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 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

4. 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.

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

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

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

8. 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

9. 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.

10. 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?

11. 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

12. 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

13. 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

14. End review