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Reversible and irreversible processes

Reversible and irreversible processes. Physics 110 -- Entropy 28 September 2011. Isolated systems. thermal reservoir may exchange heat temperature does not change. battery reversibly stores work. engine may exchange heat or work operates in a cycle. ideal gas

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Reversible and irreversible processes

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  1. Reversible and irreversible processes Physics 110 -- Entropy 28 September 2011

  2. Isolated systems • thermal reservoir • may exchange heat • temperature does not change • battery • reversibly stores work • engine • may exchange heat or work • operates in a cycle • ideal gas • may exchange heat or work

  3. Heat flow warmer warmer q q colder colder possible impossible Second Law of Thermodynamics (Clausius) No process can have as its sole result the transfer of heat from a colder system to a warmer system.

  4. Heat flow same warmer warmer q q q same colder colder possible possible impossible irreversible reversible Second Law of Thermodynamics (Clausius) No process can have as its sole result the transfer of heat from a colder system to a warmer system.

  5. Work and heat q q w w q = w q = w possible ????

  6. Work and heat warmer q possible w q = w impossible q colder Overall process violates the Second Law!

  7. Work and heat q q w w q = w q = w possible impossible irreversible

  8. Expansion of an ideal gas free expansion isothermal expansion Q = 0 W = 0 ∆E = 0 ∆T = 0 ∆E = 0 q = w q w possible possible reversible We can also perform isothermal compression.

  9. Free compression? free compression Step 1: isothermal expansion colder Q = 0 W = 0 ∆E = 0 ∆T = 0 q w q ???? warmer Note that the gas has expanded.

  10. Free compression? free compression Step 2: free compression colder Q = 0 W = 0 ∆E = 0 ∆T = 0 impossible warmer Overall process violates the Second Law!

  11. Expansion of an ideal gas free expansion isothermal expansion Q = 0 W = 0 ∆E = 0 ∆T = 0 ∆E = 0 q = w q w possible irreversible possible reversible We can also perform isothermal compression.

  12. Heat engines perfect heat engine general heat engine warmer q-q'=w q q w w q' colder impossible possible* *provided waste heat q' is not too small

  13. Refrigerators perfect refrigerator general refrigerator warmer warmer q q w q=w+q' q q' colder colder impossible possible* *provided work input w is not too small

  14. End review

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