HEAT, WORK AND INTERNAL ENERGY

HEAT, WORK AND HEAT, WORK AND INTERNAL INTERNAL ENERGYENERGY

GLOBAL GLOBAL WARMING?WARMING?

THERMODYNAMICSTHERMODYNAMICS: : the science of the science of energy, specifically heat and work, energy, specifically heat and work,

and how the transfer of energy effects and how the transfer of energy effects the properties of materials.the properties of materials.

Thermodynamics:

 "thermo":     Greek  therme heat     "dynamics":  Greek  dynamikos powerful   Physics that deals with the mechanical  action or relations between heat and work  Example 1:  Heat to work

 Heat Q from flame provides energy to do work--------------------------------------------------------- Example 2:  Work to heat.  Work done by person is converted  to heat energy via friction.

A “A “systemsystem” is the “collection of objects on ” is the “collection of objects on which attention is being focused”which attention is being focused”

The “The “surroundingssurroundings” are everything else in ” are everything else in the environmentthe environment

The system and surroundings must be The system and surroundings must be separated by walls which can either separated by walls which can either insulate or allow heat flowinsulate or allow heat flow

OPEN SYSTEMOPEN SYSTEM:  Mass and energy freely :  Mass and energy freely moves in and out between the system moves in and out between the system and the surroundingand the surrounding

ISOLATED SYSTEMISOLATED SYSTEM:  No interaction :  No interaction between the system and the surroundingbetween the system and the surrounding

CLOSED SYSTEMCLOSED SYSTEM:  fixed mass:  fixed mass

HeatHeat, , QQ, energy caused by , energy caused by temperature differencetemperature 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 EquilibriumThermal Equilibrium Systems (or objects) are said to be in thermal Systems (or objects) are said to be in thermal

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

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

Work, W,Work, W, energy caused by energy caused by physical motionphysical motion

WORKWORKW 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 RELATIONSHIP BETWEEN HEAT AND WORKWORKWhy does the volume of gas expands when it is heated?Why 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 AW = 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 INTERNAL ENERGY (U or E)E)is the total of the is the total of the kinetic energykinetic energy due to the motion due to the motion

of of moleculesmolecules ( (translationaltranslational, , rotationalrotational, , vibrationalvibrational) ) and the and the potential energypotential energy associated with the associated with the vibrational and vibrational and electricelectric energy of energy of atomsatoms within within molecules or molecules or crystalscrystals. .

The The First Law of ThermodynamicsFirst Law of Thermodynamics states that states that ::The internal energy of a system changes The internal energy of a system changes from an from an initial value initial value UUii to a final value to a final value UUff due to heat added ( due to heat added (Q)Q) and work done by and work done by the system (the system (W) W)

UU = = UUff – – UUii = = QQ – – WW

QQ is positive is positive when the when the system gains heatsystem gains heat, , and and negativenegative when the when the system loses heatsystem loses heat..

WW is positive is positive when it is when it is done BY the systemdone BY the system, , and and negativenegative when it is when it is done ON the systemdone ON the system

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 Work Done by an Expanding Gas Expanding Gas

W   = PVV =  Vf - Vi

W   =  P (Vf - Vi)

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

Work and the Pressure-Work and the Pressure-Volume CurveVolume Curve

Work Done = Area Under PV curve-------------------------------------How much work is done by the system when the system is taken from: (a)  A to B  (900 J)(b)  B to C  (0 J)(c)  C to A  (-1500 J)-------------------------------------Each "rectangle" has an area of100 Pa-m3 = 100 (N/m2)-m3                   = 100 N-m                  = 100 Joules

Expanding GasExpanding 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

Work, Rubber Bands, and Internal Work, Rubber Bands, and Internal EnergyEnergy

U = Q - W

Expand rubber band:  W < 0, Q = 0 U >0temperature increases -------------------------------------------Press thick rubber band to forehead and expand it rapidly.  The warming should be obvious. Now allow the band to contract quickly; cooling will also be evident.

ISOTHERMALISOTHERMAL--Temperature remains Temperature remains constantconstant

ISOBARIC - ISOBARIC - Pressure remains Pressure remains constantconstant

ISOMETRICISOMETRIC - - Volume remains Volume remains constantconstant (also (also ISOVOLUMETRICISOVOLUMETRIC

or or ISOCHORICISOCHORIC))

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

Adiabatic Expansion of a Adiabatic Expansion of a Ideal GasIdeal 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.

Adiabatic Expansion of a Adiabatic Expansion of a Ideal GasIdeal Gas

Blowing air through wide open mouth results to warm air. Blowing through small opening results to cooler air due to adiabatic expansion.

Both adiabatic expansion and compression of gases occur in only hundredths of a second in the cylinders of a car’s engine.

Compresses air leaking out through a small opening also results in adiabatic cooling.

PROCESS DIAGRAMS: visualize processes using properties (T, P, V, etc.)

Area underneath the slope represents the amount of work done (P x V).

CYCLECYCLE: : a system undergoes a system undergoes processes - returning to its processes - returning to its

initial stateinitial state

Area underneath the slope represents the amount of work done (P x V).

Refrigerators work by taking heat from Refrigerators work by taking heat from the interior and depositing it on the the interior and depositing it on the exteriorexterior

The compressor raises the pressure and The compressor raises the pressure and temperature of the refrigerant (freon or temperature of the refrigerant (freon or ammonia) while the coils OUTSIDE the ammonia) while the coils OUTSIDE the refrigerator allow the now hot refrigerator allow the now hot refrigerant to dissipate the heatrefrigerant to dissipate the heat

The warm refrigerant flows through an The warm refrigerant flows through an expansion valve from a high-pressure to expansion valve from a high-pressure to a low-pressure zone, so it expands and a low-pressure zone, so it expands and evaporatesevaporates

• The coils INSIDE the The coils INSIDE the refrigerator allow the refrigerator allow the cold refrigerant to cold refrigerant to absorb heat, cooling absorb heat, cooling the interiorthe interior• The cool refrigerant The cool refrigerant flows back to the flows back to the compressor, and the compressor, and the cycle repeatscycle repeats

Second Law of Thermodynamics

Heat flows naturally from a region at high temperature to a region at low temperature.  By itself, heat will not flow from a cold to a hot body. When an isolated system undergoes a change, passing from one state to another, it will do so in such a way that its entropy (disorder) will increase, or at best remain the same.

ENTROPYENTROPY

Can you beat the Can you beat the Second Law?Second Law?

So, can you cool your kitchen by So, can you cool your kitchen by leaving the refrigerator door openleaving the refrigerator door open

NO!NO! The heat removed from the The heat removed from the

interior of the refrigerator is interior of the refrigerator is deposited back into the kitchen deposited back into the kitchen by the coils on the back!by the coils on the back!

And to make matters worse, the Second Law of And to make matters worse, the Second Law of Thermodynamics says that work is needed to Thermodynamics says that work is needed to move the heat from cold to hot, so the actual move the heat from cold to hot, so the actual amount of heat added to the kitchen is MORE amount of heat added to the kitchen is MORE than the amount removed from the refrigeratorthan the amount removed from the refrigerator

Hopefully, you understand today’s Hopefully, you understand today’s lesson. Otherwise, you’ll end up lesson. Otherwise, you’ll end up

like this cow.like this cow.