Download Reversible and irreversible Processes

Reversible and irreversible Processes
intuitive approach to reversible and irreversible processes
later
introduce entropy and the 2nd law
foundation of thermodynamics
Reversible process: can be defined as one whose “direction” can be reversed by an
infinitesimal small change in some property of the system.
“Gedankenexperiment” to picture a reversible process:
1 Make a video recording of a process
Observable process
2
reversible
Run the recording backwards
Process impossible to observe
irreversible
Examples:
Process is possible
reversible
Backward recording
reversible
Backward recording
x
x
Small changes can be reversed
reversible
but
V1 ,Ts
gas
V2 ,Tf
You never observe reversed process
of free expansion
irreversible
Reversibility is an idealization (in strictest sense, almost all real processes are irreversible)
Reversibility requires equilibrium processes
but
Not every equilibrium process is reversible
Almost perfect insulation
gas in equilibrium at any time
Example
Tg
> T0
Qout
Although system in equilibrium, no small change of the system will reverse
the heat flow
Reversibility is an idealization
Dry friction between 2 objects
You never observe the reversed
process: object starts to move
without assistance
x
Friction between piston and cylinder
irreversibility
Heat Conductivity in Solids (an example for irreversibility)
Remember: Heat is an energy transferred from one system to another
because of temperature difference
T1
>
T2
System 2
System 1
Heat Q flows from
1
to
2
T1
>
T2
*(in the textbook T >T )
2
1
Heat reservoir 2
Heat reservoir 1
L
T(x)
T1
T2
0
L
x
A
Heat transfer per time interval through homogeneous solid object:
Q K
 (T1  T2 )A
t L
K: thermal conductivity of the rod
where
L
A: cross-section of the rod
Electric Systems (examples for reversibility and irreversibility)
I
#1
+
battery
VE
-
VE : work done against electrical forces per unit charge
Work: W  q VE
dq
Current I 
dt
tf
W    VE ( t ) I( t ) dt
t0
Irreversible case
#1
#2
Resistor network with total resistance
R
VE  R I
W  R I 2 t
In the steady state: Internal energy of black box unchanged
1. law:
U  Q  W
0
U  0
Q  W  R I 2 t  0
Heat leaving the system
Application as heater
#1
reversible case
Capacitor network with total Capacity C
#2
q2
- Charging an uncharged capacitor W  
2C
q2
- Discharge of the capacitor W 
done by the capacitor
2C
No heat transferred (Q=0)