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OVERVIEW ON ENERGY, ENTROPY & SECOND LAW
OF THERMODYNAMICS
THERMODYNAMICS
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• Thermodynamics: The science of energy. • Energy: The ability to cause changes.
Thermodynamics = therme + dynamis (heat) (power)
Application area of thermodynamics
SYSTEMS
• System:
A quantity of matter or a region in space chosen for study.
• Surroundings:
The mass or region outside the system
• Boundary:
The real or imaginary surface that separates the system from its surroundings.
• The boundary of a system can be fixed or movable.
• Systems may be considered to be closed or open.
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SYSTEM
SURROUNDINGS
BOUNDARY
CLOSED SYSTEM
• A fixed amount of mass, and no mass can cross its boundary. Also known as CONTROL MASS.
CLOSED system with moving boundary
CLOSED system
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m = const.
Mass NO
Energy YES
GAS2 kg1 m3
GAS2 kg3 m3
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.
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OPEN system
Mass YES
Energy YES
In Out
Imaginary Boundary
Real Boundary
OPEN system with real and imaginary boundary
PROPERTIES OF A SYSTEM
• PROPERTY: Any characteristic of a system.
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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)
e.g. Pressure (P), Volume (V), Temperature (T) and mass (m)
ENERGY
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Macroscopic energy Microscopic energy
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.
Those related to motion and the influence of some external effects such as gravity, magnetism, electricity and surface tension.
Internal energy, U: The sum of all the microscopic forms of energy.
Total energy of a system
ENERGY TRANSFER
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Energy can cross the boundaries of a closed
system in the form of heat and work.
HEAT
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Temperature difference is the driving force for heat transfer. The larger the temperature difference, the higher is the rate of heat transfer.
During an adiabatic process, a system exchanges no heat with its surroundings.
Heat: The form of energy that is transferred between two systems (or a system and its surroundings) by virtue of a temperature difference.
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.
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Specifying the directions of heat and work.
Power is the work done per unit time (kW)
Qin (+ve) Qout (-ve)Win (-ve) Wout (+ve)
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.
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Energy cannot be created or destroyed; it can only change forms.
The increase in the energy of a potato in an oven is equal to the amount of heat transferred to it.
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In the absence of any work interactions, the energy change of a system is equal to the net heat transfer.
The work (electrical) done on an adiabatic system is equal to the increase in the energy of the system.
The work (shaft) done on an 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.
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.
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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.
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Processes occur in a certain direction, and not in the reverse direction.
A process must satisfy both the first and second laws of thermodynamics to proceed.
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 that violates the second law of thermodynamics violates the first law of thermodynamics. True or false?
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.
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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.
0isolatedS
The increase of entropy principle
Entropy change of isolated system is the sum of the entropy change of the system and its surroundings which equal to entropy generation.
The entropy change of a system can be negative,
but the entropy generation
cannot.
REFERENCECengel Y.A. and Boles M.A. 2011.
Thermodynamics: An Engineering Approach. 7th Edition. New York: McGraw-Hill.
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PREPARED BY:MDM. NORASMAH MOHAMMED MANSHORFACULTY OF CHEMICAL ENGINEERING,UiTM SHAH [email protected]/019-2368303
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