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Chapter Fifteen

Chapter Fifteen. Thermodynamics. 1 st Law of Thermodynamics. The Change in a system’s internal energy is related to heat and work. Internal Energy :. 1 st Law of Thermodynamics. The Change in a system’s internal energy is related to heat and work. Conservation of energy. +Q -Q +W

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Chapter Fifteen

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  1. Chapter Fifteen Thermodynamics

  2. 1st Law of Thermodynamics The Change in a system’s internal energy is related to heat and work Internal Energy:

  3. 1st Law of Thermodynamics The Change in a system’s internal energy is related to heat and work Conservation of energy +Q -Q +W -W System GAINS heat System LOSES heat System DOES work –Loses energy Surrounding DOES work on the system

  4. Quasi-static Thermal Processes -Isobaric -Isovolumetric -Isothermal -Adiabatic Meaning that the processes take place slow enough that system has UNIFORM temperatures and pressures

  5. Isobaric Processes occurring at a CONSTANT PRESSURE -Expansion Vs. -Compression System does work Vs. Surrounding does work +Work Vs. -Work

  6. Isobaric Processes occurring at a CONSTANT PRESSURE

  7. Isovolumetric Processes that occur at a constant VOLUME.

  8. Isothermal Processes occurring at a constant TEMPERATURE • Internal energy remains constant

  9. Isothermal Processes occurring at a constant TEMPERATURE Heat transfers energy Work transfers energy

  10. Adiabatic Processes occurring with NO TRANSFER of heat

  11. Adiabatic Processes occurring with NO TRANSFER of heat

  12. Example Problem The largest glass-blown bottle is 2 m tall. The pressure used to expand the bottle is 5.1 kPa. If 3600 J of work is done in expanding the bottle, from an initial volume of 0.0 m3, what is the final volume? V=0.71 m3

  13. 2nd Law of Thermodynamics: Heat only spontaneously flows from a substance of higher temperature to a substance of lower temperature

  14. Any device that uses heat to perform work Heat Engines: Heat supplied from the relatively hot reservoir Part of the heat is used to do work Remainderis relatively cold. Goes into the cold reservoir

  15. Efficiency: In a heat engine, some energy is lost

  16. Carnot’s IDEAL Efficiency: Substance of machine is ignored This allows for a prediction of ideal efficiency TEMPERATURE IN KELVIN!!! TEMPERATURE IN KELVIN 

  17. Example problem If a gasoline engine has an efficiency of 30 % and loses 920 J of heat to the environment, how much work is done by the engine? W=394 J

  18. 2nd Law of Thermodynamics: Heat only spontaneously flows from a substance of higher temperature to a substance of lower temperature -No heat engine can ever be 100% efficient. So heat energy is lost to the environment

  19. Refrigeration:

  20. Refrigeration: Evaporation Coils Refrigerant boils at 20° F In the Evaporation Coils: -Refrigerant absorbs heat and vaporizes No work is done.

  21. Refrigeration: Compressor In the Compressor: -Gas is compressed -Work is done -Pressure and internal internal energy are increased

  22. Refrigeration: Condenser Coil In the Condenser Coil: -Heat transfers out of refrigerant -Refrigerant cools and condenses -No Work

  23. Refrigeration: Expansion Valve In the Expansion Valve: -Liquid expands -Work is done -Internal energy is decreased -No heat is transferred

  24. Refrigeration:

  25. Refrigeration: Cyclic Process: Energy is transferred (through heat and work) but there is no NET change in internal energy

  26. Entropy: • Measure of a system’s disorder • Systems left alone move toward a more disordered state • Disordered states tend to remain disordered • Greater disorder leads to less energy available for work. Entropy can be decreased by doing work

  27. 2nd Law of Thermodynamics: Heat only spontaneously flows from a substance of higher temperature to a substance of lower temperature -No heat engine can ever be 100% efficient. So heat energy is lost to the environment The entropy of the universe increases in all processes -A system’s entropy can decrease only if offset by an increase elsewhere

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