Science A 52
Lecture 20; April 24, 2006
•After the visit to the Chilled Water Plant there are
just a few electrical circuit ideas and definitions that
should allow you to compute the EER of the chilled
water plant.
Then the development of the Auto Industry,
Internal Combustion Engines
Wright Brothers and Flying
Jet engines
Spring 2006
© Harvard Science, A 52 FHA+MBM
Lecture 20, 1
Electrical Circuits
First I am going to review some
basic things in electrical circuits.
Many of the ideas that we have
developed about DC voltages and
current hold just as well with AC
circuits, but I will start by drawing
simple electrical circuits
Spring 2006
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Lecture 20, 2
Simple Circuits
Open Circuit
Closed Circuit
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Lecture 20, 3
A Way of thinking of AC
Circuits
• Let us think of our simple DC circuit
presented in the JAVA applet. Let the
resistor be a 100 watt light bulb and let
the DC voltage be a near zero value
• http://micro.magnet.fsu.edu/electr
omag/java/ohmslaw/index.html
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Lecture 20, 4
(sin t )
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2
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Lecture 20, 5
RMS of sin (t ) = ( sin t ) =1 / 2 = 0.707
2
Therefore RMS of [Vmax sin (ω t)]= 0.707 Vmax
Hence the effective value of a sine wave voltage source is
simply 0.707 of Vmax. The value of the voltage is averaged
over an entire cycle
Spring 2006
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Lecture 20, 6
Computing Power in an AC
Circuit
Power = I 2 R
I in the equation I 2 R is the RMS value
2
I
(t ) R
The power lost at any instant of time is
averaged over of a cycle
Look at the board!!
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Lecture 20, 7
Thinking of AC circuits as a rapidly time varying
DC circuit
In our Ohm’s Law DC circuit imagine the 100 ohm resistor
being replaced by a 100 watt light bulb.
We can imagine slowly increase the voltage with time, as the voltage gets near
100 volts the bulb will light, then we can slowly decrease the voltage to zero
and then go to negative values.
As we do this the voltage is negative and the direction of the current changes,
but it doesn’t matter to the bulb. We can imagine doing this again and again
and increase the cycle speed each time. The bulb glows twice each cycle
0 to +100, then to -100 and then back to zero.
If we want the bulb to burn brightly all of the time we have to increase the
voltage. If we go through a cycle of + to - in in 1/60 of a second, the bulb will
appear to have constant intensity. In reality there is a slight flicker at 120 Hertz.
To have the power be equivalent 100 watts of DC power the voltage AC must
be 141.4 V
Spring 2006
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Lecture 20, 8
In Ohm’s Law with simple Resistors
Note: Ohm’s Law is the same for DC and
AC circuits, provided you use the RMS
value of the sine voltages and current
What is the RMS value of the sine wave
time varying voltage?
Spring 2006
© Harvard Science, A 52 FHA+MBM
Lecture 20, 9
(sin t )
Spring 2006
2
© Harvard Science, A 52 FHA+MBM
Lecture 20, 10
RMS of sin (t ) = ( sin t ) =1 / 2 = 0.707
2
Therefore RMS of [Vmax sin (ω t)]= 0.707 Vmax
Hence the effective value of a sine wave voltage source is
simply 0.707 of Vmax. The value of the voltage is averaged
over an entire cycle. AC volt meters are scaled to present the
RMS value of the voltage.
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Lecture 20, 11
A few words about 3 Phase
voltage
Wave b
Wave a
Wave c
Wave a, b, and c
Each displaced
120 degrees
Three waves of voltage from a 3-phase generator
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Lecture 20, 12
3-Phase voltage Relationship
RMS voltage between phases a and b is 3 times the
voltage between any phase and neutral.
In the Chilled Water Plant the voltages displayed in the
Screen shots are the so called phase voltages.
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Lecture 20, 13
Transportation
Before looking at Henry Ford, the production line,
the low cost car, and the internal combustion engine
let us look at a brief history of transportation. The forces
that made “The Machine that Changed the World”
See the class reading list
Let us look at a Transportation Site of the
Smithsonian Institution
Look at #3,6,7, and 16
http://americanhistory.si.edu/onthemove/exhibition/index.
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Lecture 20, 14
Needs for new forms of
transportation of goods and
people lead to increased
demands for cars
Henry Ford was not the first to build a car, nor and internal
combustion engine but he was the father of modern mass
production and the first low cost car.
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Lecture 20, 15
Henry Ford
July 30, 1863 - April 7, 1947
1891 - engineer with Edison Illuminating Company
1893 - Chief engineer
1896 - Built his own self propelled car - the Quadricycle
Built racing cars and the like but the businesses failed
financially, then
1903 - with $28,000 he formed Ford Motor Company
1908 - the model T
1918 - 1/2 of all cars in the US were model T’s
The rest is history - but what were his contributions?
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Lecture 20, 16
Early developments In the
Auto Industry
•Henry Ford began his career as a manager of Detroit
Edison. The beginning of electrical power attracted the
ambitious- and that was certainly Henry Ford
•As electrical lighting began to signal the end of kerosene
for lighting - the oil industry was saved by the beginning of
the auto industry and the demand for the light fractions of
crude oil for gasoline.
•The first cars were expensive toys for the very rich
and Henry Ford changed that by developing the
moving production line.
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Lecture 20, 17
Automotive Development
•The early history of the auto industry follows the
standard history of modern industrial development
with a twist.
•The twist is that the customer for the development
was well-to-do citizens who could afford something
different from the horse drawn carriage.
•The most significant event in the development of
autos was the work of Henry Ford using the patterns
of work developed by Frederick Taylor
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Lecture 20, 18
Auto Development - continued
•Before Ford, cars bodies were made one at a time
by craftsmen who fitted the pieces together. Files and
hammers were needed to make the pieces fit.
•Engines were made by engine manufacturers - in the
pattern of the present day aircraft industry - the car makers
made the bodies and the suspensions. Tires were made
by others.
•Remember that in a beginning of a industry it is always
quicker and cheaper to purchase parts from others rather
than make them yourself as long as the business is
smaller that the suppliers.
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Lecture 20, 19
Auto Development - continued
•Using this obvious supply approach Ford contracted with
others to make parts for assembly into his cars.
•First it was necessary to get all of the suppliers - included
his own internal suppliers to use the same measurement
standards. You cannot have interchangeable parts without
having the parts made to the exact same dimensions.
•This means machinists had to check their calipers after
each shift, and there would have to be uniform standard
measuring blocks at Ford and at it suppliers.
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Lecture 20, 20
Auto Development - continued
•Without standards for length and locating holes and
the like there could not be any mass production using
relatively unskilled workers.
•Then there was the adoption of the work ideas of Frederick
Taylor. Taylorism has permeated most factory work and has
become a part of sports. Think of the professional football
and baseball for that matter.
Read about Ford and Taylor in the books on reserve.
”The Machine that Changed the World”page 265
“The Principles of Scientific Management”page 251, and also
http://www.fordham.edu/halsall/mod/1911taylor.html
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Lecture 20, 21
Auto Development - continued
• Now I want to look at engines and see how they work.
•The Four-Stroke Spark-Ignition (SI) Engine
The basic idea of a 4 stroke engine had been suggested
by Beau de Rochas in 1862
• Intake stroke - air an fuel in
• Compression stroke - to raise the temperature
• Ignition - expansion or power stroke
• Exhaust stroke - gasses out
In 1876 Otto built a 4-stroke engine and the rest is history
http://auto.howstuffworks.com/engine1.htm
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Lecture 20, 22
Auto Development - continued
• In 1892 Rudolph Diesel initially planned a new type of
engine that would be capable of burning coal dust - it
turned out that it burned a liquid fuel much more readily
•Intake stroke - air alone into the cylinder
•Compression stroke - all valves closed - to raise the air
temperature: compression of 18/1 or higher now used
•Injection of fuel - expansion stroke - at very high pressures
(In newer engines controlled by the latest generation of
Bosch EDC Electronic Diesel Control. Operate with high
pressures of up to 2,000 bar, coupled with extremely precise
fuel metering and injection sequences)
•Exhaust stroke -exhaust valve open.
http://auto.howstuffworks.com/diesel1.htm
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Lecture 20, 23
The Wright Brothers and Flight
A brief account of the Wright brothers
Wilbur 1867-1912
Orville 1871 - 1948
See” Wright Brothers and Manned Flight” in the
notes given out in class and on the course WEB page.
The account gives a brief and technically correct
account of why there is lift on a wing.
For a brief story of their work:
http://www.wam.umd.edu/~stwright/WrBr/taleplane.html.
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Lecture 20, 24
Lift on a Wing
Theory
Figure F 1.1
Calculated Air flow Over a Wing Section
from Prandtl &Tietjens
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Lecture 20, 25
More - Lift on a Wing
Bernoulli’s Equation
Conservation of Energy
Flow along a streamline
2
1
1
1
2
2
! V1 + p1 = ! V2 + p2
2
2
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Lecture 20, 26
Actual Flow Over a Wing
Figure F 1.2
Airflow over an Airfoil
after David Hazen,.
Hazen, David, Illustrated Experiments in Fluid Mechanics-Film Notes. Cambridge, MA: The MIT Press,
1972, p. 90.
Lecture 20, 27
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Surface Pressure Along the Wing
Lower Pressure
Lift is more suction on the upper surface
than + pressure on the lower surface
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Lecture 20, 28
Lift and Drag Coefficients
Lift Coefficient
Lift Coefficent = C L =
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5 X Drag Coefficient
Lift Force
1
A • !V 2
2
Drag Coefficent = C D =
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Drag Force
1
A • !V 2
2
Lecture 20, 29
End
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Lecture 20, 30
Now for some simple word of
caution about AC systems in
the house
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Lecture 20, 31
A multi-meter and my kitchen outlet
About 110 V between the two slots
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Lecture 20, 32
Red lead in the smaller slot and the black lead in the ground
~110 Volts RMS on the dial - the smaller slot is the hot side
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Lecture 20, 33
Red lead in the long slot and the black lead in the ground
No voltage difference - the long slot is the neutral
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Lecture 20, 34
Measuring Resistance
The multi meter is switched to the ohms
scale
In fact the ohm scale is 1 Ohm full scale
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Lecture 20, 35
Resistance between the touching leads is zero
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Lecture 20, 36
Resistance between the wall of the Edison socket and the
wideblade of the plug is 0. Hence they are directly connected
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Lecture 20, 37
No resistance between the hot blade and the copper
spring in the center of the bottom
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Lecture 20, 38
View of a 100 Watt bulb
Neutral
Contact
Hot contact
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Lecture 20, 39
Let us look at Electric
Generators
First let us look at a Java Applet showing a DC generator note that the output wave is just rectified AC
Batteries give essentially constant voltage, generators
don’t unless there is electronics involved to smooth the voltage
http://micro.magnet.fsu.edu/electromag/java/generator/dc.html
Spring 2006
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Lecture 20, 40
Let us look at Electric
Generators - continued
Again this is a Java applet of a AC generator.
There is another site giving detail that we will look
at in a moment
Note that the commutator rings are continuous, they
were split on the DC generator
http://micro.magnet.fsu.edu/electromag/java/generator/ac.html
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Lecture 20, 41
Lenz’s Law
• http://micro.magnet.fsu.edu/electromag/java/lenzlaw/index.html
This is again a java applet showing induction,
it is a very important idea. And as you will see it is
important for the understanding of motors.
Spring 2006
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Lecture 20, 42
Details of Electrical Motors
and Generators
•
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/mothow.html#c1
This is a very detailed and carefully explained site.
To learn about motors you must go through this slowly
your self. A quick run through doesn’t do it.
Spring 2006
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Lecture 20, 43
THE END
Spring 2006
© Harvard Science, A 52 FHA+MBM
Lecture 20, 44