Reversible and irreversible Processes

introduce entropy and the 2nd law

intuitive approach to reversible and irreversible processes

later

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:

Make a video recording of a process1

2 Run the recording backwards

Observable process

Process impossible to observe

reversible

irreversible

Examples:

Process is possible

reversible

Backward recording

Backward recording

reversible

x

Small changes can be reversed

but

gas

V1 ,Ts V2 ,Tf

You never observe reversed process of free expansion

reversible

irreversible

x

Tg

Reversibility is an idealization (in strictest sense, almost all real processes are irreversible)

Reversibility requires equilibrium processes

but

Not every equilibrium process is reversible

Example

> T0

Qout

Almost perfect insulationgas in equilibrium at any time

Although system in equilibrium, no small change of the system will reversethe heat flow

Dry friction between 2 objects

Reversibility is an idealization

You never observe the reversedprocess: 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

System 1 System 2

T1 > T2

Heat Q flows from

1 to 2

Heat reservoir 1 Heat reservoir 2

T1 > T2

L

T(x)

x

T1

T2

0 L

*(in the textbook T2>T1)

Heat transfer per time interval through homogeneous solid object:

A)TT(L

K

t

Q21

where

K: thermal conductivity of the rod

A: cross-section of the rod

A

L

Electric Systems (examples for reversibility and irreversibility)

+

-battery EV

I#1

EV : work done against electrical forces per unit charge

Work: EVqW

Currentdt

dqI

dt)t(I)t(VWf

0

t

tE

Heat leaving the system

Irreversible case

RResistor network with total resistance IRVE

tIRW 2

In the steady state: Internal energy of black box unchanged 0U

0WQU 1. law: 0tIRWQ 2

Application as heater

reversible case Capacitor network with total Capacity C

#1

#2

#1

#2

- Charging an uncharged capacitorC2

qW

2

- Discharge of the capacitorC2

qW

2 done by the capacitor

No heat transferred (Q=0)