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  2. 2. 2 THERMODYNAMICS Thermodynamics = therme + dynamis (heat) (power)• Thermodynamics: The science of energy.• Energy: The ability to cause changes.
  3. 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. 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. 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. 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. 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 energyof a system
  8. 8. 8ENERGY TRANSFER Energy can cross the boundaries of a closed system in the form of heat and work.
  9. 9. 9 HEATHeat: The form of energy that istransferred between two systems (or asystem and its surroundings) by virtueof a temperature difference. During an adiabatic process, a system exchanges no heat with its surroundings.Temperature difference is the drivingforce for heat transfer. The larger thetemperature difference, the higher isthe rate of heat transfer.
  10. 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. 11THE 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. 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 workinteractions, the energy The work (shaft)change of a system is equal to done on anthe 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. 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. 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, butsatisfying the first law does not ensure that the process can actuallyoccur. Therefore the second law of thermodynamics is introduced toidentify 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 thermodynamicsviolates the first law of thermodynamics. True or false?
  15. 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. 16The entropy of an isolated system (adiabatic closed system) during aprocess always increases, it never decreases. This is known as theincrease of entropy principle. Sisolated 0Entropy change of isolated system is the sum of the entropy change ofthe 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. 17REFERENCECengel Y.A. and Boles M.A. 2011. Thermodynamics: An Engineering Approach. 7th Edition. New York: McGraw- Hill.