8/3/2019 Heat Engines-Laws Thermo
1/19
Heat Engines and theLaws of Thermodynamics
Heat Engines and theLaws of Thermodynamics
8/3/2019 Heat Engines-Laws Thermo
2/19
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.mp48/3/2019 Heat Engines-Laws Thermo
3/19
ThermodynamicsThermodyn
amics
ThermodynamicsThermodyn
amics
Thermodynamics = study of heat and motion
Thermodynamics = study of heat and motion
Thermo = Heat
Thermo = Heat
dynamics = motion
dynamics = motion
+
+
Heat Engines and theLaws of Thermodynamics
Heat Engines and theLaws of Thermodynamics
8/3/2019 Heat Engines-Laws Thermo
4/19
Heat Engines and theLaws of Thermodynamics
Heat Engines and theLaws of Thermodynamics
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
Increase the internal
energy (temperature)
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-properties8/3/2019 Heat Engines-Laws Thermo
5/19
Heat Engines and theLaws of Thermodynamics
Heat Engines and theLaws of Thermodynamics
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
Increase the internal
energy (temperature)
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)
8/3/2019 Heat Engines-Laws Thermo
6/19
Heat Engines and theLaws of Thermodynamics
Heat Engines and theLaws of Thermodynamics
Detour: Pressure Detour: Pressure Detour: Pressure Detour: Pressure
Pressure=ForceArea
P=FA
Area
= PressureForce
i
8/3/2019 Heat Engines-Laws Thermo
7/19
Heat Engines and theLaws of Thermodynamics
Heat Engines and theLaws of Thermodynamics
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
8/3/2019 Heat Engines-Laws Thermo
8/19
Heat Engines and theLaws of Thermodynamics
Heat Engines and theLaws of Thermodynamics
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
8/3/2019 Heat Engines-Laws Thermo
9/19
Heat Engines and theLaws of Thermodynamics
Heat Engines and theLaws of Thermodynamics
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 ]
8/3/2019 Heat Engines-Laws Thermo
10/19
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
8/3/2019 Heat Engines-Laws Thermo
11/19
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-matter8/3/2019 Heat Engines-Laws Thermo
12/19
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
8/3/2019 Heat Engines-Laws Thermo
13/19
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.
8/3/2019 Heat Engines-Laws Thermo
14/19
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
8/3/2019 Heat Engines-Laws Thermo
15/19
Heat EnginesHeat Engines
waste heatwaste heat
HEAT Work
=> Some heat must be expelled from the engine=> Some heat must be expelled from the engine
8/3/2019 Heat Engines-Laws Thermo
16/19
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.shtml8/3/2019 Heat Engines-Laws Thermo
17/19
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
8/3/2019 Heat Engines-Laws Thermo
18/19
Heat EngineHeat Engine
THi
Work
TLow
RefrigeratorRefrigerator
8/3/2019 Heat Engines-Laws Thermo
19/19
PV GraphsPV Graphs
F
d
W=FdP
V
W=P VW=Fd Work is the area under the curve of a F(d) graph