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Energy and Energy Transfer
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CDB1053 Introduction to Engineering ThermodynamicsBy Dr. Oh Pei Ching
1
ENERGY AND ENERGY TRANSFER
• Introduce the concept of energy and define its variousforms.
• Discuss the nature of internal energy.
• Define the concept of heat and the terminologyassociated with energy transfer by heat.
• Discuss the three mechanisms of heat transfer:conduction, convection, and radiation.
• Define the concept of work
• Discuss the various forms of work: mechanical or non-mechanical
2
Learning Outcome
CDB1053 Introduction to Engineering Thermodynamics
QUESTION Thermodynamics Concept
Consider a room whose door and windows are tightly closed. The walls
are well-insulated so that heat loss or gain through the walls is negligible.
Place a refrigerator in the middle of the room with its door open, and plug
it into a wall outlet. Will the average temperature of air in the room
increase or decrease? Or will it remain constant?
• If we take the entire room (including the air
and the refrigerator) as the system, which
is an adiabatic closed system since the
room is well-sealed and well-insulated, 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.
The first law of thermodynamics
4
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, such as kinetic and potential
energies.
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.
The macroscopic energy of an object
changes with velocity and elevation.
• 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.
5
Total energy of a system
Energy of a system per unit mass
Potential energy per unit mass
Kinetic energy per unit mass
Potential energy
Kinetic energy
Mass flow rate
Energy flow rate
Open system (Control volume)
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.
Internal = Sensible + Latent + Chemical + Nuclear
Thermal = Sensible + LatentThe various forms of
microscopic
energies that make
up sensible energy.
The internal energy of a
system is the sum of all forms
of the microscopic energies.
7
• 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.
• A control volume (open system) can also exchange energy via mass
transfer.
FIGURE 2-15
8
ENERGY TRANSFER BY HEAT
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
9
Amount of heat transfer when heat transfer
rate changes with time:
During an adiabatic process, a system
exchanges no heat with its surroundings.
Adiabatic process: A process during
which there is no heat transfer.
Energy is recognized as heat transfer
only as it crosses the system boundary.
Heat transfer per unit mass:
Q = Amount of heat
transferred (kJ)
Amount of heat transfer when heat transfer
rate is constant:
10
Historical Background on Heat
• Kinetic theory: Treats molecules as
tiny balls that are in motion and thus
possess kinetic energy.
• Heat: The energy associated with the
random motion of atoms and
molecules.
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).
FIGURE 2-19
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 systemboundaries are all associated with work interactions
• Formal sign convention: Heat transfer to a system and work done by a systemare positive; heat transfer from a system and work done on a system arenegative.
• Alternative to sign convention is to use the subscripts in and out to indicatedirection. This is the primary approach in this text.
Specifying the directions of heat and work.
Work done per unit mass:
12
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 and have exact
differentials (d ).
Path functions have inexact differentials ( ).
FIGURE 2-22
Example: The total volume change during a process
between states 1 and 2 is:
Example: The total work done during process 1-2 is:
EXAMPLE 2-3 Burning of a Candle in an Insulated Room
A candle is burning in a well-insulated room. Taking the room (the air plus the
candle) as the system, determine
(a) If there is any heat transfer during this burning process
(b) If there is any change in the internal energy of the system
The interior surfaces of the room form the system boundary. Heat is recognized as it
crosses the boundaries. Since the room is well insulated, we have an adiabatic system and
no heat will pass through the boundaries. Therefore, Q = 0 for this process.
EXAMPLE 2-4 Heating of a Potato in an Oven
A potato initially at room temperature (25oC) is being baked in an oven
that is maintained at 200oC. Taking the potato as the system, is there any
heat transfer during this baking process?
Since the potato is the system, the outer surface of the skin of the potato can be
viewed as the system boundary. Part of the energy in the oven will pass through
the skin to the potato. Since the driving force for this energy transfer is a
temperature difference, this is a heat transfer process.
15
Electrical Work
Electrical work
Electrical power
When potential difference
and current change with time
When potential difference
and current remain constantFIGURE 2-27
V = potential difference
N = coulombs of electrical charge
I = current or
number of electrical charges
flowing per unit time
16
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.
Work = Force Distance
When force is not constantFIGURE 2-28
• In many thermodynamic problems, mechanical work is the only form of work involved.
17
MOVING BOUNDARY WORK
Moving boundary work (P dV work): The expansion and compression work
in a piston-cylinder device.
Quasi-equilibrium process: A process during which the system remains
nearly in equilibrium at all times.
The work associated with a moving boundary
(expansion and compression) is called
boundary work.
For quasi-equilibrium
process
18
Consider a gas enclosed in a piston-cylinder device.If the piston is allowed to move a distance ds in a quasi-equilibrium
manner, the differential work (boundary work) done during this process is:
Where
P = initial pressure
dV = volume change
P is the absolute pressure and is always positive.
When dV is positive, Wb is positive for expansion
When dV is negative, Wb is negative for compression
A gas does a differential
amount of work Wb as it
forces the piston to move
by a differential amount ds.
Total boundary work from initial state to final state:
To calculate total boundary work, the process by
which the system changed states must be known,
i.e. the functional relationship between P and V
during the process.
P=f (V ) should be available: equation of the process
path on a P-V diagram.
19
The area under the process curve on a P-V
diagram is equal, in magnitude, to the work
done during a quasi-equilibrium expansion
or compression process of a closed
system.
P-V diagram of quasi-equilibrium
expansion process:
The boundary work = Area under the process
curve plotted on a P-V
diagram
Differential area is equal to differential work:
dA = PdV
Total area under the process curve:
20
• A gas can follow several different paths
(with different area underneath) as it
expands from state 1 to state 2.
• Each process path gives a different value
for boundary work.
• The net work output is produced during
a cycle if the work done by the system
during the expansion process (area
under path A) is greater than the work
done on the system during the
compression part of the cycle (area
under path B).
The boundary work done during a process depends
on the path followed as well as the end states.
21
Some typical processes
P
V
1
2
P-V diagram for V = Constant
22
(b) Boundary work for a constant-pressure process
If the pressure is held constant, the boundary work equation becomes
P
V
2 1
P-V diagram for P = Constant
For the constant pressure process shown above, is the boundary work
positive or negative and why?
23
The above equation is the result of applying the ideal gas assumption for
the equation of state. For real gas undergoing an isothermal process, the
integral in the boundary work equation would be done numerically.
24
Process Exponent n
Constant pressure 0
Constant volume ∞
Isothermal & ideal gas 1
Adiabatic & ideal gas k = Cp/Cv
Where
Cp= specific heat at constant pressure;
Cv= specific heat at constant volume
For an ideal gas (PV=mRT), this equation can also be written as
For special case of n=1 the boundary work becomes
25
2
1
1dV
VC
n 1
1
1
1
2
n
VVC
nn
n
VPVP
1
1122 since C = P1V1n =P2V2
n
1,
1
12
n
n
TTmRWb
1
2
1
22
1
2
1
1 lnlnV
VmRT
V
VPVdVCVPdVWb
2
1PdVWb
nCVP
dVV
Cn
2
1
How to determine the boundary work for polytropic process?
Isothermal
process
EXAMPLE 4-1 Boundary Work for a Constant-Volume Process
A rigid tank contains air at 500 kPa and 150oC. As a result of heat transfer to the
surroundings, the temperature and pressure inside the tank drop to 65oC and 400
kPa, respectively. Determine the boundary work done during this process.
Solution:
The boundary work can be determined to be
EXAMPLE 4-2 Boundary Work for a Constant-Pressure Process
A frictionless piston-cylinder device contains 5 kg of steam at 400 kPa
and 200oC. Heat is now transferred to the steam until the temperature
reaches 250oC. If the piston is not attached to a shaft and its mass is
constant, determine the work done by the steam during this process.
m = 5 kg
P = 400 kPa
P, kPa
P0 = 400 kPa400
v1 = 0.53434 v2 = 0.59520 v, m3/kg
EXAMPLE 4-2 Boundary Work for a Constant-Pressure Process
Solution:Assumption: The expansion process is quasi-equilibrium.
Analysis: Even though it is not explicitly stated, the pressure of the steam within the cylinder
remains constant during this process since both the atmospheric pressure and the weight of the
piston remain constant. Therefore, this is a constant-pressure process, and
or
since V = mv. From the superheated vapor table, the specific volumes are determined to be
v1 = 0.53434 m3/kg at state 1 (400 kPa, 200oC) and v2 = 0.59520 m3/kg at state 2 (400 kPa, 250oC).
Substituting these values yields
Discussion: The positive sign indicates that the work is done by the system. That is, the steam
used 122 kJ of its energy to do this work. The magnitude of this work could also be determined by
calculating the area under the process curve on the P-V diagram, which is simply P0∆V for this case.
EXAMPLE 4-3 Isothermal Compression of an Ideal Gas
A piston-cylinder device initially contains 0.4 m3 of air at 100 kPa and
80oC. The air is now compressed to 0.1 m3 in such a way that the
temperature inside the cylinder remains constant. Determine the work
done during this process.
EXAMPLE 4-3 Isothermal Compression of an Ideal Gas (Cont.)
Solution:Assumption: 1 The compression process is quasi-equilibrium. 2 At specified condition, air can be
considered to be an ideal gas since it is at a high temperature and low pressure relative to its critical-
point values.
Analysis: For an ideal gas at constant temperature T0,
where C is a constant. Substituting this into boundary work equation, we have
P1V1 can be replaced by P2V2 or mRT0. Also, V2 / V1 can be replaced by P2 / P1 for this case since
P1V1= P2V2.
Substituting the numerical values yields
Discussion: The negative sign indicates that this work is done on the system (a work input), which
is always the case for compression processes.
1
211
1
22
1
2
1
2
1lnln
V
VVP
V
VC
V
dVCdV
V
CPdVWb
V
CPorCmRTPV 0
kJmkPa
kJmkPaWb 5.55
.1
1
4.0
1.0ln)4.0)(100(
3
3
31
Shaft Work
The power transmitted through the shaft is
the shaft work done per unit time
Shaft work
A force F acting through a moment arm r
generates a torque T
This force acts through a distance s
• Often the torque T applied to a rotating shaft isconstant (Force F applied is also constant).
• For a specified constant torque, the work doneduring n revolutions is determined as follows:
EXAMPLE 2-7 Power Transmission by the Shaft of a Car
Determine the power transmitted through the shaft (kW) of a car when the
torque applied is 200 N.m and the shaft rotates at a rate of 4000
revolutions per minute (rpm).
Solution:
33
Spring Work
Elongation of a spring under
the influence of a force.
• When a force is applied on a spring, the
length of the spring changes.
• 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.
34
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.
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.
Nonmechanical Forms of Work
35
Summary• Forms of energy
Macroscopic = kinetic + potential
Microscopic = Internal energy (sensible + latent + chemical + nuclear)
• Energy transfer by heat
• Energy transfer by work
• Mechanical forms of work
Moving boundary work
Constant-volume process
Constant-pressure process
Isothermal process
Polytropic process
Shaft work
Spring work
• Non-mechanical forms of work