Download Electromagnetic Induction

Lesson Plan 3 --- Electromagnetic Induction
By Fengfeng Zhou
Objectives:
1. Students will know a changing magnetic field induces electromotive force (EMF)
in a circuit. They will be able to calculate EMF and determine the direction of
induced current using the right-hand rule in certain cases.
2. Students will know how an electric generator works and how it differs from an
electric motor.
3. Students will understand Lenz’s law.
4. Students will know how a transformer works.
Activities:
1. Demonstrate what is electromagnetic induction using a galvanometer, Helmholtz
coils, and a strong magnet. Connect the galvanometer and coils to form a closed
circuit. Then move the magnet back and forth near the coils. Students will see
the pointer of the galvanometer swings.
2. Ask students to do the same using the equipments provided to them. Ask them to
pay attention to the relationship between the directions of the movements of the
magnet and the pointer. Ask them to think what induced the current in the circuit.
If they cannot figure out the reason, tell them an electromotive force will be
induced in a piece of conductor or a circuit only if a changing magnetic field
exists near the conductor or circuit.
3. Present an example problem to calculate EMF.
4. Present several slides to show applications of electromagnetic induction such as
tape recorder and “smart” traffic lights. Tell them the most important application
is electric generator.
5. Explain how an electric generator works and tell students the difference between a
generator and a motor by showing two short movies. The movies are in fact
animations of operations of electric generators and motors with sound narrative
and can be inserted in PowerPoint slides to play. They can be found in Microsoft
Encarta Encyclopedia.
6. Tell students the output voltage from a generator varies both in magnitude and
direction. The variation can be described by a sine or cosine curve. People are
usually more concerned about the “average” or effective voltage. Tell students
how the effective voltage defined and derive the formula to compute effective
voltage.
7. Make a demonstration to show the eddy-current damping effect which helps
students to understand Lenz’s law. An eddy-current pendulum will be used for
this demonstration.
8. Guide students to do an experiment to learn how a transformer works. They are
equipped with a function generator which can output alternating voltage, a set of
primary and secondary coils, an iron bar which can be inserted into the secondary
coils, and two voltmeters. Students should follow the following steps:
 Insert the secondary coils into the primary coils and the iron bar into the
secondary coils.
 Connect the function generator, primary coils, and a voltmeter to be a
closed circuit, and then connect the secondary coils and another voltmeter
to be a separate closed circuit from the first one.
 Turn on the generator and record the readings from both voltmeters.
 Increase the output from the generator and make another recording.
 Exchange the two coils and repeat what they did in last two steps.
 Remove the iron bar and record the readings from both voltmeters.
9. Ask students why there is voltage in the second circuit though there is not a power
supply in this circuit. Tell them it is due to electromagnetic induction if they
cannot get the right answer.
10. Ask students in what circumstance the voltage of the second circuit is higher than
that in the first circuit, and in what circumstance the result is opposite.
11. Ask students to summarize by what means the voltage in the second circuit can be
increased.
Assessment:
Students will take a quiz about magnetism.