Reversible and irreversible processes Physics 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 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. warmer q colder warmer q colder possibleimpossible

Heat flow warmer q colder warmer q colder irreversible same q reversible 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.

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

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

Work and heat q w q w q = w possibleimpossible irreversible

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

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

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

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

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

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

End review