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 • may exchange heat or work
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.
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.
Work and heat q q w w q = w q = w possible ????
Work and heat warmer q possible w q = w impossible q colder Overall process violates the Second Law!
Work and heat q q w w q = w q = w possible impossible irreversible
Expansion of an ideal gas free expansion isothermal expansion ∆E = 0 Q = 0 q = w W = 0 q ∆ E = 0 ∆ T = 0 w possible possible reversible We can also perform isothermal compression.
Free compression? free compression Step 1: isothermal expansion colder Q = 0 q W = 0 w ∆ E = 0 ∆ T = 0 q ???? warmer Note that the gas has expanded.
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!
Expansion of an ideal gas free expansion isothermal expansion ∆E = 0 Q = 0 W = 0 q = w ∆ E = 0 q ∆ T = 0 w possible irreversible possible reversible We can also perform isothermal compression.
Heat engines perfect heat engine general heat engine warmer q-q'=w q q w w q' impossible colder possible* *provided waste heat q' is not too small
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
End review
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