2. Objectives
• Introduce the concept of energy and define its various forms.
• Define the concept of heat and the terminology associated
with energy transfer by heat.
• Define the concept of work, including electrical work and
several forms of mechanical work.
• Introduce the first law of thermodynamics, energy balances,
and mechanisms of energy transfer to or from a system.
• Define energy conversion efficiencies.
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3. A fan running in a well-sealed
and well-insulated room
Will the room be cool or hot?
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4. A refrigerator operating with its
door open in a well-sealed and
well-insulated room
Will the room be cool or hot?
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5. Thermodynamics deals only with the change of the total energy.
∑ E = 0 (at some convenient reference point)
Total energy
Macroscopic
Microscopic
• E
• Sum of ALL energy
• Outside reference
frame
• KE and PE
• Related to molecular
structure & activity
• ∑Emic = U
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6. • Kinetic
energies of
molecules
Sensible
Energy
• Phase of a
system
Latent
Energy
• Atomic bonds
in a molecule
Chemical
Energy
• Strong bonds
within the
nucleus of
atom itself
Nuclear
Energy
Microscopic Energy
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7. • Energy resulted from
its motion relative to
some reference frame
Kinetic
Energy
• Energy resulted from
its elevation in a
gravitational field
Potential
Energy
2
2
V
m
KE mgz
PE
Macroscopic Energy
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8. Total Energy of a System
Stationary system:
Fluid flow rate: Energy flow rate:
mgz
V
m
U
PE
KE
U
E
2
2
U
E
avg
cV
A
V
m
.
. .
.
me
E
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9. 9
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.
10. Mechanical Energy
Form of energy that can be converted to
mechanical work completely and directly by an
ideal mechanical device
Forms of mechanical energy:
Kinetic, potential energy and flow energy (pressure of
flowing fluid)
Mechanical Energy of Flowing Fluid :
gz
V
P
m
Emech
2
2
.
.
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12. Form of energy
transferred between
two systems by virtue
of temperature
difference
Energy is recognized as heat transfer
only as it crosses the system boundary.
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13. • Solid to solid
• More energetic particles of a substance to the
adjacent less energetic ones as a result of
interaction between particles
Conduction
• Solid to/from fluid
• Solid surface and the adjacent fluid that is in
motion and involves the combined effects of
conduction and fluid motion
Convection
• The transfer of energy due to the emission of
electromagnetic waves (or photons)
Radiation
Heat Transfer Mechanisms
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14. Power is the
work done per
unit time (kW)
Energy transfer
associated with a force
acting through a
distance
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15. Heat transfer to a
system (Q) and from
a system (-Q)
Work done on a
system (-W) and work
done by a system (W)
Formal Sign Convention for Heat and Work
Alternative to sign
convention is to use the
subscripts in and out to
indicate direction. This
is the primary approach
in the text
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16. Tutorial on Energy 1
Consider an electric refrigerator located in
a room. Determine the direction of the
work and heat interactions (in or out)
when the following are taken as the
system:
a) The content of the refrigerator
b) All parts of the refrigerator including
the content
c) Everything contained within the room
during a winter day
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17. Tutorial on Energy 2
Consider a fan located in a 1 x 1 m
square duct. Velocities at various
points at the outlet are measured,
and the average flow velocity is
determined to be 7 m/s. Taking
the air density to be 1.2 kg/m3,
estimate the minimum electric
power consumption of the fan
motor.
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18. 18
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).
Properties are point functions; but
heat and work are path functions
(their magnitudes depend on the
path followed).
19. Path vs. Point Functions
Path function
• Clue: inexact
differentials (δ)
• Associated with process
• Magnitude depends on
path followed during a
process
• Heat (Q) and Work (W)
• Exist when the system
has experienced a
change in state.
Point function
• Clue: exact differentials
(d)
• Associated with state
• Magnitudes depend on
initial and final states
only
• Volume, Pressure,
Enthalpy
• Measurable quantities:
T, P, G, E, , U
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20. Example of Path and Point Functions
Suppose a swimmer times himself on swimming 50 m several times
using different swimming styles such as freestyle and frog-kick.
Whether he does freestyle or frog-kick he will ultimately still travel 50 meters.
The initial state and final state will be the same in each case; point function
However, the energy he use doing free-style will be much greater than that
which they will use doing frog-kick
The means or path which they take to complete the 50 meters will affect
the amount of energy they use to complete the lap
What is the point function?
What is the path function?
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21. N coulombs of electrical charge move
through a potential difference V
Electrical power in rate form
When potential difference and
current change with time
When potential difference
and current remain constant
Electrical Work
Electrons crossing the system
boundary do electrical work
on the system
VI
W e
.
VN
We
2
1
VIdt
We
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Example 2-5
22. 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.
The work done is proportional to the force
applied (F) and the distance traveled (s).
Work = Force Distance
When force is not constant
If there is no movement,
no work is done.
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23. 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
Energy
transmission with a
rotating shaft is
very common in
engineering
practice
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Shaft
Work
24. Spring Work
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.
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25. 25
Work Done on Elastic Solid Bars
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
Normal stress
Surface tension
dA = 2bdx; due to 2 surfaces in contact with air
26. Work Done to Raise or to Accelerate a Body
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.
Non-mechanical 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.
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27. A relationship between energy and energy
interaction
Also called “Conservation of Energy
Principle”
Statement: “Energy can neither be
created nor destroyed during a process; it
can only change form”
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28. 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.
Energy Balance (Conservation of Energy Principle)
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29. Tutorial on Energy Balance 1
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Consider a room that is initially at
the outdoor temperature of 20oC.
The room contains a 100-W light
bulb, a 110-W TV set, a 200-W
refrigerator and a 1000-W iron.
Assuming no heat transfer through
the walls, determine the rate of
increase of the energy content of
the room when all of these electric
devices are on.
30. Energy Change of A System, ΔEsystem
Energy change = Efinal - Einitial
1
2 E
E
E
E
E initial
final
system
Energy is a property: value DOES NOT change
unless STATE of the system change
PE
KE
U
E
E
E
1
2
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31. Mechanism of Energy Transfer (Ein and Eout)
Heat transfer, Q
• Effect internal
energy
• Heat gain and
loss increase and
decrease energy
of molecules,
respectively
Work transfer, W
• Work done ON
system increase
energy
• Work done BY
system decrease
energy
Mass flow, m
• Mass in increase
system energy as
it carries energy
• Mass out
decrease system
energy
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32. Energy Balance Equation
1
2
1
2
2
1
1
2
,
, z
z
mg
V
V
m
U
U
E
E
W
W
Q
Q out
mass
in
mass
out
in
out
in
Adiabatic: Qin-Qout=0
No work interaction: Win-Wout=0
Closed system: Emass,in-Emass,out=0
Stationary system: KE=PE=0
Closed system undergoing a
CYCLE, final and initial state
are the same ΔEsystem= 0, no
mass transfer, Q = W
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33. Efficiency: used a lot in
thermodynamics
Measure of how well an
energy conversion is
accomplished
Often cause
misunderstanding
because not properly
defined
Performance=efficiency
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34. 34
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.
35. 35
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 cooldown losses.
36. 36
• 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
incandescent
lamp.
Lighting efficacy:
The amount of light
output in lumens
per W of electricity
consumed.
Overall efficiency
of a power plant
37. 37
The efficiency of a cooking
appliance represents the
fraction of the energy
supplied to the appliance that
is transferred to the food.
• Using energy-efficient appliances conserve
energy.
• It helps the environment by reducing the
amount of pollutants emitted to the
atmosphere during the combustion of fuel.
• The combustion of fuel produces
• CO2 - causes global warming
• NOx and HCs - cause smog
• CO - toxic
• SO2 - causes acid rain.
38. 38
Efficiencies of Mechanical and Electrical Devices
The mechanical
efficiency of a fan is the
ratio of the kinetic
energy of air at the fan
exit to the mechanical
power input.
The effectiveness of the conversion process between
the mechanical work supplied or extracted and the
mechanical energy of the fluid is expressed by the
pump efficiency and turbine efficiency,
Mechanical efficiency
40. A fan running in a well-sealed
and well-insulated room
Will the room be cool or hot?
The room WILL be warmer
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41. A refrigerator operating with its
door open in a well-sealed and
well-insulated room
Will the room be cool or hot?
The room WILL be warmer
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