1 st law of thermodynamics Energy may be converted to different forms but it is neither created nor destroyed during transformations Energy from chemical bonds is converted to kinetic energy and heat body and friction from tires ID: 248317
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Slide1
ThermodynamicsSlide2
1st
law of thermodynamics
Energy may be converted to different forms, but it is neither created nor destroyed during transformations
Energy from chemical bonds is converted to kinetic energy and heat (body and friction from tires)
Amount of energy before and after transformation is the same, only the
form
of the energy has changed
ENERGY
HeatSlide3
1st Law (Contd.)
Another way to state the 1
st
law is mathematically.
D
E = 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)
Example: Can make
wood hotter by applying fire or hittingSlide4
Heat
Heat - the
ENERGY transferred
between objects of different temperature
While used a lot in our vocabulary, this term is
very misunderstood
Heat is NOT temperature. An object CANNOT
contain heat; objects contain thermal energy.
Heat is a very important type of energy transferSlide5
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
T
1
> T
2
T1 = T
2Slide6
Heat Flow
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
Heat can flow via one of three methodsSlide7
Conduction
Energy transfer by nearest molecules running into each other
Rate of heat transfer depends on
Temperature difference
D
T = T
H - TC
Thickness of material LThermal conductivity of material kSurface area A
Q k DT A
t L=Slide8
Conduction
Q
D
T A
t R
=
More familiar
If intervening material is made up of
more than one substance, add R-values
R
total = R1 + R2
+ R3 + ….
Problem: How is the rate of heat transfer affected by adding an
R-value 8 insulation to an 8’x20’ wall that has an R-value of 12when the temperature difference is 20 oF?Slide9
Convection
Heat transfer via mixing; requires some type of fluid (gas, liquid)
Things can naturally convect, especially when density changes
and more buoyant materials will riseForced convection requires energy inputSlide10
Radiation
Every object in the universe emits
electromagnetic radiation because it
has a temperature above absolute zero.
Type of radiation depends upon the
value of the temperature
Wein’s Law =>
lmax =
.003 m K
T
Problem: At what wavelength do you emit most of your radiation?Slide11
Stefan-Boltzmann Law
The rate of heat emission due to radiation depends on size and
temperature.
Q/t = e
s A T
4 where e is the emissivity of the object
Remember, the object will be absorbing radiation while it is
emitting. Therefore, the total heat transfer is
Q/t = e s A (Tobject4 - Tsurroundings4)Slide12
Heat Transfer Devices
Heat Pump
Heat Engine
Transfers heat from cold to hot using external energy W
Example: Refrigerator
Outputs useful energy W by
extracting it from heat passing
from hot to cold
Example: Car engine
In both devices,QH
= QC + WSlide13
If energy is never created or destroyed, why can’t we keep reusing the same energy source forever?
ANSWER
:
Although energy isn’t destroyed, in every energy
transfer, some of it will change to a non-usable form
This is a consequence of the 2
nd
law of thermodynamics
“In a closed system, the total entropy either increases or stays the same”
2
nd
law of thermodynamicsSlide14
Second law of thermodynamics
ENERGY
Waste
Heat
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 foreverSlide15
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
Second law of thermodynamicsSlide16
Combustion
of gasoline
Piston
movement
Axle
turns
Wheels
turn
Heat loss during
combustion
E
Friction
with pistons
E
Friction
with axle
E
Friction
of tires
with road
E
E
E
E
E
Energy in
gasoline
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
Usable ESlide17
Efficiency
A measure of how well energy is converted
Efficiency =
useful energy out
total energy input
Examples
Internal combustion engine car is about 10% efficient
Electric car is about 20% efficient
Incandescent light bulb is about 1% efficientSlide18
Efficiency Example
A power plant consumes 80,000 Joules of coal energy to
produce 30,000 Joules of electricity. What is the efficiency?
Efficiency =
30,000 J
80,000 J
= .375 = 37.5 %
= 10,000 JSlide19
Heat Engine Efficiency
Energy input = Q
H
Usable energy output = W
Efficiency =
W
Q
H
Since
QH = QC + W => W = QH - QC
Efficiency = 1 -
Q
C
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?Slide20
Heat Pump COP
For heat pumps, it is not proper to discuss
efficiency since there is no “usable energy
ouput”. Instead, define “coefficient of
performance” to discuss how much energy
it moves per energy paid for.
COPheater =
COPa.c.
= QH
W
Q
C
W
Note: COP
heater is always greater than 1. Why?Slide21
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
Maximum efficiency = 1 -
T
C
T
H
where TC is the temperature of the cold reservoir and TH
is the temperature of the hot reservoir in the Kelvin temperaturescaleSlide22
Maximum Efficiency Example
An inventor proposes a heat engine that will produce electricity
by extracting heat from ocean surface water at 20
oC (293 K) and dumping the waste heat to the deep ocean at 5oC (278 K). What is the maximum efficiency?
Maximum efficiency = 1 -
278 K
293 K
= 1 - .95 = .05
At most, this device will be 5% efficient. In reality, it will
probably only be about half of this, or 2-3% efficient.Slide23
Recapping
2nd LAW
:
Energy is transformed from high quality to low quality
1st LAW
:
Energy is neither created nor destroyed, only transformed
RESULT
:
Low quality heat cannot do substantial work, requiring a new source of high quality energy