Transcript U3e L8 Electricity and Magnetism 1

March 04, 2010
DRILL
1.
What does the following schematic symbol represent?
DC Power Supply
2.
What does the following schematic symbol represent?
AC Power Supply
3.
4.
How many volts are typical in a residential electrical
outlet? 120 volts
What do graphs of DC and AC power sources look
like? Why?
U3e-L8
Direct Current
• If voltage is constantly 1.5 V, what does its
graph over time look like?
2
1
Voltage (volts)
3
1.5 V
0
2
4
6 8 10 12 14
Time (seconds)
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Alternating Current
0
120
+/-120 V
-120
Voltage (volts)
• If voltage is alternating 120 V to -120 V, what
does its graph over time look like?
0
2
4 6 8 10 12 14
Time (seconds)
U3e-L8
Electric Motors
•
Michael Faraday (1791-1867)
–
London, England.
–
Physicist and Chemist
•
First person to successfully
create an electric motor.
•
Discovered that electric currents
create a magnetic field, and vice
versa.
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Purpose of Lesson
•
•
To demonstrate the principle of
electromagnetism.
To use principles of electromagnetism to
produce a mechanical result.
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ELECTRIC MOTORS
•
•
Electric motors are used everyday in many ways.
A motor’s purpose is to turn electrical energy into
mechanical energy.
Motors work on simple principles of magnetism:
• Opposite poles _____________
ATTRACT
• Like poles _____________
REPEL
N
S
S
S
N
N
U3e-L8
ELECTRIC MOTORS
•
•
An electric current produces a magnetic field.
Electric current through a coil of wire creates an
electromagnet.
• What is the benefit of creating a magnet with
electricity?
– It can be turned on and off.
– The strength and direction of the magnetic
poles can be controlled by altering the strength
and direction of the electric current.
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ELECTRIC MOTORS
–
What will happen now?
–
The electromagnet and the permanent magnet will
ATTRACT each other.
N
S
•
The electromagnet and the permanent magnet will
REPEL each other.
S
N
•
•
Back to our magnet:
Suppose we create an electromagnet like below.
What will happen?
U3e-L8
ELECTRIC MOTORS
•
•
•
What would happen if we put an axle through the
middle of the permanent magnet, then turned on
the electromagnet?
This is how a motor rotates.
The electromagnet is turned on and off so that the
permanent magnet is attracted, then repelled,
then attracted, then repelled….and so on.
N
S
U3e-L8
ELECTRIC MOTORS
What is a schematic?
A schematic is a diagram that represents the
elements of a system without using realistic pictures.
How would a schematic which includes an electric
motor look?
Switch = Commutator
Motor
V = 9V
U3e-L8
I
ELECTRIC MOTORS
•
Now that you know what switches look like in
electrical schematics, draw an electrical schematic
for a lamp plugged into a wall outlet.
•
Remember that AC power supplies look like
in electrical schematics.
U3e-L8
ELECTRIC MOTORS
Lamp
120 V
U3e-L8
HOMEWORK
• Finish reading the article about
magnetism and motors (Maglev
Trains) and answering the
questions.
• Due tomorrow.
U3e-L8