Heat Engines-Laws Thermo

8/3/2019 Heat Engines-Laws Thermo

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Heat Engines and theLaws of Thermodynamics



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Heat Engine: A device that converts heat intomechanical energy

Heat Engine: A device that converts heat into

mechanical energy

Example: Automobile (Car) Engine

Example: Automobile (Car) Engine

(Heat) Engines and theLaws of Thermodynamics

(Heat) Engines and theLaws of Thermodynamics

Movie Clip: Animation Car Engine

Movie Clip: Animation Car Engine
http://opt/scribd/conversion/tmp/scratch6355/Car%20engine_%20main%20structure%20components%20-%203D%20animation.mp4http://opt/scribd/conversion/tmp/scratch6355/Car%20engine_%20main%20structure%20components%20-%203D%20animation.mp4http://opt/scribd/conversion/tmp/scratch6355/Car%20engine_%20main%20structure%20components%20-%203D%20animation.mp4


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ThermodynamicsThermodyn

amics

ThermodynamicsThermodyn

amics

Thermodynamics = study of heat and motion

Thermodynamics = study of heat and motion

Thermo = Heat

Thermo = Heat

dynamics = motion

dynamics = motion

+

+




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First Law of Thermodynamics First Law ofThermodynamics First Law of Thermodynamics First Law ofThermodynamics

Energy, even thermal energy, is conserved.Energy, even thermal energy, is conserved.

OR, you can't get something from nothingOR, you can't get something from nothing

When you heat a gas you can:When you heat a gas you can:

Do work

(i.e. move something)

Do work

(i.e. move something)OROR

Increase the internal

energy (temperature)

of the gas



of the gas

AnimationAnimation
http://phet.colorado.edu/en/simulation/gas-propertieshttp://phet.colorado.edu/en/simulation/gas-propertieshttp://phet.colorado.edu/en/simulation/gas-properties


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First Law of Thermodynamics First Law ofThermodynamics First Law of Thermodynamics First Law ofThermodynamics

Heat added to a gas = Q:Heat added to a gas = Q:

Do work = W(i.e. move something)

Do work = W(i.e. move something) OR(+)

OR(+)

Increase the internalenergy (temperature)

of the gas = U



of the gas = U

Q = W + UQ = W +

U

(Remember: = change in = final initial, so U = Uf- Ui)(Remember: = change in = final initial, so U = Uf- Ui)


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Detour: Pressure Detour: Pressure Detour: Pressure Detour: Pressure

Pressure=ForceArea

P=FA

Area

= PressureForce

i


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Pressure=Force

Area P=F

A

Units for Pressure:

SI unit is the Pascal [Pa]

Units for Pressure:

SI unit is the Pascal [Pa]

P=FA1 [Pascal]=1 [Newton][1 meter2] or [Pa]=

[N][m2]

other units:

1.0 [atmosphere] = 1.01325 x 105 [Pa]

1.0 [pound per square inch] = 1.0 [psi] = 6.895 x 103 [Pa]

H t E i d th


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Work=Pressure Change in Volume W=PV=PVfVi

Remember: W=K=1

2 mv f2

1

2 mv i2

Work=Force distance W=FdWork=Force Area Area distance W=Fd

H t E i d th


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W=K=KfKi=12 mv f212 mv i2

Example: Pressure inside a Car Engine Piston

Let's calculate the work done on a car going from0 mph to 60 mph (~27 m/s)

Assume a car mass of 850 kg

W= 12(850 kg)(27 m /s)2 1

2(850 kg)(0 m /s)2=3.1105 [J]

Volume change in piston: 8.7 x 10-4 m3

W=PVP= WV= (3.1105 [J])

(8.7104 [m3 ])=4.02108 [Pa ]


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First Law of

Thermodynamics

First Law of

Thermodynamics

is generally expressed as:

(Heat added) = (Work Done) +(Change in Thermal Energy)

is generally expressed as:

(Heat added) = (Work Done) +

(Change in Thermal Energy)

is really just energy conservation:

Initial Energy = Final Energy

is really just energy conservation:

Initial Energy = Final Energy


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Second Law of ThermodynamicsSecond Law of Thermodynamics

Without external action, Heat flows from Hot to

Cold.

Without external action, Heat flows from Hot to

Cold.

Which is more disorganized (disorderly):

a solid, a liquid, or a gas?

Which is more disorganized (disorderly):

a solid, a liquid, or a gas?

animationanimation

Most organized

SolidsLiquids

Gases

Least organized

Most organized

SolidsLiquids

Gases

Least organized
http://phet.colorado.edu/en/simulation/states-of-matterhttp://phet.colorado.edu/en/simulation/states-of-matterhttp://phet.colorado.edu/en/simulation/states-of-matter


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Second Law of ThermodynamicsSecond Law of Thermodynamics

Hot systems are more disorderly (disorganized) than

cold systems.

Hot systems are more disorderly (disorganized) than

cold systems.

Which is hotter: a solid, a liquid, or a gas?Which is hotter: a solid, a liquid, or a gas?

Lowest Temperature

Solids

Liquids

Gases

Highest Temperature

Lowest Temperature

Solids

Liquids

Gases

Highest Temperature

Most organized

Solids

Liquids

Gases

Least organized

Most organized

Solids

Liquids

Gases

Least organized


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Second Law of Thermodynamics

Without external action, Cold things become hotWithout external action, Cold things become hot

Hot systems are more disorderly than cold systems.Hot systems are more disorderly than cold systems.

=> Without external action, ordered things tend

toward disorder

=> Without external action, ordered things tendtoward disorder

Entropy = amount of disorder in a systemEntropy = amount of disorder in a system

=> Without external action, entropy increases.=> Without external action, entropy increases.


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Second Law of Thermodynamics

Consider a machine that converts heat to work:Consider a machine that converts heat to work:

=> It is impossible for heat to be completely converted

into useful work.

=> It is impossible for heat to be completely converted

into useful work.

========> VIOLATES 2nd LAW VIOLATES 2nd LAW


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Heat EnginesHeat Engines

waste heatwaste heat

HEAT Work

=> Some heat must be expelled from the engine=> Some heat must be expelled from the engine


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Heat EnginesHeat Engines

waste heat

waste heat

THi

Work

TLow

efficiency=1 THiTLow

Animated EnginesAnimated Engines
http://www.animatedengines.com/otto.shtmlhttp://www.animatedengines.com/otto.shtmlhttp://www.animatedengines.com/otto.shtml


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Heat Engines Heat Engines

THi

Work

TLow

efficiency=1 THiTLow

TLow

0 => efficiency increases

TLow

= 0 => infinite efficiency!!!

=> Third law of Thermodynamics:

You cant get to T = 0 K

=> Third law of Thermodynamics:

You cant get to T = 0 K


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Heat EngineHeat Engine

THi

Work

TLow

RefrigeratorRefrigerator


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PV GraphsPV Graphs

F

d

W=FdP

V

W=P VW=Fd Work is the area under the curve of a F(d) graph

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Heat Engines-Laws Thermo