(1) Thermodynamics is the science of energy, energy transfer, and its relation to changes in temperature, volume, pressure and other properties in a system. (2) The first law of thermodynamics states that energy can change forms but cannot be created or destroyed during a process. (3) The second law introduces the concept of entropy and states that processes occur spontaneously in the direction of increasing entropy or decreasing quality of energy. (4) Entropy is a measure of molecular disorder or randomness that always increases for isolated systems undergoing a process.

1. OVERVIEW ON ENERGY,
ENTROPY & SECOND LAW OF
THERMODYNAMICS

2. 2
THERMODYNAMICS
Thermodynamics = therme + dynamis
(heat) (power)
• Thermodynamics: The science of energy.
• Energy: The ability to cause changes.

3. 3
SYSTEMS
• System:
A quantity of matter or a region in
space chosen for study.
SURROUNDINGS
• Surroundings:
The mass or region outside the system
• Boundary:
The real or imaginary surface that SYSTEM
separates the system from its
surroundings.
• The boundary of a system can be fixed
or movable.
BOUNDARY
• Systems may be considered to be closed
or open.

4. 4
CLOSED SYSTEM
• A fixed amount of mass, and no mass can cross its boundary. Also
known as CONTROL MASS.
Mass NO
GAS
m = const. GAS
2 kg
3 m3
2 kg
Energy YES 1 m3
CLOSED system CLOSED system with moving
boundary

5. 5
OPEN SYSTEM
• A properly selected region in space. Also known as CONTROL
VOLUME.
• Boundary of open system is called CONTROL SURFACE.
• E.g. Water heater, nozzle, turbine, compressor.
Real Boundary
Mass YES
In Out
Energy YES
Imaginary Boundary
OPEN OPEN system with real and
system imaginary boundary

6. 6
PROPERTIES OF A SYSTEM
• PROPERTY: Any characteristic of a system.
e.g. Pressure (P), Volume (V), Temperature (T) and mass (m)
Intensive : Independent on mass of system.
- e.g. Temperature (T), Pressure (P)
Extensive : Dependent on size/extent of system.
- e.g. Total mass, total volume
Specific : Extensive properties per unit mass.
- e.g. Sp. Vol (v=V/m), Sp. Enthalpy (h=H/m)

7. 7
ENERGY
Macroscopic energy Microscopic energy
Kinetic energy, KE: The Those related to motion
energy that a system possesses as and the influence of some
a result of its motion relative to external effects such as
some reference frame. gravity, magnetism,
electricity and surface
tension.
Potential energy, PE: The
Internal energy, U: The
energy that a system possesses as
sum of all the microscopic
a result of its elevation in a
forms of energy.
gravitational field.
Total energy
of a system

8. 8
ENERGY TRANSFER
Energy can cross the boundaries
of a closed system in the form of
heat and work.

9. 9
HEAT
Heat: The form of energy that is
transferred between two systems (or a
system and its surroundings) by virtue
of a temperature difference.
During an adiabatic process, a
system exchanges no heat with
its surroundings.
Temperature difference is the driving
force for heat transfer. The larger the
temperature difference, the higher is
the rate of heat transfer.

10. 10
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 system boundaries are all associated with work
interactions
• Formal sign convention: Heat transfer to a system (Qin) and
work done by a system (Wout) are positive; heat transfer from a
system (Qout) and work done on a system (Win) are negative.
Qin (+ve) Qout (-ve)
Win (-ve) Wout (+ve)
Power is the
work done per
unit time (kW) Specifying the directions
of heat and work.

11. 11
THE FIRST LAW OF THERMODYNAMICS
• The first law of thermodynamics (the conservation of energy
principle) provides a basic to study the relationships among various forms of
energy and energy interactions.
• The first law states that energy can be neither created nor destroyed
during a process; it can only change forms.
Energy
cannot be
created or
destroyed; The increase in the energy of a potato in
it can only an oven is equal to the amount of heat
change transferred to it.
forms.

12. 12
The work
(electrical) done
on an adiabatic
system is equal to
the increase in
the energy of the
system.
In the absence of any work
interactions, the energy The work (shaft)
change of a system is equal to done on an
the net heat transfer. adiabatic system
is equal to the
increase in the
energy of the
system.
The energy change of a system
during a process is equal to the
net work and heat transfer
between the system and its
surroundings.

13. 13
THE SECOND LAW OF THERMODYNAMICS
• The second law of thermodynamics asserts that energy has quality as
well as quantity, and actual processes occur in the direction of
decreasing quality of energy.
• A process must satisfy the fist law to occur. However, satisfying the first
law alone does not ensure that the process will actually take place.
A cup of hot coffee left on a table eventually
cools off.
First law: amount of energy lost by the
coffee is equal to the amount gained by the
surrounding air.
BUT a cup of cool coffee in the same room
never gets hot by itself.
This process never takes place. Doing so
would not violate the first law as long as
the amount of energy lost by the air is equal
to the amount gained by the coffee.
Heat flows in the direction of
decreasing temperature.

14. 14
Processes occur in a certain
direction, and not in the
reverse direction.
The first law places no restriction on the direction of a process, but
satisfying the first law does not ensure that the process can actually
occur. Therefore the second law of thermodynamics is introduced to
identify whether a process can take place.
A process must satisfy both
the first and second laws of
thermodynamics to proceed.
A process that violates the second law of thermodynamics
violates the first law of thermodynamics. True or false?

15. 15
ENTROPY
• Entropy is a measure of molecular disorder, or molecular
randomness. As a system becomes more disordered, the positions of
the molecules becomes less predictable and the entropy increases.
• Entropy is defined as
• The entropy change can be obtained from integration
• Entropy change for internally reversible isothermal heat transfer
process:
where To is the constant temperature of the system and Q is the heat
transfer for the internally reversible process. 15

16. 16
The entropy of an isolated system (adiabatic closed system) during a
process always increases, it never decreases. This is known as the
increase of entropy principle.
Sisolated 0
Entropy change of isolated system is the sum of the entropy change of
the system and its surroundings which equal to entropy generation.
The increase of entropy principle
The entropy change of a
system can be negative, but
the entropy generation
cannot.

17. 17
REFERENCE
Cengel Y.A. and Boles M.A. 2011. Thermodynamics: An
Engineering Approach. 7th Edition. New York: McGraw-
Hill.

18. PREPARED BY:
MDM. NORASMAH MOHAMMED MANSHOR
FACULTY OF CHEMICAL ENGINEERING,
UiTM SHAH ALAM.
norasmah@salam.uitm.edu.my
03-55436333/019-2368303