Physics Lesson Plan
Teacher
Howard
Unit Title
Length
Goal(s)/PLO(s):
Course
Grade Level
Block/Period
 qualitatively describe how a generator uses
induction to produce an electric current
 define back emf
 with respect to DC motors, solve problems
involving
– current
– back emf
– armature resistance
– voltage to motor
Phys 12
12
Date
Class Size
Lesson #, of
21-05
 give examples of current fluctuations due to
back emf in motors (e.g., overheating)
Materials:
Generators and motors
Timeline
Class Activities
Introduction
Body
Notes 21-05 to 21-06
Closure
Questions 9-10 , Problems 25-29 odd
21-05 Electric Generators
An electric generator, or dynamo normally generates alternating
current.
• exact opposite of a motor
• converts mechanical energy into
electric energy
• faraday_en.jar
The armature is turned by steam, wind,
water…which induces a current in the armature.
• Each half rotation the current flows in the opposite direction
o 60Hz in North America
DC generators can be made by
replacing the slip rings with
commuters.
• the voltage still varies but is
either zero or in one direction
• capacitors or diodes can also
be used to smooth out the
current
21-06 Back EMF and Counter Torque; Eddy Currents
As the armature in a DC motor spins there is a change in the flux
of the armature
• an emf is generated which opposes motion (Lenz’s Law)
o called the back EMF or counter EMF
Even with no load, a motor’s speed will not increase forever…it
will max out when the back EMF is equal to the input voltage. If
there is a mechanical load then the motor will rotate slower and
there is less back EMF.
http://www.stumbleupon.com/su/1d11ps/www.youtube.com/watch
%253Fv%253Dnrw-i5Ku0mI
Example 21-9, 21-10
Counter torque
• the equivalent of back EMF in a generator
• The more current is drawn from the generator the more the
induced current fights the motion of the generator
o the external torque will have to be greater to keep the
generator spinning
o more mechanical energy is required to make more
electrical energy…duh!
Questions 9-10 , Problems 25-29 odd