The document discusses various topics relating to energy, including: - The main forms of energy are kinetic, potential, internal, thermal, chemical, and nuclear energy. Heat and work are the only ways for energy to transfer across system boundaries. - Heat transfer is caused by a temperature difference, while work involves a force acting through a distance. Both heat and work are path dependent and occur at boundaries. - Common types of work include shaft work, spring work, and electrical work. The total work on a system equals the sum of all individual work terms. - The first law of thermodynamics states that energy cannot be created or destroyed, only transferred or changed in form.

1. 1
CHAPTER 2aCHAPTER 2a
ENERGY, ENERGY TRANSFER
& GENERAL ENERGY
ANALYSIS
ENERGY, ENERGY TRANSFER
& GENERAL ENERGY
ANALYSIS

2. 2
CONTENTSCONTENTS
 Forms of Energy
 Energy Transfer by Heat
 Energy Transfer by Work
 Mechanical Forms of Work
 The First Law of Thermodynamics
 Energy Conversion Efficiencies

3. 3
LESSON OBJECTIVESLESSON OBJECTIVES
At the end of this lesson, you should be able
to:
State the various forms of energy
Describe the nature of internal energy
Describe the energy transfer by heat and work
Explain mechanical work

4. FORMS OF ENERGYFORMS OF ENERGY

5. 5
FORMS OF ENERGYFORMS OF ENERGY
 Energy exists in numerous forms (thermal, mechanical, chemical, kinetic,
potential, electric, magnetic & nuclear)
 The sum of the energies is the total energy, E (kJ)
 Or for a unit mass,
Grouping of
Energy forms
macroscopic
microscopic
energy of a system as a whole with respect to some
outside reference frames, e.g. KE, PE
• related to molecular structure of a system and the
degree of molecular activity
• independent of outside reference frames
• The sum is the Internal Energy, U
m
E
e  (kJ/kg)

6. 6
FORMS OF ENERGY (cont’d)FORMS OF ENERGY (cont’d)
Kinetic energy (KE)
- result of motion relative to some
reference frame (unit J)
where
= velocity of the system relative to
some fixed reference frame (m/s)
m = mass of an object (kg)
Potential energy (PE)
- due to elevation in a gravitational
field (unit J)
where
g = gravitational acceleration, 9.81 m/s2
h = elevation of center of gravity of a
system relative to some arbitrarily
plane (m)
Macroscopic forms of energy
 2
1
2
212
2
2
1
2
1




mKEKEKE
mKE

mgzPE 
 1212 zzmgPEPEPE 

7. 7
Sensible Energy
- Kinetic energy of molecules
Latent Energy
- Associated with
phase of a
system
Microscopic forms of energy
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.
The internal energy of a system is the sum of all forms of
the microscopic energies.
Internal = Sensible + Latent + Chemical + Nuclear
Thermal = Sensible + Latent
FORMS OF ENERGY (cont’d)FORMS OF ENERGY (cont’d)

8. 8
Total Energy of a system
mgzmUPEKEUE  2
2
1

gzupekeue  2
2
1

Total Energy of a system per unit mass
Most of the closed system remains stationary, so for that
system;
FORMS OF ENERGY (cont’d)FORMS OF ENERGY (cont’d)

9. Energy in Closed SystemEnergy in Closed System
Most of the closed system
remains stationary, so for that
system;
0 PEKE
PEKEUE 

10. Energy Interaction
 Forms of energy not stored in the system
 It is also called dynamic forms of energy
 Recognized as it crosses boundary,
represent energy lost or gain
 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.
Energy can cross the
boundaries of a closed system
in the form of heat and work.
FORMS OF ENERGY (cont’d)FORMS OF ENERGY (cont’d)

11. 11
ENERGY TRANSFER BY HEATENERGY TRANSFER BY HEAT
Heat Transfer Means of energy transfer caused by temperature
difference between the system and the surroundings
Direction of heat transfer: Higher T
to Lower T
Energy is recognized as heat
transfer only as it crosses the
system boundary
Temperature difference is the driving force
for heat transfer. The larger the temperature
difference, the higher is the rate of heat
transfer.

12. 12
ENERGY TRANSFER BY HEAT
(cont’d)
ENERGY TRANSFER BY HEAT
(cont’d)
 Adiabatic process is a process during
which there is no heat transfer, Q=0
 How a process can be adiabatic?
 Well insulated system
 No temperature difference
 Adiabatic is not necessarily means
isothermal process. Temperature of the
system can still be changed by other
means

13. 13
 Amount of heat transferred is denoted by Q (kJ)
 For a unit mass:
m
Q
q  (kJ/kg)
Q positive indicates heat input
Q negative indicates heat lost
 Sign Convention for Heat:
Specifying the directions
of heat using in and out
ENERGY TRANSFER BY HEAT
(cont’d)
ENERGY TRANSFER BY HEAT
(cont’d)

14. ENERGY TRANSFER BY HEATENERGY TRANSFER BY HEAT
14

15. 15
MECHANISMS OF HEAT TRANSFER
Conduction Convection Radiation
The transfer of energy due to the
emission of electromagnetic waves
(or photons).
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.
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.
ENERGY TRANSFER BY HEAT
(cont’d)
ENERGY TRANSFER BY HEAT
(cont’d)

16. 16
ENERGY TRANSFER BY WORKENERGY TRANSFER BY WORK
 Work is an energy interaction between a system and its
surroundings that is not caused by temperature difference
WORK Energy transfer associated with a force acting
through a distance, e.g. rotating shaft, rising piston
 The work done by, or on, a system is defined as (unit kJ):

2
1
.
s
s
dsFW or
m
W
w Work done per unit mass
 The work done per unit time is Power (unit kJ/s or kW):

17. 17
 Sign convention for work:
 The work done by, or on, a
system is defined as (unit
kJ):
W positive indicates work done by system (work output)
W negative indicates work done on the system (work input)
Specifying the directions
of work using in and out
ENERGY TRANSFER BY WORK
(cont’d)
ENERGY TRANSFER BY WORK
(cont’d)

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).
ENERGY TRANSFER BY WORK
(cont’d)
ENERGY TRANSFER BY WORK
(cont’d)

19. 19
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
ENERGY TRANSFER BY WORK
(cont’d)
ENERGY TRANSFER BY WORK
(cont’d)

20. 20
MECHANICAL FORMS OF
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.
Work = Force  Distance When force is not constant
If there is no movement, no
work is done.
 In thermo, most of the work is mechanical work, which
associated with moving boundary work
 Other common forms of work
 Shaft work
 Spring work

21. 21
A force F acting through a moment arm r
generates a torque T
Shaft work is
proportional to the
torque applied and the
number of revolutions
of the shaft.
This force acts through a distance s
The power transmitted through the shaft is the shaft work done per unit time
Shaft work:
SHAFT WORKSHAFT WORK

22. 22
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.
SPRING WORKSPRING WORK

23. 23
Different forms of work transfer could occur in a system simultaneously
during a process.
The total or net work done by the system = algebraic sum of all work
NET WORK DONE BY A
SYSTEM
NET WORK DONE BY A
SYSTEM
...WWWWW sshebtotal


24. 24
1. How can a closed system and its surroundings interact?
2. What are the sign conventions used for energy transfer?
3. Express total energy and for each of the terms, indicate the unit.
Class TakeawayClass Takeaway