1. 1
CHAPTER 2bCHAPTER 2b
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 First Law of Thermodynamics
Explain the conservation of energy principle
(Energy Balance)
4. 4
Energy BalanceEnergy 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.
CONSERVATION OF ENERGYCONSERVATION OF ENERGY
5. 5
Energy Change of a System,
Esystem
Energy Change of a System,
Esystem
Total Energy = sum of internal, kinetic, and potential energy changes
6. 6
Closed System Energy BalanceClosed System Energy Balance
In a process of a closed system the energy of the system
increases and decreases by an amount equal to the
amount of energy transferred across its boundary
WQEE 12
(kJ)
Change in Energy
of the system
Net amount of
heat transfer in
Net amount of
work transfer out
= -
WQPEKEU
7. 7
RATE OF ENERGY TRANSFERRATE OF ENERGY TRANSFER
(kJ)
The energy balance can then be expressed compactly as:
Or in a rate form:
Or on a unit mass basis:
8. 8
EXAMPLESEXAMPLES
A closed system containing 20 kg of air undergoes a process in
which there is a heat transfer of 1000 kJ from the system to the
surroundings. The work done on the system is 200 kJ. If the initial
specific internal energy of the system is 300 kJ/kg, what is the
final specific internal energy? Neglect KE and PE. (260 kJ/kg)
9. 9
Class TakeawayClass Takeaway
1. For any system, the first law can be written as:
a. ____________________________ (kJ) or,
b. ____________________________ (kJ/kg) or,
c. ____________________________ (kW)
2. Fill in the blank spaces in the table.
Process Q W E1 E2 E
A +50 -20 +50
B +50 +20 +20
C -40 +60 +20
D -90 +50 0
E +50 +20 -100
10. 10
Summary of Chapter 2Summary of Chapter 2
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
The first law of thermodynamics
Energy balance
Energy change of a system
Mechanisms of energy transfer (heat, work, mass flow)
