Lecture 3: 09.12.03 The first law of thermodynamics

 


           

Today:                                                

 

Last Time............................................................................................................................................................... 2

Work and heat change the internal energy of a system......................................................................................... 3

The first law: conservation of energy in thermodynamic calculations................................................................................ 3

Defining work and heat............................................................................................................................................ 3

Equivalence of work and heat.................................................................................................................................... 4

Heat and work in reversible processes......................................................................................................................... 5

Path dependence vs. path independence of work, heat, and other thermodynamic functions............................................................................................................................................................... 7

Path independent variables: State functions.................................................................................................................. 7

Path dependent variables: heat and work...................................................................................................................... 7

Application example............................................................................................................................................... 9

References........................................................................................................................................................... 10

           


           

 

 

Reading:                                  Mortimer, Ch. 3 ŒWork, Heat, and Energy: The First Law of Thermodynamics,ı pp. 45-60

                                                          

 

Supplementary Reading:        Dill and Bromberg, Ch. 7 pp. 119-126 (further discussion of the first law, work, heat, and reversible vs. irreversible processes)

 

PLANNING NOTES:

 

Need to fill in conversion of work to heat examples

           

 

 

 

 

 

 

 

 

Last Time

 

Work and heat change the internal energy of a system

 

 

The first law: conservation of energy in thermodynamic calculations

 

 

(Eqn 1)                                    

 

·       Internal energy, like kinetic and potential energy that you first encounter in physics, is conserved. Energy lost from a system is not destroyed, it is passed to its surroundings. The first law of thermodynamics is simply a statement of this conservation. The first law, mathematically, is:

 

 

 

 

 

 

Defining work and heat

 

Definition of work

 

 

 

 

 

 

Examples of work in materials

Force, F

Change in systemıs extent dx in response to this force (displacement):

Pressure, P

Volume dV

Electric field, E

Charge

Chemical potential of A atoms, µA

Number of A atoms, NA

Surface tension, g

Surface area of system, dA

 

 

Definition of heat

 

 

 

 

 

 

Heat and work only refer to processes of energy transfer (!!!)

 

 

 

 

 

 

 

 

 

Equivalence of work and heat

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Heat and work in reversible processes

 

P-V work in reversible processes

 

 

 

 

 

 

(Eqn 7)                        

 

 

 

 

Heat transfer is linked to an entropy change in reversible processes

 

 

 

 

 

 

 

 

 

 

 

 

(Eqn 9)                             for a reversible process

 

 

 

 

Path dependence vs. path independence of work, heat, and other thermodynamic functions

 

Path independent variables: State functions

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Path dependent variables: heat and work

 

 

A simple example: path dependence of P-V work

 

 

(Eqn 12)                                  

 

 

 

 

 

(Eqn 13)                                  

 

 

 

 

 

 

 

 

Application example

 

References