
a) saturated solid state
b) saturated liquid state
c) saturated vapour state
Question
Answer: B
Answer: D
3. Superheated vapour behaves
A. exactly as gas
B. as steam
C. as ordinary vapour
D. approximately as a gas
E. as average of gas and vapour.
Answer: D
1. At critical point, value of Vg-Vf is
a) Two c) zero
b) One d) infinity

Equations of State
The relationship among the state variables, temperature, pressure, and
specific volume is called the equation of state. We now consider the
equation of state for the vapor or gaseous phase of simple compressible
substances.
Ideal Gas
Based on our experience in chemistry and physics we recall that the
combination of Boyle’s and Charles’ laws for gases at low pressure result
in the equation of state for the ideal gas as
where R is the constant of proportionality and is called the gas constant
and takes on a different value for each gas. If a gas obeys this relation, it is
called an ideal gas. We often write this equation as
Pv RT

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The gas constant for ideal gases is related to the universal gas constant valid
for all substances through the molar mass (or molecular weight). Let Ru be
the universal gas constant. Then,
R
R
M
u

The molar mass is the ratio of mass to moles and has the same value
regardless of the system of units.
M
g
gmol
kg
kmol
lbm
lbmol
air   
2897 2897 2897
. . .
Since 1 kmol = 1000 gmol or 1000 gram-mole and 1 kg = 1000 g, 1 kmol of
air has a mass of 28.97 kg or 28,970 grams.
m N M

The ideal gas equation of state may be written several ways.
Pv RT
V
P RT
m
PV mRT



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Here
P = absolute pressure in MPa, or kPa
= molar specific volume in m3/kmol
T = absolute temperature in K
Ru = 8.314 kJ/(kmolK)
v
Universal Gas Constant, Ru
8.314 kJ/(kmolK)
8.314 kPam3/(kmolK)
1.986 Btu/(lbmolR)
1545 ftlbf/(lbmolR)
10.73 psiaft3/(lbmolR)
The ideal gas equation of state can
be derived from basic principles if
one assumes
1. Intermolecular forces are small.
2. Volume occupied by the
particles is small.
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Answer: A
Question
Answer: B

Chapter Three: Work and Heat
 If we take the entire room—including the air and the refrigerator (or fan)—as the
system, which is an adiabatic closed system since the room is well-sealed and well-
insulated, what will happen to the average temperature of air in the room?
 the only energy interaction involved is the electrical energy crossing the system
boundary and entering the room.
 As a result of the conversion of electric energy consumed by the device to heat,
the room temperature will rise.
 If this heating effect is greater than the cooling effect.
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A refrigerator
operating with its
door open in a well-
sealed and well-
insulated room
A fan running in a
well-sealed and
well-insulated room
will raise the
temperature of air in
the room.

FORMS OF ENERGY
 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, they are related to motion and the influence of some
external effects such as gravity, magnetism, electricity, and surface tension, 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. They are independent of outside reference
frames.
 Internal energy, U: The sum of all the microscopic forms of energy.
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The macroscopic energy of an object
changes with velocity and elevation.
• 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.

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Total energy
of a system
Energy of a system
per unit mass
Potential energy
per unit mass
Kinetic energy
per unit mass
Potential energy
Total energy
per unit mass
Kinetic energy
Mass flow rate
Energy flow rate

Some Physical Insight to Internal Energy
The internal energy of a
system is the sum of all forms
of the microscopic energies.
The various forms of
microscopic
energies that make
up sensible 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.
Internal = Sensible + Latent + Chemical + Nuclear
Thermal = Sensible + Latent

Question
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1) The total energy of a stationary(store) system is equivalent to:
(A) The potential energy of the system
(B) The kinetic energy of the system
(C) The internal energy of the system
(D) The sum of the kinetic and potential energies of the system.
Answer: C
2) Portable electric heaters are commonly used to heat small rooms.
Explain the energy transformation involved during this heating
process.
a) electrical energy is converted to sensible internal energy.
b) electrical energy is converted to latent internal energy.
c) electrical energy is converted to chemical internal energy.
d) heat energy is converted to electrical energy.
Answer: A
3) Which of the following is NOT a type of internal energy associated
with the
microscopic forms of energy in a system?
(A) mass flow energy
(B) latent energy
(C) chemical energy
(D) sensible energy

The macroscopic kinetic energy is an
organized form of energy and is much
more useful than the disorganized
microscopic kinetic energies of the
molecules.
• 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
and 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.
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More on Nuclear Energy
The fission of uranium and the fusion of
hydrogen during nuclear reactions, and
the release of nuclear energy.
• The best known fission reaction
involves the split of the uranium atom
(the U-235 isotope) into other elements
and is commonly used to generate
electricity in nuclear power plants (440
of them in 2004, generating 363,000
MW worldwide), to power nuclear
submarines and aircraft carriers, and
even to power spacecraft as well as
building nuclear bombs.
• Nuclear energy by fusion is released
when two small nuclei combine into a
larger one.
• The uncontrolled fusion reaction was
achieved in the early 1950s, but all the
efforts since then to achieve controlled
fusion by massive lasers, powerful
magnetic fields, and electric currents to
generate power have failed.
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Mechanical Energy
50
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.
Mechanical energy of a
flowing fluid per unit mass
Rate of mechanical
energy of a flowing fluid
Mechanical energy change of a fluid during incompressible flow per unit mass
Rate of mechanical energy change of a fluid during incompressible flow
The mechanical work supplied to
the fluid (if emech > 0) or extracted
from the fluid (if emech<0).

ENERGY TRANSFER BY HEAT
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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.
Heat: The form of energy that is
transferred between two
systems (or a system and its
surroundings) by virtue of a
temperature difference.

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Energy is
recognized
as heat
transfer only
as it crosses
the system
boundary.
During an adiabatic process, a system
exchanges no heat with its surroundings.
Heat transfer
per unit mass
Amount of heat transfer
when heat transfer rate
changes with time
Amount of heat transfer
when heat transfer rate
is constant

Energy Transfer By Work
 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.
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Specifying the directions
of heat and work.
Formal sign convention:

Heat vs. Work
 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 (i.e., their
magnitudes depend on the path followed
during a process as well as the end
states).
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Properties are point functions; but
heat and work are path functions
(their magnitudes depend on the
path followed).

Question
55
1) Heat flow into a system is ____, and heat flow out of the system is ________
a) positive, positive c) negative, positive
b) negative, negative d) positive, negative
Answer: D
2) Work done is zero for the following process
A. constant volume C. throttling
B. free expansion D. all Of the above
E. none of the above.
Answer: D
3) Which of the following involves work done BY a system?
a) increasing internal energy c) expansion
b) Compression d) Cooling
Answer: C
4) In a free expansion process
A. work done is zero
B. heat transfer is zero
C. both A. and B. above
D. work done is zero but heat increases
E. work done is zero but heat decreases.
Answer: C

Electrical Work
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Electrical power in terms of
resistance R, current I, and
potential difference V.
Electrical work
Electrical power
When potential difference
and current change with time
When potential difference
and current remain constant
 The electrons crossing the system boundary do electrical work on the system.
 In an electric field, electrons in a wire move under the effect of (emf) electromotive
forces, doing work

MECHANICAL FORMS OF WORK
 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.
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The work done is proportional to the force
applied (F) and the distance traveled (s).
Work = Force  Distance
When force is not constant

Shaft
Work
58
Energy transmission through rotating shafts
is commonly encountered in practice.
Shaft work is proportional to the
torque applied and the number
of revolutions of the shaft.
A force F acting through
a moment arm r
generates a torque T
This force acts through a distance s
The power transmitted through the shaft
is the shaft work done per unit time
Shaft
work

Spring Work
59
Elongation
of a spring
under the
influence of
a force.
When the length of the spring changes by
a differential amount dx under the influence
of a force F, the work done is
For linear elastic springs, the displacement
x is proportional to the force applied
k: spring constant (kN/m)
Substituting and integrating yield
x1 and x2: the initial and the final
displacements
The
displacement
of a linear
spring doubles
when the force
is doubled.

Work Done on Elastic Solid Bars
60
Solid bars
behave as
springs
under the
influence of
a force.
Stretching
a liquid film
with a
movable
wire.
Work Associated with the Stretching of a Liquid Film
 Consider a liquid film such as soap film suspended on a wire
frame

Work Done to Raise or to Accelerate a Body
61
1. The work transfer needed to raise a body is equal
to the change in the potential energy of the body.
2. The work transfer needed to accelerate a body is
equal to the change in the kinetic energy of the
body.
The energy
transferred to
a body while
being raised
is equal to
the change in
its potential
energy.
Nonmechanical Forms of Work
Electrical work: The generalized force is the
voltage (the electrical potential) and the
generalized displacement is the electrical charge.
Magnetic work: The generalized force is the
magnetic field strength and the generalized
displacement is the total magnetic dipole moment.
Electrical polarization work: The generalized
force is the electric field strength and the
generalized displacement is the polarization of the
medium.

Question
62
1) room is heated as a result of solar radiation coming in
through the windows. Is this a heat or work interaction for the
room?
a) Heat b) work c) mass carrying energy d)
all
Answer: A
Explanation: It is a heat interaction since it is due to the
temperature difference between the sun and the room.
2) A gas in a piston-cylinder device is compressed, and as a
result its temperature rises. Which form of energy is interact?
a) Heat b) work c) mass carrying energy d)
all
Answer: B
3) A room is heated by an iron that is left plugged in. Which
form of energy is interact? Take the entire room, including the
iron, as the system.
a) Heat b) work c) mass carrying energy d) all
Answer: B
Explanation: It is a work interaction since the electrons are

The First Law of Thermodynamics
 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.
 The 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.
63
Energy cannot
be created or
destroyed; it
can only
change forms.
The increase in the energy of
a potato in an oven is equal to
the amount of heat transferred
to it.

Energy Balance
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.
64
The work (boundary) done on
an adiabatic system is equal
to the increase in the energy
The energy
change of a
system during
a process is
equal to the
net work and
heat transfer
between the
system and its
surroundings.

Energy Change of a System,
Esystem
65
Internal, kinetic, and
potential energy
changes

Energy Conversion Efficiencies
Efficiency is one of the most frequently used terms in thermodynamics, and it
indicates how well an energy conversion or transfer process is accomplished.
66
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.
The definition of performance is not
limited to thermodynamics only.

67
Heating value of the fuel: The amount of heat released when a unit amount
of fuel at room temperature is completely burned and the combustion
products are cooled to the room temperature.
Lower heating value (LHV): When the water leaves as a vapor.
Higher heating value (HHV): When the water in the combustion gases is
completely condensed and thus the heat of vaporization is also recovered.
The definition of the
heating value of gasoline.
The efficiency of space heating systems
of residential and commercial buildings
is usually expressed in terms of the
annual fuel utilization efficiency
(AFUE), which accounts for the
combustion efficiency as well as other
losses such as heat losses to unheated
areas and start-up and cool down
losses.

68
• Generator: A device that converts mechanical energy
to electrical energy.
• Generator efficiency: The ratio of the electrical
power output to the mechanical power input.
• Thermal efficiency of a power plant: The ratio of
the net electrical power output to the rate of fuel
energy input.
A 15-W
compact
fluorescent
lamp
provides as
much light
as a 60-W
incandesce
nt lamp.
Lighting
efficacy:
The amount
of light
output in
lumens per
W of
electricity
consumed.
Overall efficiency of
a power plant

69
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 is the velocity of the
center of mass.

V
Heat
Work
z
Closed
System
Reference Plane, z = 0

V
For the closed system shown above, the conservation of energy principle or the
first law of thermodynamics is expressed as
Energy Analysis of Closed Systems

70
or
E E 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
Q W E
net net system
  
Where
2
1
( )
net in out
net out in other b
b
Q Q Q
W W W W
W PdV
 
  
 
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, Esystem, is given as

71
E Internal energy Kinetic energy Potential energy
E U KE PE
= + +
= + +
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.
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


 
 


The change in stored energy for the system is
   
E U KE PE
  
Now the conservation of energy principle, or the first law of thermodynamics for
closed systems, is written as
Q W U KE PE
net net
   
  

72
If the system does not move with a velocity and has no change in elevation, the
conservation of energy equation reduces to
Q W 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
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. The first law for a closed system operating in a
thermodynamic cycle becomes
Q W U
Q W
net net cycle
net net
 



73
Energy balance for a constant-pressure expansion or compression
process
General analysis for a closed system undergoing a quasi-
equilibrium constant-pressure process. Q is to the system
and W is from the system
An example of constant-pressure pro
Since

Question
74
1) For a cycle (a system’s initial state is the same as the final state), the work done
is equivalent to:
(A) The distance travelled by the system
(B) The change in temperature of the system
(C) The heat added to the system
(D) None of the above
Answer C
2) What is net transfer of energy by heat?
a) It is the sum of all energy transfers by heat into the system.
b) It is the product of all energy transfers by heat into the system.
c) It is the sum of all energy transfers by heat into and out of the system.
d) It is the product of all energy transfers by heat into and out of the system.
Answer: C

75
Conservation of Mass for General Control Volume
The conservation of mass principle for the open system or control volume is
expressed as
or
   ( / )
m m m kg s
in out system
 
  
If the fluid density and velocity are constant over the flow cross-sectional area, the
mass flow rate is
ave
ave
V A
m V A
v

 
where  is the density, kg/m3 ( = 1/v), A is the cross-sectional area, m2; and is the
average fluid velocity normal to the area, m/s.
Energy Analysis of Open Systems
 Energy can cross the boundaries of an open system in the form of
heat, work or mass carrying energy. Energy transfer across a system
boundary due solely to the temperature difference between a system
and its surroundings is called heat.

76
0
CV
CV
dm
m
dt
  
Steady-state, Steady-Flow Conservation of Mass:
Since the mass of the control volume is constant with time during the steady-state,
steady-flow process, the conservation of mass principle becomes
or
  ( / )
m m kg s
in out
 

Special Case: Steady Flow of an Incompressible Fluid
The mass flow rate is related to volume flow rate and fluid density by
m V


For one entrance, one exit steady flow control volume, the mass flow rates are
related by
Steady-state

77
in out
in in out out
in out
in out
in in out out
incompressible assumption
(kg/s)
m m
V V
V V
V A V A
 
 





Word of caution: This result applies only to incompressible
fluids. Most thermodynamic systems deal with processes
involving compressible fluids such as ideal gases, steam, and
the refrigerants for which the above relation will not apply.

78
The total energy of flowing fluid
The total energy carried by a unit of mass as it crosses the control surface is the sum
of the internal energy, flow work, potential energy, and kinetic energy.
    
  
u Pv
V
gz
h
V
gz


2
2
2
2
Here we have used the definition of enthalpy, h = u + Pv.
Energy transport by mass
Amount of energy transport across a control surface:
2
(kJ)
2
mass
V
E m m h gz

 
   
 
 
The term Pv is called the
flow work done on the unit
of mass as it crosses the
control surface.

79
Rate of energy transport across a control surface:
2
( )
2
mass
V
E m m h gz kW

 
   
 
 
Conservation of Energy for General Control Volume
The conservation of energy principle for the control volume or open system has the
same word definition as the first law for the closed system. Expressing the energy
transfers on a rate basis, the control volume first law is
or
Rate of net energy transfer Rate change in internal, kinetic,
by heat, work, and mass potential, etc., energies
( )
in out system
E E E kW
  
Considering that energy flows into and from the control volume with the mass, energy
enters because net heat is transferred to the control volume, and energy leaves
because the control volume does net work on its surroundings, the open system, or
control volume, the first law becomes

80
where  is the energy per unit mass flowing into or from the control volume. The
energy per unit mass, , flowing across the control surface that defines the control
volume is composed of four terms: the internal energy, the kinetic energy, the
potential energy, and the flow work.
The total energy carried by a unit of mass as it crosses the control surface is
    
  
u Pv
V
gz
h
V
gz


2
2
2
2
Where the time rate change of the energy of the control volume has been written as
 
ECV

81
Steady-State, Steady-Flow Processes
Most energy conversion devices operate steadily over long periods of time. The
rates of heat transfer and work crossing the control surface are constant with time.
The states of the mass streams crossing the control surface or boundary are constant
with time. Under these conditions the mass and energy content of the control volume
are constant with time.
dm
dt
m
dE
dt
E
CV
CV
CV
CV
 
 




0
0
Steady-state, Steady-Flow Conservation of Mass:
  ( / )
m m kg s
in out
 

Steady-state, steady-flow conservation of energy
Since the energy of the control volume is constant with time during the steady-state,
steady-flow process, the conservation of energy principle becomes

82
  
E E E kW
in out system
 
Rate of net energy transfer
by heat, work, and mass
Rate change in internal, kinetic,
potential, etc., energies
( )
 
 
 
 


0
or
or
Considering that energy flows into and from the control volume with the mass, energy
enters because heat is transferred to the control volume, and energy leaves because
the control volume does work on its surroundings, the steady-state, steady-flow first
law becomes

83
Often this result is written as
where
  
  
Q Q Q
W W W
net in out
net out in
 
 
 
 
Steady-state, steady-flow for one entrance and one exit
A number of thermodynamic devices such as pumps, fans, compressors, turbines,
nozzles, diffusers, and heaters operate with one entrance and one exit. The steady-
state, steady-flow conservation of mass and first law of thermodynamics for these
systems reduce to

84
When compared to the enthalpy of steam (h  2000 to 3000 kJ/kg) and the enthalpy
of air (h  200 to 6000 kJ/kg), the kinetic and potential energies are often neglected.
When the kinetic and potential energies can be neglected, the conservation of energy
equation becomes
  ( ) ( )
Q W m h h kW
  
2 1
We often write this last result per unit mass flow as
q w h h kJ kg
  
( ) ( / )
2 1
q
Q
m



w
W
m



Where and .
Some Steady-Flow Engineering Devices
Below are some engineering devices that operate essentially as steady-state, steady-
flow control volumes.

85
 
V V
2 1

 
V V
2 1


V1

V1
Nozzles and Diffusers
For flow through nozzles, the heat transfer, work, and potential energy are normally
neglected, and nozzles have one entrance and one exit. The conservation of energy
becomes

86
Solving for

V2
 
V h h V
2 1 2 1
2
2
  
( )
Example 5-4
Steam at 0.4 MPa, 300oC, enters an adiabatic nozzle with a low velocity and leaves
at 0.2 MPa with a quality of 90%. Find the exit velocity, in m/s.
Control Volume: The nozzle
Property Relation: Steam tables
Process: Assume adiabatic, steady-flow
Conservation Principles:
Conservation of mass:
For one entrance, one exit, the conservation of mass becomes
 
  
m m
m m m
in out
 

 
1 2

87
Conservation of energy:
According to the sketched control volume, mass crosses the control surface, but no
work or heat transfer crosses the control surface. Neglecting the potential energies,
we have
Neglecting the inlet kinetic energy, the exit velocity is

V h h
2 1 2
2
 
( )
Now, we need to find the enthalpies from the steam tables.
1 1 2 2
1 2
Superheated Saturated Mix.
300 3067.1 0.2
0.4 0.90
o kJ
T C h P MPa h
kg
P MPa x
 
 
  
 
 
  

At 0.2 MPa hf = 504.7 kJ/kg and hfg = 2201.6 kJ/kg.

88
2 2
= +
= 504.7 + (0.90)(2201.6) = 2486.1
f fg
h h x h
kJ
kg
2 2
2
1000 /
2(3067.1 2486.1)
/
1078.0
kJ m s
V
kg kJ kg
m
s
 

Turbines
If we neglect the changes in kinetic and potential energies as fluid flows
through an adiabatic turbine having one entrance and one exit, the
conservation of mass and the steady-state, steady-flow first law becomes

90
Question
1) First law of thermodynamics
(A) Enables to determine change in internal energy of the
system
(B) Does not enable to determine change in entropy
(C) Provides relationship between heat, work and internal
energy
(D) All of the above
Answer: D
2) Energy can cross the boundaries of an open system in the form of
.
a) heat
b) work
c) flow energy
d) all
e) none
Answer: D

The Second Law of Thermodynamics
91
A cup of hot coffee
does not get hotter in
a cooler room.
Transferring
heat to a wire
will not
generate
electricity.
Transferring
heat to a
paddle wheel
will not cause
it to rotate.
a process must
satisfy the first law
to occur.

92
Processes occur in a
certain direction, and not
in the reverse direction.
A process must satisfy both
the first and second laws of
thermodynamics to proceed.
 The first law places no restriction on the direction of a process, but
satisfying the first law does not ensure that the process can actually
occur.
MAJOR USES OF THE SECOND LAW
1.The second law may be used to identify the direction of processes.
2.The second law also asserts that energy has quality as well as quantity.
3. The second law of thermodynamics is also used in determining the theoretical
limits for the performance of commonly used engineering systems, such as
heat engines and refrigerators, as well as predicting the degree of completion
of chemical reactions.

93
1) Transfer of heat takes place according to which of the following
law..
A. First law of thermodynamics
B. Second law of thermodynamics
C. Zeroth law of thermodynamics
D. None of the above
Answer: B
Questions
2) . of thermodynamics is sometimes called as law of degradation
of the energy.
C. Zeroth law of thermodynamics
Answer: B

Thermal Energy Reservoirs
94
Oceans, lakes, and rivers as well as the atmospheric air can be modeled as
reservoirs because of their large thermal energy storage capabilities
A hypothetical body that can supply or absorb finite amounts of heat without
undergoing any change in temperature is called a reservoir
The atmosphere, for example, does not warm up as a result of heat losses
from residential buildings in winter.
The air in a room, for example, can be treated as a reservoir, since the
amount of heat transfer from the TV set to the room air is not large enough
to have effect on the room air temperature.

95
A reservoir that supplies energy in the form of heat is called a source
A reservoir that absorbs energy in the form of heat is called a sink
Example : Industrial furnace.
Example : Oceans, lakes, and rivers
Heat transfer from industrial sources to the environment is of major
concern to engineers. Irresponsible management of waste energy can
significantly increase the temperature of portions of the environment,
causing what is called thermal pollution.

96
1) Which among the following is/are example/s of sink?
a. River
b. Sea
c. Atmosphere
d. all of the above
Answer: D
2) The thermal energy reservoir from which heat is transferred to
the system which works on heat engine cycle is called as
a. source
b. sink
c. atmosphere
d. all of the above
Answer: A
Questions
3) A thermal energy reservoir (TER) has
a. a finite heat capacity
b. an infinite heat capacity
c. a finite mass
d. none of the above
Answer: B

97
HEAT ENGINES
Work can always be converted to heat directly
and completely, but the reverse require the use
of special device called heat engine
1. They receive heat from a high-
temperature source (solar energy, oil
furnace, nuclear reactor, etc.).
2. They convert part of this heat to
work (usually in the form of a
rotating shaft.)
3. They reject the remaining waste
heat to a low-temperature sink (the
atmosphere, rivers, etc.).
4. They operate on a cycle.
The devices that convert heat to work.
 The work-producing device that
best fits into the definition of a heat
engine in a thermodynamic cycle is
the steam power plant.

98
A Steam power plant
 A heat-engine cycle cannot be
completed without rejecting some
heat to a low-temperature sink.
Can We Save Qout?

99
Thermal efficiency
Even the most
efficient heat
engines reject
almost one-half
of the energy
they receive as
waste heat.
net work output of a heat engine is always
less than the amount of heat input, Wnet< Q

100
The Second Law of Thermodynamics:
Kelvin–Planck Statement
A heat engine that violates the
Kelvin–Planck statement of the
second law.
Under ideal conditions,
a heat engine must
reject some heat to a
low-temperature
reservoir in order to
complete the cycle
No heat engine can
convert all the heat it
receives to useful work
It is impossible for any device that operates on a cycle to
receive heat from a single reservoir and produce a net
amount of work.
No heat engine can
have a thermal
efficiency of 100
percent

101
1) According to kelvin Planck statement of second law of thermodynamics.
A. It is impossible to construct an engine working on a cyclic process whose
main purpose is to convert heat energy into the work
B. It is possible to construct an engine working on a cyclic process whose
sole purpose is to convert heat into work
C. Heat engine can have a thermal efficiency of 100 percent
Answer: A
2) The law of Kelvin Planck statement about the
A. Conservation of energy
B. Conservation of heat
C. Conservation of work
D. Conversion of heat into work
Answer: D
Questions

102
• The most frequently used refrigeration
cycle is the vapor-compression
refrigeration cycle.
• The refrigerant enters the compressor and
is compressed at a relatively high
temperature
• Flows through the coils of the
condenser-cools down and
condenses by rejecting heat to the
surrounding medium
• Enters a capillary tube where its
pressure and temperature drop
drastically
• low-temperature refrigerant then
enters the evaporator, where it
evaporates by absorbing heat from
the refrigerated space.
In a household refrigerator, the freezer
compartment serves as the evaporator,
and the coils serve as the condenser.
The Second Law of Thermodynamics: Clausius
Statement

103
Reversible Heat Engine
Refrigerators and Heat Pumps
Refrigerators and heat pumps operate on the same cycle but differ in their
objectives
The objective of a
refrigerator is to
maintain the
refrigerated space at a
low temperature by
removing heat from it.
The objective of a refrigerator is to
remove QL from the cooled space.
The objective of a heat pump is to
supply heat QH into the warmer space
The objective of a heat
pump, however, is to
maintain a heated space at a
high temperature

104
COEFFICIENT OF PERFORMANCE
Can the value of COPR be
greater than unity?
REFRIGERATORS HEAT PUMS
Can the value of COPHP
be lower than unity?
What does COPHP=1
represent?

105
When installed backward,
an air conditioner
functions as a heat pump.
• Most existing heat pumps use
the cold outside air as the heat
source in winter.
• Air conditioners are basically
refrigerators whose refrigerated
space is a room or a building
instead of the food compartment.
• Air conditioning unit cools a
room by absorbing heat from the
room air and discharging it to the
outside.
• The same air-conditioning unit
can be used as a heat pump in
winter by installing it backwards
• The unit absorbs heat from the
cold outside and delivers it to the
room

106
The Second Law of Thermodynamics:
Clasius Statement
A refrigerator that
violates the Clausius
statement of the second
law.
This heat transfer process occurs in nature in the
direction from high temperature to low temp.
The reverse process, from a low-temperature to a
high-temperature one requires special devices called
refrigerators.
The working fluid used in the refrigeration
cycle is called a refrigerant
It is impossible to construct a device that operates
in a cycle and produces heat from a lower-
temperature body to a higher-temperature body.

107
Equivalence of the Two Statements
Proof that the violation of the Kelvin–Planck statement leads to
the violation of the Clausius statement.
Thus, the combination of
these two devices can be
viewed as a refrigerator that
transfers heat in an amount
of QL from a cooler body to a
warmer one without requiring
any input from outside
The heat engine converts all
the heat QH it receives to
work.
Therefore W = QH
The high temperature reservoir
receives a net amount of heat QL
(QL+ QH) - QH.

108
1) The refrigerator and heat pump is work on which principle.
C. Third law of thermodynamics
D. Zeroth law of thermodynamics
Answer: B
3) Which of the following is a reversed heat engine?
a. Heat pump
b. Refrigerator
c. Steam Power Plant
d. both a. and b.
e. none of the above
Answer: D
Questions
2) Clausius statement is related to
a. heat engine operating in a cycle
b. heat pump operating in a cycle
c. both a. and b.
Answer: B

109
Perpetual-Motion Machines
 Any device that violates either law is called a
perpetual-motion machine.
 A device that violates the first law of
thermodynamics (by creating energy) is called
a perpetual-motion machine of the first kind
(PMM1).
 PMMI is the first temperature delivers work
continuously without any input.
 It violates firs law of thermodynamics and
impossible to construct an engine working
with this principle.
 A device that violates the second law of
thermodynamics is called a perpetual-motion
machine of the second kind (PMM2)
Perpetual-motion machine of
the first kind (PMM1).
Perpetual-motion machine of the
second kind (PMM2).

110
2) PMM2 is the machine which violates ___________
a. Kelvin-Planck statement
b. Clausius statement
c. both a. and b.
Answer: C
1) If a heat engine produces net work output by exchanging heat
with only one reservoir, then the heat engine will be,
a. perpetual motion machine of first kind (PMM1)
b. perpetual motion machine of second kind (PMM2)
c. perpetual motion machine of third kind (PMM3)
Answer: B
Questions
3) A device that violates the first law of thermodynamics (by creating
energy)
a. perpetual motion machine of first kind (PMM1)
b. perpetual motion machine of second kind (PMM2)
c. perpetual motion machine of third kind (PMM3)
Answer: A

111
REVERSIBLE AND IRREVERSIBLE PROCESSES
Two familiar
reversible processes.
Reversible process: A process that can be reversed
without leaving any trace on the surroundings.
• Do not occur in nature
• Can be appoximated, but can never be achieve
The second law of thermodynamics states that no heat engine can have an
efficiency of 100 percent.
What is the highest efficiency that a heat engine can possibly have?
Irreversible process: A process that is not reversible.
• All the processes occurring in nature are
irreversible
• Once a cup of hot coffee cools, it will not heat
up by retrieving the heat it lost from the
surroundings.

112
(1) they are easy to analyze
(2) they serve as idealized models (theoretical limits) to which actual
processes can be compared.
Engineers are interested in reversible processes because
Why are we interested in reversible processes?
work-producing devices such as car engines and gas or steam
turbines deliver the most work,
work-consuming devices such as compressors, fans, and pumps
consume the least work instead of irreversible ones
THE MOST REASON
The better the design, The more closely we approximate a reversible process,
the lower the irreversibilities and higher the efficiency.

113
Irreversibilities
Friction
renders a
process
irreversible.
(a) Heat transfer
through a
temperature
difference is
irreversible, and
(b) the reverse
process is
• The factors that cause a process to be irreversible include friction,
unrestrained expansion, mixing of two fluids, heat transfer across a finite
temperature difference, electric resistance, inelastic deformation of solids,
and chemical reactions.
• When the
membrane is
ruptured, the
gas fills the
entire tank
• compress it
to its initial
volume
• Heat is lost

114
Internally and Externally Reversible
Processes
 Reversible process involves no irreversibilities associated with either of
them.
 A process is called internally reversible if
no irreversibilities occur within the
boundaries of the system during the
process. During an internally reversible
process, a system proceeds through a
series of equilibrium states and when the
process is reversed, the system passes
through exactly the same equilibrium states
while returning to its initial state
Example: The quasi-equilibrium process
 A process is called externally reversible if no irreversibilities occur outside the
system boundaries during the process.
 A process is called totally reversible, or
simply reversible, if it involves no
irreversibilities within the system or its
surroundings
 no heat transfer, no non quasi-equilibrium
changes, and no friction or other dissipative

115
3) The irreversibility in the system caused by friction is an example of
a. internal irreversibility
b. external irreversibility
c. frictional irreversibility
d. chemical irreversibility
Answer A
Questions
1) A process becomes reversible when
a. it undergoes in the absence of any dissipative effect
b. all the points through the path are at thermodynamic
equilibrium
c. it undergoes as a very slow quasi-static process
d. all of the above
Answer: D
2) All spontaneous processes are
a. reversible
b. irreversible
c. quasi-static
Answer: B

116
THE CARNOT CYCLE
• Heat engines are cyclic devices
• The efficiency of a heat-engine cycle greatly
maximized by using processes that require the least
amount of work and deliver the most, that is, by using
reversible processes
• The most efficient cycles are reversible cycles
• Probably the best known reversible cycle is the
Carnot cycle, first proposed in 1824 by French
engineer Sadi Carnot

117
THE CARNOT CYCLE
Reversible Isothermal Expansion (process 1-2, TH = constant)
Reversible Adiabatic Expansion (process 2-3, temperature drops from TH to
TL)
Reversible Isothermal Compression (process 3-4, TL = constant)
Reversible Adiabatic Compression (process 4-1, temperature rises from TL

118
P-V diagram of the Carnot cycle.
The direction
of the process
at P-V diagram
of the reversed
Carnot cycle
are reverse
The Reversed Carnot
Cycle
The Carnot Cycle
The Carnot heat-engine cycle
is a totally reversible cycle.
Therefore, all the processes that comprise
it can be reversed, in which case it
becomes the Carnot refrigeration cycle.
• Heat in the amount of QL is absorbed
from the low-temperature reservoir,
• heat in the amount of QH is rejected to a
high-temperature reservoir,
• and a work input of Wnet,in is required
to accomplish all this.

119
THE CARNOT PRINCIPLES
1. The efficiency of an irreversible
heat engine is always less than
the efficiency of a reversible one
operating between the same
two reservoirs.
2. The efficiencies of all reversible
heat engines operating between
the same two reservoirs are the
same.
The Carnot principles.

120
THE CARNOT HEAT ENGINE
The Carnot heat engine is the most
efficient of all heat engines
operating between the same high-
and low-temperature reservoirs.
Any heat engine, Carnot heat
engine
This is the highest efficiency a heat
engine operating between the two
thermal energy reservoirs
at temperatures TL and TH can have

%
70
100
30
1
1 




L
H
Q
Q

%
55
100
45
1
1 




L
H
Q
Q

%
70
1000
300
1
1
max 




H
L
T
T

%
80
100
20
1
1 




L
H
Q
Q

No heat engine can have a higher
efficiency than a reversible heat engine
operating between the same high- and
low-temperature reservoirs.
QL=45
QH=100
QL=20
QH=100
1. Reversible heat engine
Carnot Efficiency
2. Irreversible heat engine
3.Impossible heat engine
QH=100
QL=30

1) According to the Carnot's theorem, the efficiency of a reversible heat
engine operating between a same given constant temperature source
and a given constant temperature sink is
a) higher than any other irreversible heat engine
b) less than any other irreversible heat engine
c) equal to any other irreversible heat engine
d) none of the above
Answer: A
Questions
122
2) An inventor claims to have developed a heat engine that
receives 700 kJ of heat from a source at 500 K and produces
300 kJ of net work while rejecting the waste heat to a sink at
290 K. what claim is it .
a) Irreversible (Actual) heat engine
b) Reversible (Ideal) heat engine
c) Impossible (False) heat engine
d) None of the above
Answer: C

123
The Quality of Energy
These efficiency values
show that energy has
quality as well as
quantity.
The higher the temperature,
the higher its quality.
more of the high-temperature
thermal energy can be
converted to work.

124
The efficiency of a Carnot heat engine
increases as TH is increased, or as TL is
decreased.
As TL approaches zero, the Carnot efficiency
approaches unity
The thermal efficiency of actual heat engines
can be maximized by
supplying heat to the engine at the highest possible
temperature (limited by material strength)
and rejecting heat from the engine at the
lowest possible temperature (limited by the
temperature of the cooling medium such as rivers,
lakes, or the atmosphere).

125
THE CARNOT REFRIGERATOR
AND HEAT PUMP
No refrigerator can have a higher COP
than a reversible refrigerator operating
between the same temperature limits.
Any refrigerator or heat pump
Carnot refrigerator or heat pump
How do you increase the COP
of a Carnot refrigerator or
heat pump? How about for
actual ones?

126
a) 20MW and 32.5% b) 30MW and 37.5% c) 20MW and 32.5% d) n

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