Chapter 15Chapter 15

ThermodynamicsThermodynamicsThermodynamicsThermodynamics The name we give to the study of The name we give to the study of

processes in which energy is processes in which energy is transferred as transferred as heatheat and as and as workwork

There are 4 Laws of There are 4 Laws of Thermodynamics: 0, 1, 2, 3Thermodynamics: 0, 1, 2, 3

1 & 2 have the most meaning and 1 & 2 have the most meaning and will be studied the mostwill be studied the most

Heat and Work-Heat and Work-Both forms of Energy (J)Both forms of Energy (J)

Heat and Work-Heat and Work-Both forms of Energy (J)Both forms of Energy (J)

HeatHeat- a transfer of energy due to a - a transfer of energy due to a difference in temperaturedifference in temperature

WorkWork- a transfer of energy NOT due to - a transfer of energy NOT due to temperature differencetemperature difference

SystemSystem- object on set of objects we wish - object on set of objects we wish to considerto consider

EnvironmentEnvironment- everything else NOT in our - everything else NOT in our systemsystem

Closed SystemClosed System- no mass enter or leaves - no mass enter or leaves our system (energy may be exchanged w/ our system (energy may be exchanged w/ environment)environment)

Open SystemOpen System- mass may enter - mass may enter or leave (also energy)or leave (also energy)

Isolated SystemIsolated System- No energy of - No energy of any form leaves the systemany form leaves the system

Continued…Continued…

11stst Law of Thermodynamics Law of Thermodynamics

ΔU = Q – W ΔU = Q – W – Derived from the law of Derived from the law of

conservation of energyconservation of energyU= internal energy of a system (J)U= internal energy of a system (J)Q = heat (J)Q = heat (J)W = Net Work (J)W = Net Work (J)

We would expect internal energy to We would expect internal energy to INCREASEINCREASE if work were done or if heat if work were done or if heat were added.were added.

Likewise internal energy Likewise internal energy DECREASESDECREASES if if heat flows out or if work were done by heat flows out or if work were done by the systemthe system– Signs +/-Signs +/-

+Q heat added+Q heat added-Q heat lost-Q heat lost-W work -W work ONON system system+W Work +W Work BYBY system system

ΔU = Q – W applies for a ΔU = Q – W applies for a closedclosed system, it also system, it also applies to an applies to an openopen system if system if we take into account the we take into account the change in internal energy change in internal energy due to the increase or due to the increase or decrease in the amount of decrease in the amount of mattermatter

For an isolated system, No work For an isolated system, No work is done and NO heat enters or is done and NO heat enters or leaves, leaves, W = Q = O W = Q = O therefore therefore ΔU = OΔU = O

Properties of a system are U, P, Properties of a system are U, P, V, T, mass, n(# of moles)…V, T, mass, n(# of moles)…Work Work and Heat are NOT properties of a and Heat are NOT properties of a systemsystem but they are involved in a but they are involved in a system changesystem change!!

ExampleExampleAn amount of heat equal to An amount of heat equal to

2500J is added to a system, and 2500J is added to a system, and 1800J of work is done on the 1800J of work is done on the system, what is the change in system, what is the change in internal energy of the system?internal energy of the system?

Q= 2500JQ= 2500J

Work on System= -1800JWork on System= -1800J

1800 is done on the system: (-)1800 is done on the system: (-)

Q= 2500JQ= 2500J

Work on System= -1800JWork on System= -1800J

1800 is done on the system: (-)1800 is done on the system: (-)

ΔU = 2500J - (-1800J)ΔU = 2500J - (-1800J)

ΔU = 4300JΔU = 4300J

ΔU = 2500J - (-1800J)ΔU = 2500J - (-1800J)

ΔU = 4300JΔU = 4300J

It seems intuitive to add them It seems intuitive to add them but keep up with signs because but keep up with signs because

not every problem will be not every problem will be intuitiveintuitive

It seems intuitive to add them It seems intuitive to add them but keep up with signs because but keep up with signs because

not every problem will be not every problem will be intuitiveintuitive

ExampleExample

What would be the internal What would be the internal energy change if 2500J of energy change if 2500J of heat is added to the heat is added to the system and 1800J of work system and 1800J of work is done by the system.is done by the system.

What would be the internal What would be the internal energy change if 2500J of energy change if 2500J of heat is added to the heat is added to the system and 1800J of work system and 1800J of work is done by the system.is done by the system.

Heat is being added so Q = +2500J. Heat is being added so Q = +2500J. but now work is being done by the but now work is being done by the

systemsystem

Heat is being added so Q = +2500J. Heat is being added so Q = +2500J. but now work is being done by the but now work is being done by the

systemsystem

ΔU = 2500J – 1800J =ΔU = 2500J – 1800J =

700J700J

ΔU = 2500J – 1800J =ΔU = 2500J – 1800J =

700J700J

ΔU is much less here because ΔU is much less here because work was output.work was output.

READ PAGE 445- 447READ PAGE 445- 447

Simple SystemsSimple Systems

Prefix – ISO (same)Prefix – ISO (same) Isothermal- same temperatureIsothermal- same temperature Isobaric- same pressureIsobaric- same pressure Isochoric- same volume (isovolumetric)Isochoric- same volume (isovolumetric)

Pressure volume Pressure volume diagrams…diagrams…

Pressure volume Pressure volume diagrams…diagrams…

IsothermalIsothermal

PP

VV

A’A’

AA

BB

B’B’A’- B’ LowerA’- B’ Lower

Constant TempConstant Temp

Curve is called an Curve is called an isothermisotherm

A’- B’ LowerA’- B’ Lower

Constant TempConstant Temp

Curve is called an Curve is called an isothermisotherm

IsobaricIsobaric

PP

VV

IsochoricIsochoric

V

P

11stst Law is seen Law is seen

T = constantT = constant

ΔU = OΔU = O

Q= WQ= W

P= constantP= constant

W = PΔVW = PΔV

V= constant V= constant

W= OW= O

Adiabatic process is one which NO heat Adiabatic process is one which NO heat is allowed to flow into or out of the is allowed to flow into or out of the system Q=0system Q=0

1.1. Happens fast, no time to transfer (I.e. Happens fast, no time to transfer (I.e. engine)engine)

2.2. Happens in a very well insulated Happens in a very well insulated containercontainer

• Slow adiabatic expansion follows curve Slow adiabatic expansion follows curve AC since Q=0AC since Q=0

AB isothermalAB isothermal

AC adiabaticAC adiabatic

AA

BB

CC

ΔU = -W U decreases as gas ΔU = -W U decreases as gas expands therefore temperature expands therefore temperature decreases as welldecreases as well

Adiabatic compression follows curve Adiabatic compression follows curve AC , work is done ON gas, as U AC , work is done ON gas, as U increases therefore temperature increases therefore temperature increasesincreases

In diesel engines air/fuel is compressed so fast In diesel engines air/fuel is compressed so fast that fuel explodes spontaneously due to increase that fuel explodes spontaneously due to increase in U and Tin U and T

F = PA F = PA W = FdW = FdW = PadW = PadW= PW= PΔVΔV

Concepts pg. 447Concepts pg. 447Read Example 15-4 pg. 448Read Example 15-4 pg. 448

area x dist. = volumearea x dist. = volume

In BDA only 1 leg does work, BDIn BDA only 1 leg does work, BDA.A. W = PW = PΔV (2x10ΔV (2x105 5 n/mn/m22)(2x10)(2x10-3-3mm33 - 10x10 - 10x10-3-3mm33) )

W= -1.6 x 10W= -1.6 x 1033JJ negative means work negative means work done on the gasdone on the gas

B.B. There is NO temperature change from B to There is NO temperature change from B to A (along isotherm) so there is NO change in A (along isotherm) so there is NO change in internal energy ΔU = 0 internal energy ΔU = 0 U = Q-W U = Q-W 0 = Q-W 0 = Q-W Q= W Q= W Q= 1.6 x 10Q= 1.6 x 1033 J J

1600J flows out of the gas, total heat loss for 1600J flows out of the gas, total heat loss for process BDAprocess BDA

In BDA only 1 leg does work, BDIn BDA only 1 leg does work, BDA.A. W = PW = PΔV (2x10ΔV (2x105 5 n/mn/m22)(2x10)(2x10-3-3mm33 - 10x10 - 10x10-3-3mm33) )

W= -1.6 x 10W= -1.6 x 1033JJ negative means work negative means work done on the gasdone on the gas

B.B. There is NO temperature change from B to There is NO temperature change from B to A (along isotherm) so there is NO change in A (along isotherm) so there is NO change in internal energy ΔU = 0 internal energy ΔU = 0 U = Q-W U = Q-W 0 = Q-W 0 = Q-W Q= W Q= W Q= 1.6 x 10Q= 1.6 x 1033 J J

1600J flows out of the gas, total heat loss for 1600J flows out of the gas, total heat loss for process BDAprocess BDA

ProblemsProblems

Example problems Example problems 15-5, 15-6 (pg. 448)15-5, 15-6 (pg. 448)

Class Work pg. 471 - 472 Class Work pg. 471 - 472 #’s 1, 3, 5, 7#’s 1, 3, 5, 7

Homework pg. 471- 472 Homework pg. 471- 472 #’s 2, 4, 6, 8 #’s 2, 4, 6, 8

1.1. ΔU = Q-WΔU = Q-W= -3.42 x 10= -3.42 x 1033J - (-1.6 x 10J - (-1.6 x 1033J) = -1.8x 10J) = -1.8x 1033JJ

00 11 22

.5.5

1.01.03.3. AA

CC

BB

P P (Atm)(Atm)

V(L)V(L)

5.5.AA

BB

CC

Vo/ 2Vo/ 2 VoVo00

Po/ 2Po/ 2

PoPo

VV

PP

77

AA Adiabatic ProcessAdiabatic Process

No Heat FlowNo Heat Flow

Q=0Q=0

BB 11stst Law Law ΔU = Q-WΔU = Q-W

= 0 - (-1350)= 0 - (-1350)

= +1350J= +1350J

CC Internal Energy of ideal gas depends Internal Energy of ideal gas depends only on the temperature, U= 3/2nRTonly on the temperature, U= 3/2nRT

Increase in U means…Increase in U means…INCREASE IN TINCREASE IN T

22ndnd Law of Thermodynamics Law of ThermodynamicsSpeaks to the direction of heat flowSpeaks to the direction of heat flowThere is a natural direction that does There is a natural direction that does

not reverse itselfnot reverse itself– Example: you will never see a broken glass Example: you will never see a broken glass

cup on the floor reassemble itself, cup on the floor reassemble itself, accelerate up to a table top and come to accelerate up to a table top and come to rest. rest.

There is an irreversible direction to There is an irreversible direction to some process.some process.

Just to look at the 1Just to look at the 1stst law of law of thermodynamics there is NO thermodynamics there is NO direction expressed or implied to direction expressed or implied to which way heat flows. which way heat flows.

The second law states directionThe second law states direction““Heat flows naturally from a hot Heat flows naturally from a hot

object to a cold object, heat will NOT object to a cold object, heat will NOT flow spontaneously from a cold flow spontaneously from a cold object to a hot object.” – Clausiusobject to a hot object.” – Clausius

22ndnd law came about partly due to the law came about partly due to the study of heat enginesstudy of heat engines

Heat EnginesHeat EnginesAny device that changes thermal Any device that changes thermal

energy into mechanical work (steam energy into mechanical work (steam engine, automobile engine)engine, automobile engine)– Fig 15-9 pg. 451Fig 15-9 pg. 451– Steam engineSteam engine– reciprocatingreciprocating– TurbineTurbine– Internal combustion engineInternal combustion engine

4 cycle pg. 4524 cycle pg. 452

OverheadOverhead

FIG. 15-6 PV diagram for different FIG. 15-6 PV diagram for different processprocess

FIG 15-9 Energy transfers for a heat FIG 15-9 Energy transfers for a heat engineengine

FIG 15-10 Steam EngineFIG 15-10 Steam EngineFIG 15-11 the 4 cycle internal FIG 15-11 the 4 cycle internal

combustion engine (otto cycle)combustion engine (otto cycle)

Why is a temperature Why is a temperature difference required?difference required?

If the steam were at the same If the steam were at the same temperature on both sides of temperature on both sides of the piston or turbine were the the piston or turbine were the same, that would mean the same, that would mean the pressure was the same on pressure was the same on both sides and then NO work both sides and then NO work would be donewould be done

If the steam were at the same If the steam were at the same temperature on both sides of temperature on both sides of the piston or turbine were the the piston or turbine were the same, that would mean the same, that would mean the pressure was the same on pressure was the same on both sides and then NO work both sides and then NO work would be donewould be done

Efficiency of Heat Engine Efficiency of Heat Engine ((e)e)

e= W/Qe= W/QHH

– W = work output (J) –what you get W = work output (J) –what you get from enginefrom engine

– QQHH = what you put in = what you put in

Since energy is conserved, heat input Since energy is conserved, heat input QQHH must equal work done plus heat must equal work done plus heat

that flows out at the low temp (Qthat flows out at the low temp (QLL))

QQHH = W + Q = W + QLL

W = QW = QHH – Q – QLL

e= W/Qe= W/QHH = Q = QHH - Q - QLL/ Q/ QHH = 1 - Q = 1 - QLL/Q/QHH

** to write as % must still multiply by 100**** to write as % must still multiply by 100**

ExampleExample

A 20% efficient car engine A 20% efficient car engine produces an average of produces an average of 23,000J of mechanical 23,000J of mechanical work per second. How work per second. How

much heat is discharged much heat is discharged per second from this per second from this

engine?engine?

QQLL = ? Q = ? QLL/ Q/ QHH = 1 - e = .80 Q = 1 - e = .80 QLL = .8Q = .8QHH

QQHH = W/e 23000J/.20 = 1.15 x 10 = W/e 23000J/.20 = 1.15 x 1055JJ

QQLL = .80Q = .80QHH = .8(1.15 x 10 = .8(1.15 x 1055J) = 9.2 x 10J) = 9.2 x 1044JJ

=92KW=92KW

QQLL = ? Q = ? QLL/ Q/ QHH = 1 - e = .80 Q = 1 - e = .80 QLL = .8Q = .8QHH

QQHH = W/e 23000J/.20 = 1.15 x 10 = W/e 23000J/.20 = 1.15 x 1055JJ

QQLL = .80Q = .80QHH = .8(1.15 x 10 = .8(1.15 x 1055J) = 9.2 x 10J) = 9.2 x 1044JJ

=92KW=92KW

The Ideal EngineThe Ideal Engineis the Carnot engineis the Carnot engine

One does not really exist. One does not really exist. This concept proves that This concept proves that 100% efficient engines 100% efficient engines cannot exist and cannot exist and efficiencies can be efficiencies can be determined by a ratio of determined by a ratio of temperaturestemperatures

One does not really exist. One does not really exist. This concept proves that This concept proves that 100% efficient engines 100% efficient engines cannot exist and cannot exist and efficiencies can be efficiencies can be determined by a ratio of determined by a ratio of temperaturestemperatures

Carnot (Ideal) Carnot (Ideal) EfficiencyEfficiency

eeidealideal = T = THH - T - TLL/ T/ THH = 1 - T = 1 - TLL/T/THH

Pg. 454 – 455 Pg. 454 – 455

example 15-9, 15-10example 15-9, 15-10

For a 100% efficient engine to For a 100% efficient engine to happen Thappen TLL would need to be would need to be absolute 0 … 0 Kelvinabsolute 0 … 0 Kelvin

100% efficiency is a violation 100% efficiency is a violation of the 3of the 3rdrd law of law of thermodynamics- thermodynamics- “absolute “absolute zero is unattainable”zero is unattainable”

For a 100% efficient engine to For a 100% efficient engine to happen Thappen TLL would need to be would need to be absolute 0 … 0 Kelvinabsolute 0 … 0 Kelvin

100% efficiency is a violation 100% efficiency is a violation of the 3of the 3rdrd law of law of thermodynamics- thermodynamics- “absolute “absolute zero is unattainable”zero is unattainable”

OverheadOverhead

Fig. 15-12 The Carnot CycleFig. 15-12 The Carnot Cycle

22ndnd Law Restrained Law Restrained Kelvin- Plank StatementKelvin- Plank Statement No device is possible No device is possible

whose sole effect is to whose sole effect is to transform a given amount transform a given amount of heat completely into of heat completely into workwork

Refrigerators, Air Refrigerators, Air Conditioners & Heat PumpsConditioners & Heat Pumps

The above three operate on the same The above three operate on the same principle just in the reverse direction as a principle just in the reverse direction as a heat engine. heat engine.

They transfer heat out of an environment to They transfer heat out of an environment to make it coldmake it cold

By doing work, heat is taken from a low By doing work, heat is taken from a low temp, Ttemp, TLL (inside the fridge) and a greater (inside the fridge) and a greater amount of heat is exhausted at a high temp, amount of heat is exhausted at a high temp, TTHH (the room), you can feel this warmth and (the room), you can feel this warmth and that’s why behind the fridge is so warm.that’s why behind the fridge is so warm.

The work (W) is normally done by a The work (W) is normally done by a compressor.compressor.

A perfect refrigerator is NOT possible A perfect refrigerator is NOT possible – See Clausius statement of 2See Clausius statement of 2ndnd Law Law

Heat Pump: these can heat a house in Heat Pump: these can heat a house in winter (Qwinter (QLL outside to Q outside to QHH inside) by doing inside) by doing

work. True heat pumps can be turned work. True heat pumps can be turned around and used to cool a house in the around and used to cool a house in the summersummer

OVERHEADOVERHEAD

FIG. 15-15FIG. 15-15

Typical Refrigerator systemTypical Refrigerator system

FIG. 15-15FIG. 15-15

Typical Refrigerator systemTypical Refrigerator system

EntropyEntropy This is the term that defines the This is the term that defines the

general statement of 2general statement of 2ndnd Law of Law of Thermodynamics.Thermodynamics.

Entropy is a Entropy is a function of the state function of the state of a systemof a system

Entropy is a measure of the Entropy is a measure of the disorder or order of a systemdisorder or order of a system

ΔS = Q/T T in KelvinΔS = Q/T T in Kelvin

The total entropy of any system The total entropy of any system plus that of its environment plus that of its environment increases as a result of any natural increases as a result of any natural process. (2process. (2ndnd Law, General Law, General Statement)Statement)

This is quite different that the other This is quite different that the other laws of physics. Most are equalities:laws of physics. Most are equalities:– V = d/t p= mv PE = mghV = d/t p= mv PE = mgh

Entropy is Entropy is NOTNOT conserved it conserved it ALWAYSALWAYS increasesincreases

Phase Changes and EntropyPhase Changes and Entropy

During a phase change, if energy flows out (water freezes) entropy decreases.

Even though the Waters entropy decreases, the entropy of the universe has still increased. (think about the electrical energy used to cool the water)

Clockwise and Counter Clockwise and Counter clockwiseclockwise

On a PV graph, when a complete cycle is shown, Clockwise means that heat is removed from the gas (substance).

Counterclockwise would indicate heat is added to the gas (substance).

Natural Processes tend to Natural Processes tend to move toward a state of move toward a state of

greater disorder.greater disorder.ORDERORDER DISORDERDISORDER

PepperPepper

SaltSalt Shake Shake

it up!it up!

FallsFalls

CRASH!CRASH!

With heat it goes like this…With heat it goes like this…

High heatHigh heat

time time same heatsame heat

same average KEsame average KE

Low HeatLow Heat

Order DisorderOrder Disorder

(could do work) (cant do (could do work) (cant do work)work)

Disorder can be associated with Disorder can be associated with randomnessrandomness

Order associates with Order associates with informationinformation

Heat death Energy ResourcesHeat death Energy Resources