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
next