Orange Coast College
Physics 280
Experiment #7
Electromagnetic Induction
The purpose of this experiment is to help you become familiar with some of the principles of
Faraday’s law of electromagnetic induction.
Apparatus:
Two flat wound coils
Two voltmeters
One ammeter
One DC Power supply
One AC power supply
One Galvanometer
One soft-iron U-shaped bar that will hold the two coils
Theory
Electromagnetic induction theory predicts that a voltage will be induced (also called an “emf”)
every time the magnetic flux ! B through a conducting loop changes, as given by the relation:
emf = !
d" B
dt
The magnetic flux ! B through a coil can be expressed as:
!B =
#
! !
B " dA =
#
B dA cos (! )
A change in the magnetic flux ! B through a coil can be generated by:
(i) changing the magnetic field through the coil, or
(ii) changing the area of the coil (either by stretching or compressing the coil), or
(iii) by rotating the coil relative to the direction of the magnetic field.
The negative sign in the equation above means that the direction of the induced current in the
conducting loop is such as to oppose whatever caused it. This is known as Lenz’s law.
In this activity you will become familiar with the qualitative response of coils when subjected to
a changing magnetic flux through them. You will also build a simple transformer.
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Procedure
Part I.
Connect the galvanometer, as shown in figure 1. Watch the galvanometer needle while you
thrust the north pole of a bar magnet through the stationary coil. Describe what you observe?
Use the following symbols in this report to answer the pertinent questions:
L = Left,
R = Right,
N = North,
S = South.
Questions: (Circle the best choice, if given)
(1a) What is the direction of the galvanometer needle when you thrust the north pole of the bar
magnet (at constant speed) toward the stationary coil? _______L, R_______
when you move the coil toward the stationary bar magnet? __________L, R_______
move both the coil and bar magnet toward each other? _____________L, R______________
(1b) What general statements can you make concerning the preceding observations?
______________________________________________________________________________
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Part II.
Next, with the same circuit of figure 1 above, place the bar magnet in the center of the coil, as
shown in figure 2 below. The coil now has a maximum amount of magnetic flux through it.
Questions:
(2a) What is the galvanometer reading? _______L, R,
none_____________________
(2b) How do you account for the observed reading?
______________________________________________________________________________
______________________________________________________________________________
(2c) Observe the galvanometer needle as you withdraw the bar magnet to the left at constant
speed. What is the direction of the needle deflection as the bar magnet is withdrawn? _L, R_
(2d) Vary the speed with which you withdraw the magnet from the coil. What conclusions can
you draw from this?
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
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Part III.
From the preceding discussion, you should be convinced by now that it is not the magnetic flux,
but the change in the magnetic flux that induces a voltage (or emf) across the coil. If we had a
method to change the magnetic flux through the coil rapidly, we would be able to observe the
generation of a rather large induced current. We can achieve this by substituting the permanent
bar magnet with an electromagnet. Arrange the set-up as shown in figure 3 below.
Use the DC power supply and set its voltage output at about 10.0 Volts.
(3a) Observe the galvanometer needle deflection as you close the circuit. What happens?
(3b) Keep a steady current through the electromagnet. What is the galvanometer needle
deflection or reading when there is a steady current in the electromagnet?
______________________________________________________________________________
______________________________________________________________________________
(3c) As you approach the coil with the electromagnet, observe the galvanometer needle
deflection and describe what happens.
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
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(3d) As you cut the current off (by opening the circuit), what happens to the galvanometer
reading?
______________________________________________________________________________
______________________________________________________________________________
(3e) Now insert the soft-iron through the center of the two coils and describe what happens when
the circuit is closed,
______________________________________________________________________________
______________________________________________________________________________
and when the circuit is opened,
______________________________________________________________________________
______________________________________________________________________________
(3f) Replace the soft-iron core with various other materials if they are available; a wooden
pencil, an aluminum rod, a carbon rod, etc. Describe the results using each different material.
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________
Part IV.
The Transformer
When an alternating current passes through a coil of wire, it produces a time-changing magnetic
field that alternates in direction and varies in magnitude. This is precisely one of the conditions
needed for the electromagnetic induction to take place in a secondary coil of wire. Here you will
investigate how the number of turns and the input voltage in a primary coil (input coil) affects
the induced voltage (output voltage) in a secondary coil (output coil) as a function of the number
of turns in the secondary coil.
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Set up the coils as shown in figure 4, noting that the DC power supply has been replaced by an
AC power supply. In the diagram, the coil to the left will be referred to as the primary coil, and
the coil to the right will be referred to as the secondary coil. Note that you will put in an
alternating current to the primary coil at one voltage level, and reading the output voltage at the
secondary coil.
(4a) With the 400-turn coil as the primary coil and the 400-turn coil as the secondary coil, adjust
the input voltage to about 10 Volts. Measure the output voltage and record the result in the table
below. Repeat this for input voltages of about 13 volts and for 15 volts.
(4b) Repeat (4a) with the 400-turn coil as the primary coil and the 800-turn as the secondary coil.
(4c) Repeat (4a) with the 400-turn coil as the primary coil and the 1600-turn as the secondary
coil.
(4d) Repeat (4a) with the 400-turn coil as the primary coil and the 3200-turn as the secondary
coil.
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Number of
Number of
turns in
turns in
primary
secondary
coil
coil
N1
N2
Input
Output
Voltage
Voltage
ΔV1 (Volts)
ΔV2 (Volts)
!V2
!V1
!V2 N 2
=
!V1 N1
(experimental)
(Theory)
1
400
400
1
1
2
400
800
2
2
4
400
1600
4
4
8
400
3200
8
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For a transformer, the ratio of the voltages in the coils is equal to the ratio of the number of turns
in the coils. The working transformer equation is
!V2 N 2
.
=
!V1 N1
Essential Question: Discuss in your report how the experimental values of the ratio ΔV2/ΔV1
compare with the theoretical values (already entered in the table above under N2/N1).
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