SLIDE 1
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
Reversible and irreversible processes Physics 110 -- Entropy 28 - - PowerPoint PPT Presentation
Reversible and irreversible processes Physics 110 -- Entropy 28 - - PowerPoint PPT Presentation
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
SLIDE 2
SLIDE 3
Heat flow
Second Law of Thermodynamics (Clausius) No process can have as its sole result the transfer
- f heat from a colder system to a warmer system.
warmer
q
colder warmer
q
colder possible impossible
SLIDE 4
Heat flow
warmer
q
colder warmer
q
colder irreversible same
q
same reversible possible possible impossible Second Law of Thermodynamics (Clausius) No process can have as its sole result the transfer
- f heat from a colder system to a warmer system.
SLIDE 5
Work and heat
????
q w
q = w
q w
q = w possible
SLIDE 6
Work and heat
q w q
warmer colder q = w Overall process violates the Second Law! possible impossible
SLIDE 7
Work and heat
q w q w
q = w q = w possible impossible irreversible
SLIDE 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 possible reversible
SLIDE 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.
SLIDE 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?
SLIDE 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 possible irreversible
SLIDE 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
SLIDE 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
SLIDE 14