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CHAPTER 3
ENERGY, ENERGY
TRANSFER AND GENERAL
ENERGY ANALYSIS
DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003
CHAPTER 3 : ENERGY, HEAT AND WORK
OBJECTIVES
The objectives of this chapter
DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003
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CHAPTER 3 : ENERGY, HEAT AND WORK
WHAT IS ENERGY?
The word `ENERGY’ comes from a Greek word ἐνέργεια (Energeia)
Definitions in physics field :
Energy is the capacity of a physical system to perform work.
Energy is a property or characteristic of matter that makes things happen
(Dave Watson )
Definition in Thermodynamics : Energy can be defined as an extensive property
which has an ability to change the condition (or state) of a system and its
surrounding through interaction that occurs through system boundary.
• Examples of changes in condition : changes in shape, volume, chemical
composition pressure, temperature, density and phase changes.
• Without energy, nothing would ever change, nothing would ever happen. You
might say energy is the ultimate agent of change, the mother of all change
agents
DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003
CHAPTER 3 : ENERGY, HEAT AND WORK
INTRODUCTION
Energy terms commonly used : ENERGY and ENERGY TRANSFER
ENERGY
Symbol, E
Unit, kJ (kJ/kg)
A function of other
properties
E = f (T, p , V, etc)
Can be stored
in a body
Energy can be stored within system in various form, can be converted from one
form to another and can be transferred between systems
Generally, in thermodynamics, energy equation can be written as
E = U + KE + PE
Internal Energy Kinetic Energy
Potential Energy
DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003
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CHAPTER 3 : ENERGY, HEAT AND WORK
WORK AND HEAT ARE PROCESSES
(AND FORMS OF ENERGY?)
There are just two ways that energy is transferred - by work or
by heat.
We often use the words work and heat as if they are forms of
energy. I do it all the time and I'm not sorry.
But some thermodynamic text books say that work and heat are
processes or methods of energy transfer, not forms of energy.
When describing energy transfers in this way, we should say
something like, "energy transferred during a work process". Or
when talking about a heat process, we would be more correct to
say, "energy transferred by heat flow".
Energy transfer is not a thermodynamics property because it
depends on the history of the system. e.g Heat and Work.
DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003
CHAPTER 3 : ENERGY, HEAT AND WORK
KINETIC ENERGY
The energy that a system possesses as a result of its motion relative to some
reference frame. When all parts of the system move with the same velocity,
the kinetic energy is expressed as
KE = 21 mV 2
Kinetic Energy (J)
Velocity (m/s)
Mass (kg)
Initial State
When a body with a mass m,
moving from initial state vith a
velocity of ⎯V1 to a final state with
a velocity of ⎯V2, then
1
2
V1
m
Change in Kinetic Energy, ∆KE =
Or per unit mass, ∆ke =
Final State
(V
2
2
1
2
(
m
V2
)
m V22 − V12 J
)
− V12 J/kg
DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003
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CHAPTER 3 : ENERGY, HEAT AND WORK
POTENTIAL ENERGY
The energy that a system possesses as a results of its elevation in a
gravitational field and is expressed as
PE = mgz (J)
Mass
(kg)
When a work is done to a system
with a mass of m, from a distance
from datum z1 to a distance from
datum z2, the change in potential
energy can be expressed as
Height
(m)
Gravitational
Accelaration
(m/s2)
∆PE = mg (z2 - z1 ) J
Initial
State
m
Final
State
m
z2
Or per unit mass,
∆pe = g (z2- z1 )
z1
J/kg
Datum
DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003
CHAPTER 3 : ENERGY, HEAT AND WORK
POTENTIAL ENERGY
Hanging apple = Potential energy
When it's hanging it's potential. When it's falling the
potential energy is turned into kinetic energy.
What happens to the energy when it hits the head?
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CHAPTER 3 : ENERGY, HEAT AND WORK
INTERNAL ENERGY
Translation
Energy
• The internal energy of a denoted by U, is the total of the
kinetic energy due to the motion of particles
(translational, rotational, vibrational) and the potential
energy associated with the vibrational and electric
energy of atoms within molecules.
Rotational
Kinetic
Energy
• It includes the energy in all of the chemical bonds, and
the energy of the free, conduction electrons in metals.
Vibrational
kinetic
Energy
Rotational
Kinetic
Energy
• It is a state function of a system, and is an extensive
quantity
• The unit is kJ or per unit mass kJ/kg
Spin
Energy
• The internal energy is proportional to the temperature
of the system, at higher temperature, the system
possess higher U, and vice versa.
Spin
Energy
• Any gas undergoes any process which doesn’t
involves any temperature different, the change in
internal energy is zero.
• Method to analyze internal energy depends on the type
of the working fluid (vapor or gas)
DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003
CHAPTER 3 : ENERGY, HEAT AND WORK
ENERGY TRANSFERRED BY HEAT
n
nitio
Defi
Fl
ow
Di
re
ct
io
n
Heat Transfer
Mode
He
at
A form of energy
transferred between two
bodies (two systems or
a system and its surroundings)
by virtue of a temperature
difference
HEAT TRANSFER
Direction of heat
Transfer is always from
the higher Temperature
body to The lower
temperature body
Symbol
Q (kJ) , q (kJ/kg)
& (kW) = Q (kJ)
Q
Heat is Rejected
Q negatif
∆t (s)
Conduction
Convection
Radiation
System
Surrounding
Heat is Supplied,
Q positif
A process without heat transfer is called
adiabatic process. i.e the system is well
insulated or both the system and the
surrounding are at the same temperature
DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003
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CHAPTER 3 : ENERGY, HEAT AND WORK
ENERGY TRANSFERRED BY WORK
WORK
ion
nit
Work sign
fi
De
The energy transfer
associated with a force
acting through a
distance
Symbol
W (kJ) , w (kJ/kg)
& (kW) = W (kJ)
W
∆t (s)
(Power)
Work done by system
W positive
System
Surrounding
Work done on system
W negative
DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003
CHAPTER 3 : ENERGY, HEAT AND WORK
ENERGY TRANSFERRED BY WORK
δW = Fdx
F
W12 =
x2
∫x
Work to Lift a Mass
F dx
Pulley
1
dx
Work to Push A Mass
Initial State
F
Datum
x=0
m
Final
state
m
Initial
State
m
Final State
F
m
z2
z1
Datum
W12 = mg(z2 - z1)
x1
x2
W12 = F(x2 - x1)
DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003
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CHAPTER 3 : ENERGY, HEAT AND WORK
FORMAL SIGN CONVENTION OF HEAT
AND WORK
Heat transfer to a system and work
done by a system are positive
(Positive machine)
Heat transfer from a system and work
done on a system are negative
(Negative machine)
Heat is transferred
from system
Work done by
system
System
System
Surrounding
Work done on
system
Heat is transferred to
system
Similarities between heat transfer and work
• Both are recognized at the boundaries of
a system as they cross the boundaries
• System possess energy but not heat or
work
• Both associated with a process not a
state
• Both are path function
Surrounding
Path Function
• Have inexact differentials, differential
amount of heat or work is represented by
δQ or δW instead of dQ or dW
• Work done during process 1-2 = W12
• Heat transferred during process 1-2 = Q12
2
∫1 δW = W12 (not W2 - W1
or ∆W )
2
∫1 δQ = Q 12 (not Q 2 - Q1 or ∆Q)
DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003
CHAPTER 3 : ENERGY, HEAT AND WORK
EXAMPLE 2-3 pg 64
DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003
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CHAPTER 3 : ENERGY, HEAT AND WORK
EXAMPLE 2-4 pg 64
No heat is
transferred to
the system
Heat is
transferred to
potato
through
boundary
DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003
CHAPTER 3 : ENERGY, HEAT AND WORK
EXAMPLE 2-5/2-6 pg 64
DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003
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CHAPTER 3 : ENERGY, HEAT AND WORK
ELECTRICAL WORK
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CHAPTER 3 : ENERGY, HEAT AND WORK
SHAFT WORK
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CHAPTER 3 : ENERGY, HEAT AND WORK
EXAMPLE 2-7
DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003
CHAPTER 3 : ENERGY, HEAT AND WORK
SPRING WORK
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CHAPTER 3 : ENERGY, HEAT AND WORK
THE FIRST LAW OF THERMODYNAMICS
The 1st Law of Thermodynamics also
known as the conservation of energy
The 1st Law of Thermodynamics state that
energy can be either created nor destroyed
during a process, it can only change forms
PE1 =100 kJ
KE1 = 0 kJ
Total Energy
= 100 kJ
PE2 = 50 kJ
KE2 = 50 kJ
Total Energy
= 100 kJ
PE3 = 0 kJ
KE3 = 100 kJ
Total Energy
= 100 kJ
z/2
z
Other ways to say it:
Energy doesn't pop into existence from nowhere.
Energy doesn't pop out of existence into
nowhere.
"Energy in" equals "energy stored" plus "energy
out"
(if no energy is stored, then "Energy in" equals
"Energy out").
The net change in the total energy of system
during a process is equal to the difference
between the total energy entering and the total
energy leaving the system
DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003
CHAPTER 3 : ENERGY, HEAT AND WORK
THE FIRST LAW OF THERMODYNAMICS
⎛ Total energy
⎞ ⎛ Total energy
⎞ ⎛ Change in the total
⎞
⎜⎜ entering the system ⎟⎟ − ⎜⎜ leaving the system ⎟⎟ = ⎜⎜ energy of the system ⎟⎟
⎝
⎠ ⎝
⎠ ⎝
⎠
E in − E out = ∆E system
Energy change = Energy at final state – Energy at initial state
∆Esystem = ∆Efinal - ∆Einitial = E2 – E1
Where ∆E = ∆U + ∆KE ∆PE = m(u2 − u1 ) +
1
2
(V
2
2
)
− V12 + mg(z 2 − z 1 )
For closed system KE and PE are very small compared to internal energy
and can be neglected, so
DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003
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CHAPTER 3 : ENERGY, HEAT AND WORK
EXAMPLE 2-10
DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003
DAH HABIS DA…….
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