Thermodynamics

ThermodynamicsM. D. Eastin

Forms of EnergyEnergy comes in a variety of forms…

Potential

Mechanical

Chemical Electrical

Internal Kinetic

Heat

ThermodynamicsM. D. Eastin

The Concept of WorkWork is a Mechanical form of Energy:

DistanceForceWork

xFdW

ForceDistance

x

THERMODYNAMICS: the science of energy, specifically heat and work, and how the transfer of energy effects the properties of materials.

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Thermodynamics: The science of energy. Energy: The ability to cause changes. The name thermodynamics stems from

the Greek words therme (heat) and dynamis (power).

Conservation of energy principle: During an interaction, energy can change from one form to another but the total amount of energy remains constant.

Energy cannot be created or destroyed. The first law of thermodynamics: An

expression of the conservation of energy principle.

The first law asserts that energy is a thermodynamic property.

THERMODYNAMICS AND ENERGY

Energy cannot be created or destroyed; it can only change forms (the first law).

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The second law of thermodynamics: It asserts that energy has quality as well as quantity, and actual processes occur in the direction of decreasing quality of energy.

Classical thermodynamics: A macroscopic approach to the study of thermodynamics that does not require a knowledge of the behavior of individual particles.

It provides a direct and easy way to the solution of engineering problems and it is used in this text.

Conservation of energy principle for the human body.

Heat flows in the direction of decreasing temperature.

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Application Areas of Thermodynamics

Heat, Q, energy caused by temperature difference

Heat ... is the amount of internal energy entering or leaving a system... occurs by conduction, convection, or radiation.... causes a substance's temperature to change... is not the same as the internal energy of a substance  ... is positive if thermal energy flows into the substance... is negative if thermal energy flows out of the substance... is measured in joules

                   

       

Thermal Equilibrium Systems (or objects) are said to be in

thermal equilibrium if there is no net flow of thermal energy from one to the other.  A thermometer is in thermal equilibrium with the medium whose temperature it measures, for example.

If two objects are in thermal equilibrium, they are at the same temperature.

Work, W, energy caused by physical motion

WORKW is positive if work is done by system.

                           

Air does work on the environment:   W > 0.

W is negative if work is done on the system.

                                             Environment (man) does work on system:  W < 0 (Alternative:  system does negative work because force by air pressure on thumb is opposite to the direction of motion of the thumb.)

RELATIONSHIP BETWEEN HEAT AND WORKWhy does the volume of gas expands when it is heated?

W = F x d Pressure (P) = (Force) F or F = P A (Area) AVolume (V) = L x W x H or A x dd = V A

W = P A V = P V A

Internal Energy (U or E) :  (measured in joules) - Sum of random translational, rotational, and vibrational kinetic energies U:  change in U U > 0 is a gain of internal energyU < 0 is a loss of internal energy----------------------------------Thermal Energy:   same as internal energy

                      Vibrational kinetic energy in solids. The hotter the object, the larger the vibrationalkinetic energy

                     

Motions of a diatomic  molecule in a fluid

INTERNAL ENERGY (U or E)is the total of the kinetic energy due to the motion

of molecules (translational, rotational, vibrational) and the potential energy associated with the vibrational and electric energy of atoms within molecules or crystals.

Example:  1000 J of thermal energy flows into a system (Q = 1000 J).  At the same time, 400 J of work is done by the system (W = 400 J). What is the change in the system's internal energy U?----------------------------------------------------------

Solution: U = Q - W       = 1000 J - 400 J       = 600 J

                        

Example:  800 J of work is done on a system (W = -800 J) as 500 J of thermal energy is removed from the system (Q = -500 J). What is the change in the system's internal energy U?-----------------------------------------------------Solution:

U = Q - W        = -500 J - (-800 J)        = -500 J + 800 J        =   300 J

                        

Work Done by an Expanding Gas                     

       

W   = PV V =  Vf - Vi

W   =  P (Vf - Vi)

             

                     

  Area under pressure-volume curve is  the work done  ----------------------------------------- Isobaric Process: "same pressure"         Greek:  barys, heavy

Expanding Gas

                                    

Example:   If a gas expands at a constant pressure, the work done by the gas is:  

W = PV

10 grams of steam at 100oC at constant pressure rises to 110oC:P   = 4 x 105 Pa           T = 10oC   V = 30.0 x 10-6 m3        c = 2.01 J/g oC

What is the change in internal energy?  U = Q - WW = (4 x 105)(30.0 x 10-6) = 12 JQ = mcT = (10)(2.01)(10) = 201 J

 U = Q - W = 201 J - 12 J = 189 J

ISOTHERMAL-Temperature remains constant

ISOBARIC - Pressure remains constant

ISOMETRIC - Volume remains constant (also ISOVOLUMETRIC or ISOCHORIC)

                                  

         

 Since ΔV = 0, W = 0 then U = Q - W = Q

 

Adiabatic Expansion of a Ideal Gas No heat transfer therefore no temperature change (Q=0).Generally obtained by surrounding the entire system with a strongly insulating material or by carrying out the process so quickly that there is no time for a significant heat transfer to take place.                                              

If Q = 0 then ΔU = - WA system that expands under adiabatic conditions does positive work, so the internal energy decreases. A system that contracts under adiabatic conditions does negative work, so the internal energy increases.