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Chapter 18 Heat, Work, and the First Law of Thermodynamics

Chapter 18 Heat, Work, and the First Law of Thermodynamics. Heat and work Thermodynamic cycle. Heat and work Work is done by the system: Work is done on the system :. The first law of thermodynamics Work and heat are path-dependent quantities

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Chapter 18 Heat, Work, and the First Law of Thermodynamics

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  1. Chapter 18 Heat, Work, and the First Law of Thermodynamics

  2. Heat and work Thermodynamic cycle

  3. Heat and work • Work is done by the system: • Work is done on the system :

  4. The first law of thermodynamics • Work and heat are path-dependent quantities • Quantity Q + W = ΔEint(change of internal energy) is path-independent • 1st law of thermodynamics: the internal energy of a system increases if heat is added to the system or work is done on the system

  5. The first law of thermodynamics • Adiabatic process: no heat transfer between the system and the environment • Isochoric (constant volume) process • Free expansion: • Cyclical process:

  6. Chapter 18 Problem 19 In a certain automobile engine, 17% of the total energy released in burning gasoline ends up as mechanical work. What’s the engine’s mechanical power output if its heat output is 68 kW?

  7. Work done by an ideal gas at constant temperature • Isothermal process – a process at a constant temperature • Work (isothermal expansion)

  8. Work done by an ideal gas at constant volume and constant pressure • Isovolumetric process – a process at a constant volume • Isobaric process – a process at a constant pressure

  9. Molar specific heat at constant volume • Heat related to temperature change: • Internal energy change:

  10. Molar specific heat at constant pressure • Heat related to temperature change: • Internal energy change:

  11. Adiabatic expansion of an ideal gas

  12. Adiabatic expansion of an ideal gas

  13. Chapter 18 Problem 24 How much work does it take to compress 2.5 mol of an ideal gas to half its original volume while maintaining a constant 300 K temperature?

  14. Free expansion of an ideal gas

  15. Degrees of freedom and molar specific heat • Degrees of freedom: • 3 translations, 3 rotations, + oscillations

  16. Degrees of freedom and molar specific heat • Degrees of freedom: • 3 translations, 3 rotations, + oscillations • In polyatomic molecules different • degrees of freedom contribute at • different temperatures

  17. Chapter 18 Problem 26 A gas mixture contains 2.5 mol of O2 and 3.0 mol of Ar. What are this mixture’s molar specific heats at constant volume and at constant pressure?

  18. Questions?

  19. Answers to the even-numbered problems Chapter 18 Problem 22 1.2 kJ

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