Thermodynamics

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Thermodynamics
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1st law of thermodynamics
Energy may be converted to different forms, but
it is neither created nor destroyed during
transformations
Amount of energy before and after transformation
is the same, only the form of the energy has
changed
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1st Law (Contd.)
Another way to state the 1st law is
mathematically.
DE Q W
This equation says that the only way to change
the energy of a system is to add heat to it (Q)
or to do work on it (W)
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Heat
While used a lot in our vocabulary, this term is
very misunderstood
Heat - the ENERGY transferred between objects of
different temperature
Heat is NOT temperature. An object
CANNOT contain heat objects contain thermal
energy.
Heat is a very important type of energy transfer
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Heat Versus Temperature
Temperature - the property that two objects have
in common when NO heat is transferred between them
Temperature is a relative property. We define it
in relationship to other things
T1 gt T2
T1 T2
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Heat Flow
Heat can flow via one of three methods

• Conduction - energy transfer by next-nearest
  molecule interaction
• Convection - energy transfer by mixing can be
  naturalor forced (fan, stirring, etc.)
• Radiation - energy transfer by electromagnetic
  radiation

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Conduction
Energy transfer by nearest molecules running into
each other
Rate of heat transfer depends on

• Temperature difference DT TH - TC
• Thickness of material L
• Thermal conductivity of material k
• Surface area A

Q k DT A

t L
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Conduction
More familiar
Q DT A

t R
If intervening material is made up of more than
one substance, add R-values
Rtotal R1 R2 R3 .
Problem How is the rate of heat transfer
affected by adding anR-value 8 insulation to an
8x20 wall that has an R-value of 12when the
temperature difference is 20 oF?
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Convection
Heat transfer via mixing requires some type of
fluid (gas, liquid) Things can naturally
convect, especially when density changesand more
buoyant materials will rise Forced convection
requires energy input
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Radiation
Every object in the universe emitselectromagnetic
radiation because it has a temperature above
absolute zero.
Type of radiation depends upon the value of the
temperature
.003 m K
Weins Law gt lmax
T
Problem At what wavelength do you emit most of
your radiation?
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Stefan-Boltzmann Law
The rate of heat emission due to radiation
depends on size and temperature.
Q/t e s A T4 where e is the
emissivity of the object
Remember, the object will be absorbing radiation
while it isemitting. Therefore, the total heat
transfer is
Q/t e s A (Tobject4 - Tsurroundings4)
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Heat Transfer Devices
Heat Pump
Heat Engine
Outputs useful energy W by extracting it from
heat passing from hot to cold Example Car engine
Transfers heat from cold to hot using external
energy W Example Refrigerator
In both devices, QH QC W
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2nd law of thermodynamics
If energy is never created or destroyed, why
cant we keep reusing the same energy source
forever?
This is a consequence of the 2nd law of
thermodynamics
In a closed system, the total entropy either
increases or stays the same
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Second law of thermodynamics
When a chemical bond is broken, you get some high
quality ENERGY capable of doing work, and some
low quality wasted energy
No energy was lost or created in the transfer,
but the usability of the energy declined in the
transformation.
This low quality energy cannot be effectively
harnessed to do any more work, so you cannot use
one energy source forever
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Second law of thermodynamics
Example powering your car
Breaking chemical bonds in gas during combustion
yields high quality energy which produces kinetic
energy to move car
Also produces waste energy as heat with little
ability to do work
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Usable E
Amount of high quality energy declines with each
step (width of orange arrows)
No energy is lost, it simply is converted to
low-quality heat that cannot be used for further
work
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Efficiency
A measure of how well energy is converted
useful energy out
Efficiency
total energy input
Examples
Internal combustion engine car is about 10
efficient Electric car is about 20
efficient Incandescent light bulb is about 1
efficient
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Efficiency Example
A power plant consumes 80,000 Joules of coal
energy to produce 30,000 Joules of electricity.
What is the efficiency?
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Heat Engine Efficiency
Energy input QH Usable energy output W
W
Efficiency
QH
Since QH QC W gt W QH - QC
QC
Efficiency 1 -
QH
Problem A car takes in 20,000 J of gasoline and
outputs 19,000 J of heat. What is the efficiency
of the car?
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Heat Pump COP
For heat pumps, it is not proper to
discussefficiency since there is no usable
energyouput. Instead, define coefficient of
performance to discuss how much energyit moves
per energy paid for.
QH
COPheater
W
QC
COPa.c.
W
Note COPheater is always greater than 1. Why?
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Maximum Efficiency
Unfortunately, the 2nd law of thermodynamics
limits the maximum efficiency that a device can
have. No device will ever be 100 efficient.
For a heat engine, the limit is given by
where TC is the temperature of the cold reservoir
and TH is the temperature of the hot reservoir
in the Kelvin temperature scale
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Maximum Efficiency Example
An inventor proposes a heat engine that will
produce electricity by extracting heat from ocean
surface water at 20oC (293 K) and dumping the
waste heat to the deep ocean at 5oC (278 K).
What is the maximum efficiency?
At most, this device will be 5 efficient. In
reality, it will probably only be about half of
this, or 2-3 efficient.
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Recapping