physics 105 physics for decision makers: the global energy crisis

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Physics 105 Physics for Decision Makers:The Global Energy CrisisLecture 8 Thermodynamics IIFall 2011

Physics 105 Fall 111Energy AuditEnergy Audit Project: Download assignment info sheet from ElmsGroup project When you meet -> Each person should have a role one person needs to be responsible for organization.Project Due Dates:Assignment #1 - Sept 30 only one person needs to turn something in, but the whole group needs to be involvedAssignment #2 - Oct 7Assignment #3 - Oct 21Assignment #4 - Oct 28Assignment #5 (Presentations in Discussion) Oct 29 - Nov 1. Page 2Physics 105 Fall 11Food Supply and ClimateCNN today - Why is 'food security' sparking unrestRussian Drought wiped out 25% of their wheat cropThis caused them to stop grain exportsBread prices went up dramatically 30% in some countriesRiots ensued in MozambiqueDecisions we make about usingbiofuels can affect people around the world Page 3

Physics 105 Fall 11Activities: Guided Tours Thermodynamics:

What is Thermodynamics?Physics 105 Fall 11 Page 5

Physics 105 Fall 11The Zeroth Law of ThermodynamicsTemperature - if two objects are in thermal equilibrium with a third object (like a thermometer) then they are in thermal equilibrium with each otherAnother way of saying it is that temperature is a measurable quantity and it tells us about the energy content of an objectthis law asserts that we can define a temperature function, or more informally, that we can 'construct a thermometer'

Physics 105 Fall 11Thermal Equilibrium if Q=0 then we are in thermal equilibrium

TA = TB ABQPhysics 105 Fall 11Heat TransferThree methodsConductionTransfer Of Energy Through MatterAir Is A Poor Conductor - Metal is a good oneOnly important at the earth's surface

ConvectionTransfer of energy by movement of massCan only take place in liquids & gases - e.g. Air Convection on a global scale creates worldwide atmospheric circulationRadiationProportional to the 4th power of the temperatureHow we get energy from the sunWhy it gets cold on a clear night

Physics 105 Fall 118

Fig. 2-2, p. 30Physics 105 Fall 119

Physics 105 Fall 1110Radiation

Physics 105 Fall 1111The First Law of Thermodynamics Energy ConservationMany statements:Energy is conservedHeat is a form of energyThe energy of an isolated system (e.g. the universe) is constantEnergy is conserved during any change in state.Specifically:Heat absorbed by a system + work done on the system = change in internal energy of the systemMathematically:Q+W=DUQ is heat, W is work and U is internal energy

Physics 105 Fall 1112The internal energy of a system

does NOT depend on which of the following:The temperature of the systemThe amount of material in the systemThe type of materialThe amount heat that has been put into the systemIt depends on all of themPhysics 105 Fall 11First Law - energy is conservedWhere did energy to power the light come from?

Physics 105 Fall 11Heat CapacityThe specific heat is the amount of heat per unit mass required to raise the temperature by one degree Celsius (or Kelvin)Heat Capacity = Q/THeat Capacity of a lake depends on the size of the lake (extrinsic) Specific Heat = Heat Capacity/kg - property of the water (intrinsic)

Physics 105 Fall 1115Latent HeatLatent Heat of vaporizationIf we add heat to water - its temperature goes up1 Kcal - will raise 1kg of water 1oCUntil water hits 100oC - then the temperature stops going upWhat happens?It vaporizes (boils). It takes 539 kcal to boil 1kg of waterEventually once its all steam the temp will start going up again 539 kcal/kg is the latent heat of vaporization

Physics 105 Fall 1116Latent HeatLatent heat of fusionTake 1kg of ice at -20oC and add heat at a rate of 1kcal/minThe Ice will warm up at 2oC/min - (why 2)?Once it hits 0oC - (after 10 min) the ice will start to meltIt will take 80 minutes to melt the iceSo the latent heat of fusion of ice is 80kcal/kg

Physics 105 Fall 1117Does the 1st law prohibit a lake from freezing on a warm day?YesNoDoesnt applyPhysics 105 Fall 1118The 2nd Law of ThermodynamicsMany formulations:It is impossible to convert heat completely into work. No perfect engineCant just pull heat out of the environmentHeat cannot spontaneously flow from a material at lower temperature to a material at higher temperature.No perfect refrigeratorIn an isolated system, a process can occur only if it increases the total entropy of the system.

Physics 105 Fall 1119The Perfect Heat EngineTakes heat out of theenvironment and turns it into work.The solution to all of ourproblemsreduce global warmingSolve the energy problem 2nd Law

T1Q1WoutPhysics 105 Fall 1120Perfect RefrigeratorT1 HotT2 ColdQPhysics 105 Fall 1121RestatementZeroth Law Lunch exists (temperature)First Law TANSTAAFL (energy conserved)Second Law (heat flows from hot to cold)Lunch will be expensive

Physics 105 Fall 1122EntropyToss a penny N times

What is probability that you get Heads every time?

ans. (1/2)(1/2)(1/2)(1/2). = (1/2)N

if N =2, p = 0.25if N= 10, p = .001if N= 100, p = 8 X 10-31if N = 104 , p = 5 X 10-3011 !!!!

We expect about N/2 heads

What is probability that we get within 1% of N/2?

if N = 10, p = 0.25if N=1000, p = .248if N = 105, p = .998if N = 108, p = 1 - 3 X 10-2174 !!!!

Physics 105 Fall 11I flip a coin 10 times - the first 9 come up heads..

What is the probability that the next toss is also heads?

.0011 - 9/10 = 0.1(9/10)(1/2) = 0.450.59/10 = 0.9Physics 105 Fall 11Entropy - microstatesWhy is all heads so rare and ~50/50 so common?

Lets call each possible outcome a microstateThere is only one microstate that is all headsHHHHHHHHHHThere are many microstates that are ~ 50/50HHHHHTTTTTHHHHTHTTTTHHHTHHTTTTHHTHHHTTTTTTTTTHHHHHTHTHTHTHTHEach microstate is equally likely.Physics 105 Fall 11Entropy and Particles in a boxParticles start all on leftWhat happens?What are the odds that later we findall of them on the left?Just like the coin tossing!

After mixing, chance of all on the left is (1/2)N

It would take work push all the molecules back to the left sidePhysics 105 Fall 1126Entropy and the macrostateWe can define macroscopic propertiesmore-or-less uniform distribution between right and leftWhen a physical system is allowed to evolve in isolation, some single macroscopic outcome is overwhelmingly more probable than any otherSecond Law: If a system of many particles is permitted to change, it will evolve to the macrostate made of the largest number of microstates, and stay there.Physics 105 Fall 11EntropyWe define a quantity called entropy

Entropy = S = kB ln (no. of microstates)

With this definition, it can be shown that:

Sfinal - Sinitial = energy input from heating/T = Q/TSecond Law: S > Q/T-Physics 105 Fall 11Definition of EntropyChange in entropy S=DQ/TTake an example -

Q=30JScold=+30J/283oK= +0.106Swarm=-30J/333ok= -0.09Stotal= +.07J/okEntropy of the entire systemincreases

10o60oQ=30JPhysics 105 Fall 1129The 2nd Law of ThermodynamicsMost systems are microscopically reversibleParticles in a boxThe 2nd Law tells us the direction of time

Physics 105 Fall 11Reversible and Irreversible Processes








Physics 105 Fall 1138Heat EnginesHeat engines take heat from a hot reservoir and does work and expels heat to the cold reservoirNote that the first law saysQ1=W+Q2 The 2nd law tells us the amount of work we can get from a temperature differenceEfficiency = (work output)/(energy in)

Physics 105 Fall 1139Carnot EngineThe Carnot Engine is an idealized engine that works in a reversible wayWhat is a reversible engine?A refrigeratorBy adding work we can take heat from the cold reservoir and deposits it to the hot reservoirAgain - 1st law worksW+Q1 = Q2 Notice more heat is delivered than work done!

Physics 105 Fall 1140Heat PumpCools in summerHeats in winter

What is temperature of the air blowing out in summer and winter?Physics 105 Fall 11Heat Pump Page 42Winter

SummerOutsideOutsidePhysics 105 Fall 11Carnot EfficiencyThe efficiency (work/energy in) of a Carnot Engine T1 -T2)/T1When is the efficiency high?When T2 is lowExampleT1= 500oC = 773KT2= 0oC = 273KT1 -T2)/T1= 500/773= 65%This says W=65% Q2= 35%So if we take 100J from T1 we get 65J of workRedo if T1 is 100oC = 373kT1 -T2)/T1= 100/373= 26% so our 100J of energy only gives us 26J of work

Physics 105 Fall 1143Proof that no engine can have efficiency greater than CarnotAssume Engine 1 is higher efficiency than Carnot Operating between T1 & T2 it produces W1.W1 is greater than Wc would produce for the same Q1. (i.e. Q2 is less)Since the Carnot is reversible run it backwards powered by W1. This will pump more heat out of the cold reservoir than engine 1 put in as Qc >Q2 Net result - a perfect refrigerator - which violates the 2nd law since we are moving heat from cold to hot with no external work (lake freezing)

11Physics 105 Fall 1144What is the net effect of putting a refrigerator in a room opening the door an turning it on?

The room cools down a littleThe room cools down a lotThe room heats up a littleThe room heats up a lotPhysics 105 Fall 1145System PropertiesExtensive quantities (depend on amount of material)U = Internal energy V = Volume Ni = # of Moles Heat Capacity

The intensive quantities (do NOT depend on amount of material)PressureTemperatureSpecific HeatPhysics 105 Fall 1146Real Heat EnginesReal heat engines always are less efficient than Carnot enginesIn all heat engines there is waste heat from the 2nd lawElectric motors are not governed by the 2nd although generally the production of electricity isIn real engines there is additional waste heatThe Carnot engine gives us a goal

Physics 105 Fall 1147Real EngineT1T2Q1Q2WoutQwastePhysics 105 Fall 1148Car Enginesefficiency of about 25% The efficiency may be as high as 37% at the optimum operating point. Most internal combustion engines waste about 35% of the energy in gasoline as heat lost to the cooling system and another 35% through the exhaust. The rest, about 5%, is lost to friction

Physics 105 Fall 1149Car Engine

Physics 105 Fall 1150Does it cost you fuel to use your car heater while driving?YesNoPhysics 105 Fall 11

Q1Work outQ2 outQWastePhysics 105 Fall 1152Real Engine heat recoveryT1T2Q1Q2WoutQwasteT3T2Q3Q2WoutQwastePhysics 105 Fall 1153Its possible to reduce the entropy of an object? TrueFalsePhysics 105 Fall 11

physics 105 physics for decision makers: the global energy crisis

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