Prepared by Dr. A. Mekki
 
Summary of chapter 19

 

 

1.                 If two bodies are in thermal equilibrium with each other they must have the same temperature.

 

2.                 The Zeroth-law of thermodynamics states that if two bodies A and B are separately in thermal equilibrium with a third body, C,

 then A and B are in thermal equilibrium with each other when placed in thermal contact.

 

3.                 Change of temperature scale:

 

TC = TK 273

 

TF = 9/5 TC + 32

 

TK = 5/9 TF + 255

 

                        Important:                              and      

 

4.                 When a substance is heated, it generally expands. The change in length, DL is related to the change in temperature DT

and the proportionality constant is called a (coefficient of linear expansion)

 

5.                 Expansion and contraction of a solids

 

Ø     In one dimension

 

The change in length is given by             and    

 

The final length is                          

 

Li is the initial length, Lf is the final length, and a is the coefficient of linear expansion

 

Ø     In two dimensions

 

The change in the area is given by            and    

 

The final area is                              

 

Ø     In three dimensions

 

The change in the volume is                                   and                            

The final volume is                                     

 

 This equation is valid for solids and liquids.

 

Coefficient of volume expansion .

 

 

6.                 The heat gained or lost by a substance is given by:

 

(i)                If there is a change in temperature and there is no change in the phase

 

                  

c is the specific heat of the substance and DT = Tf - Ti

 

(ii)               If there is a change in phase and the temperature of the system remains the same

 

 

L is called the heat of transformation.

 

If there is fusion (solid liquid), then we use Lf, if there is vaporization (liquid   gas), then we use Lv.

 

 

7.                 This section is related to GASES ONLY

 

A gas may exchange energy with the surroundings through work.

The work done on or by a gas as it expands or contract from Vi to Vf is  

 

                                     

 

The work can also be calculated from a PV diagram.

 

 

 

 

 

 

 

The work is the area under the curve in a PV diagram as shown in the above figure.

 

For a cyclic process the work is the area enclosed by the cycle.

 

 

 

 

 

 

 

 

 

 

 

 

 


8.                 The first law of thermodynamics is the law of conservation of energy and given by

 

 

where DEint is the internal energy of the gas.

 

Ø     Q > 0 if the gas absorbs (gains) heat

Ø     Q < 0 if the gas expels (lose) heat

Ø     W > 0 if the gas does work

Ø     W < 0 if external work is done on the gas

 

9.                 Special cases of the first law of thermodynamics

 

Ø     Adiabatic process:       Q = 0 and    DEint = - W

 

Ø     Constant volume process: W = 0    and    DEint = Q

 

 

 

 

 

 


Ø     Cyclic process:   DEint = 0, Q = W

 

 

 

 

 

 

 


Ø     Free expansion:          DEint = Q = W = 0

 

10.             Heat can be transferred between a system and the environment in three ways; by conduction, by convection, and by radiation.

 

Ø     In the case of transfer of heat by conduction, the rate of heat flow is given by:

 

              (Watts)

 

k is the thermal conductivity of the material through which heat is conducted.

 

 

Ø     Radiation is heat transfer through the emission of electromagnetic energy. The power of the radiating heat source is given by:

 

                   (Watts)

 

          s = 5.6703 10-8 W/m2K is Stefan-Boltzman constant.

          e is the emissivity of the object and A is its surface area of the heat source.

          T is the temperature in Kelvin.