Reversibility
Reversible Process
Quasi-static processes
meant that each step was
slo enough to maintain
equilibrium.
If the process is reversed the
work and heat also just
reverse sign.
Heat lost to friction is not
reversible.
Reversible process is
infinitely slow; system stays
at equilibrium.
Irreversible process has
heat loss to friction; system
has net loss of useful
energy.
Plotting Reversibility
The PV diagram can plot a quasi-static, reversible
process.
Irreversible processes cannot be plotted.
P
P
P2 ,V2 , T2
P2 ,V2 , T2
P1 ,V1 , T1
P1 ,V1 , T1
V
V
Reversible Cycle
A series of reversible
processes can be plotted on
a PV diagram.
A reversible cycle is a set of
reversible processes that
return to the initial state.
P
P1 ,V1 , T1
P3 ,V3 , T3
P2 ,V2 , T2
V
Carnot Engine
An ideal Carnot engine
consists of four processes.
1) expand gas isothermally.
2) expand gas adiabatically.
3) compress gas isothermally.
4) compress gas adiabatically.
Expansion
During the isothermal
expansion there is work
done with heat in.
W12 QH nRTH ln
V2
V1
There is no heat flow during
the adiabatic expansion, but
work is done.
P
P1 ,V1 , T1
P2 ,V2 , T2
P3 ,V3 , T3
P2V2
CP / CV
P3V3
CP / CV
V
Compression
During the isothermal
compression work is
returned with heat out.
W34 QL nRTL ln
V3
V4
P
P1 ,V1 , T1
There is no heat flow during
the adiabatic compression,
but work is returned.
P4V4
C P / CV
P1V1
C P / CV
P4 ,V4 , T4
P3 ,V3 , T3
V
Carnot Efficiency
The heat and temperatures are related in a Carnot
engine.
• |QL| / |QH| = TL / TH
This is the ideal engine efficiency.
eideal 1
TL
TH
Underwater
A nuclear plant produces
540 MW of power while the
fuel releases 1590 MW.
Steam enters the turbine at
556 K and is discharged at
313 K.
What is the ideal and actual
efficiency?
The ideal efficiency assumes
a Carnot engine.
T
eideal 1 L 43.7%
TH
The actual efficiency is found
from the power usage.
e
W
QH
540 MW - s
34%
1590 MW - s
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