Reversible and irreversible processes

Mechanics

Oct 27, 2013 (4 years and 6 months ago)

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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

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

possible

impossible

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
of heat from a colder system to a warmer system.

Work and heat

????

q

w

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

q = w

possible

impossible

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

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

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