ThermodynamicsThermodynamicsThermodynamicsThermodynamics

Mechanical Equivalent of HeatMechanical Equivalent of HeatMechanical Equivalent of HeatMechanical Equivalent of Heat

Heat produced by other forms of energyHeat produced by other forms of energy Internal EnergyInternal Energy: total available potential & : total available potential &

kinetic energy of particleskinetic energy of particlesAdding heat increases internal energyAdding heat increases internal energyJoule’s experiment with the paddles Joule’s experiment with the paddles

stirring the water proved heat is form of stirring the water proved heat is form of energyenergy

Expansion and WorkExpansion and WorkExpansion and WorkExpansion and Work

Many thermodynamic processes involve Many thermodynamic processes involve expansion or compression of gasesexpansion or compression of gases

Expanding gases exert pressure Expanding gases exert pressure (force/area) and can do work (car engine)(force/area) and can do work (car engine)

Work done equals pressure times volume Work done equals pressure times volume change for constant pressurechange for constant pressure

For expansion with pressure change, work For expansion with pressure change, work equals area under curve of pressure vs. equals area under curve of pressure vs. volume graphvolume graph

First Law of ThermodynamicsFirst Law of ThermodynamicsFirst Law of ThermodynamicsFirst Law of Thermodynamics

Heat energy supplied to closed system Heat energy supplied to closed system equals work done by system plus change equals work done by system plus change in internal energy of system.in internal energy of system.

Q = Q = U + WU + W1st Law is actually conservation of energy 1st Law is actually conservation of energy

restated to include heat energyrestated to include heat energy If no work is done by system, heat added If no work is done by system, heat added

equals change in internal energyequals change in internal energy

Adiabatic ProcessesAdiabatic ProcessesAdiabatic ProcessesAdiabatic Processes

A process where no heat is added or A process where no heat is added or allowed to escapeallowed to escape

Process must happen very quickly or be Process must happen very quickly or be well insulatedwell insulated

Adiabatic compression of gas causes Adiabatic compression of gas causes temp. increasetemp. increase

Adiabatic expansion of gas causes coolingAdiabatic expansion of gas causes cooling

Isothermal ProcessesIsothermal ProcessesIsothermal ProcessesIsothermal Processes

No temperature change occursNo temperature change occurs Isothermal expansion requires heat input Isothermal expansion requires heat input

from surroundingsfrom surroundings Isothermal compression requires heat Isothermal compression requires heat

emission to surroundingsemission to surroundings

Specific Heats of GasesSpecific Heats of GasesSpecific Heats of GasesSpecific Heats of Gases

Gases have two specific heats, one for Gases have two specific heats, one for constant volume (constant volume (ccv v )) , and one for , and one for

constant pressure (constant pressure (ccp p ))

((ccv v )) < < ((ccp p )) becausebecause work must be done work must be done

against pressure to change volumeagainst pressure to change volume

Second Law of Second Law of ThermodynamicsThermodynamicsSecond Law of Second Law of

ThermodynamicsThermodynamics It is impossible to convert all heat energy It is impossible to convert all heat energy

into useful workinto useful work Device that uses heat to do work can never Device that uses heat to do work can never

be 100% efficientbe 100% efficient Some heat will remain and must be expelled Some heat will remain and must be expelled

into low temp. sinkinto low temp. sink Heat will never flow from a cold object to a Heat will never flow from a cold object to a

hot object by itself without work inputhot object by itself without work input Absolute zero is unattainableAbsolute zero is unattainable

Heat EnginesHeat EnginesHeat EnginesHeat EnginesAny device that turns heat into mechanical Any device that turns heat into mechanical

energyenergyAnything that burns fuel or uses steam to Anything that burns fuel or uses steam to

move or do workmove or do work Ideal heat engine cycle: Ideal heat engine cycle:

takes heat from high temp. source, takes heat from high temp. source, does work using part of the energy, does work using part of the energy, expels remaining heat energy into low temp. expels remaining heat energy into low temp.

heat sink.heat sink.

Ideal Heat Engine DiagramIdeal Heat Engine Diagram

Efficiency of Heat EnginesEfficiency of Heat EnginesEfficiency of Heat EnginesEfficiency of Heat Engines

Efficiency = work done/heat input Efficiency = work done/heat input Work done = heat energy usedWork done = heat energy used Sadi Carnot (1796 – 1832) showed max. Sadi Carnot (1796 – 1832) showed max.

efficiency for any heat engine = temp. efficiency for any heat engine = temp. difference between hot source and cold difference between hot source and cold sink divided by temp. of source sink divided by temp. of source

eeidealideal = (T = (Thothot - T - Tcold cold )/ T)/ Thothot

For max. efficiency, industry uses high For max. efficiency, industry uses high temp. source, large body of water for sink.temp. source, large body of water for sink.

Types of Heat EnginesTypes of Heat EnginesTypes of Heat EnginesTypes of Heat Engines

Steam Engines: first heat engines; Watt, Steam Engines: first heat engines; Watt, Fulton, Newcomen; external combustionFulton, Newcomen; external combustion

Steam Turbine: uses high pressure steam Steam Turbine: uses high pressure steam to turn wheel with many cupped fan-like to turn wheel with many cupped fan-like bladesblades

Gasoline Engines: internal combustion Gasoline Engines: internal combustion engine; heat from expanding combustion engine; heat from expanding combustion gases drive pistongases drive piston

Types of Heat EnginesTypes of Heat EnginesTypes of Heat EnginesTypes of Heat Engines

Diesel Engines: heat from compression Diesel Engines: heat from compression ignites fuel; efficient, powerful, but heavyignites fuel; efficient, powerful, but heavy

Gas Turbines: air compressed by turbine Gas Turbines: air compressed by turbine forced through combustion chamber; used forced through combustion chamber; used on airplaneson airplanes

Types of Heat EnginesTypes of Heat EnginesTypes of Heat EnginesTypes of Heat Engines

Jet Engines: expanding combustion gases Jet Engines: expanding combustion gases forced out rear of engine; action-reaction & forced out rear of engine; action-reaction & conservation of momentum create forward conservation of momentum create forward thrustthrust

Rockets: like jets, but carries own oxidizer Rockets: like jets, but carries own oxidizer to work outside atmosphere. Thrust to work outside atmosphere. Thrust depends on velocity of exhaust gasesdepends on velocity of exhaust gases

Heat PumpsHeat PumpsHeat PumpsHeat Pumps

Reverse cycle of heat engine; work from Reverse cycle of heat engine; work from electric motor causes heat to flow from electric motor causes heat to flow from cool area to warm areacool area to warm area

Uses easily condensed vapor (freon); Uses easily condensed vapor (freon); motor condenses vapor in compressor; motor condenses vapor in compressor; When allowed to vaporize, it extracts its When allowed to vaporize, it extracts its heat of vaporizationheat of vaporization

Basis for refrigerators, air conditioners Basis for refrigerators, air conditioners

Heat Pump DiagramHeat Pump Diagram

EntropyEntropyEntropyEntropy

A measure of the disorder of a systemA measure of the disorder of a system Is related to the amount of energy that Is related to the amount of energy that

cannot be converted into mechanical workcannot be converted into mechanical workNatural systems tend toward greater Natural systems tend toward greater

disorder (greater entropy)disorder (greater entropy)Controls the direction of timeControls the direction of time

EntropyEntropyEntropyEntropy

Work must be done to decrease entropy of Work must be done to decrease entropy of systemsystem

Heat is disordered energy, increased Heat is disordered energy, increased entropyentropy

Change in entropy of system equals heat Change in entropy of system equals heat added divided by absolute temperatureadded divided by absolute temperature

S = S = Q/T Q/T (J/K)(J/K)

EntropyEntropyEntropyEntropy

Entropy increased in melting, evaporation, Entropy increased in melting, evaporation, organic decayorganic decay

Total energy of universe is constant, but Total energy of universe is constant, but usable energy decreases due to increased usable energy decreases due to increased entropyentropy