Alabama Science in Motion
Electricity: Electromagnetic Induction
Electromagnetic Induction–Magnet and a Coil
Purpose: To measure the voltage or emf across a coil of wire as a cylindrical magnet
moves through the coil of wire.
Equipment:
ITEM
GLX
Voltage Probe
Cylindrical Magnet (set of two)
Coil
Tape
QTY
1
1
1
1
1
ITEM
Large Base and Support Rod
Three Finger Clamp
90° Clamp
Pad or Styrofoam Cup
QTY
1
1
1
1
Background:
Most of you have wrapped a wire around a nail, hooked
up a battery to the wire and made your own electromagnet. Did
you ever wonder if you could use a magnet to make electricity?
Question:
Michael Faraday was one of the first scientists to show that
electricity can be produced from magnetism. The essence of his
discovery is described in the following statement:
A changing magnetic field in the presence of a conductor
induces a voltage in the conductor.
Michael Faraday
For example, if a coil of wire (a conductor) is near a magnet, and the magnetic field due
to the magnet somehow changes, there will be a voltage across the coil of wire as a
result. One way to change the magnetic field near a coil of
wire is by moving the magnet relative to the coil. Can you
think of another way to change the strength of the magnetic
field near a coil?
Because electricity is induced by a changing magnetic field,
this process is called electromagnetic induction. It’s the
concept behind the electric generator (and countless other
electrical devices).
Faraday discovered several factors that determine how
much voltage is induced. One is the strength of the
magnetic field. A second is how fast the magnetic field
changes. Another factor is the number of turns (loops) of
wire that are in the coil.
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Fig. 1: Electromagnetic
induction
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Alabama Science in Motion
Electricity: Electromagnetic Induction
Procedure:
Use a Voltage Probe on the GLX to measure and graph the voltage across a coil of wire
as a cylindrical magnet moves through the coil of wire. Examine the graph of voltage
versus time to determine the amount of voltage.
Prediction: Before continuing, turn to the Student Response section and sketch your
prediction for the shape of the Voltage vs Time curve as the magnet falls through the
coil.
Safety Precaution

Keep the magnets away from the GLX, watches and calculators.
GLX Setup
Turn on the GLX ( ) and open the GLX setup file
Faraday.
1. In the Home screen, highlight Data Files and
press
.
2. In the Data Files screen, use the arrow keys
to navigate up and over to the Flash folder. A list
of labs should fill the screen. Use the arrow keys
to highlight the file, Faraday.
Fig. 2: GLX Graph
3. Press
to open the file. (Open) should appear
next to the name of the lab.
4. Press the Home button (
) to return to the Home Screen.
5. Press
to open the Graph. The GLX displays a Graph screen of ‘Voltage (V)’
versus ‘Time (s)’. The file is set to measure voltage 500 times per second (500 Hz).
6. Plug a Voltage Probe into the voltage input port
on the left side of the GLX.
Equipment Setup:
1.
Set up the coil so that you can
drop a magnet vertically through
the center of the coil. A rolled
up index card or piece of paper
through the coil can act as a
guide tube for the magnet.
2.
Connect the Voltage Probe
banana plugs into the coil. Does
it matter which slot the red lead
plugs into? See question (6).
For now, place the red lead into
the top position when the coil
number of turns(3200) is up.
Fig. 3: Equipment setup
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Alabama Science in Motion
Electricity: Electromagnetic Induction
3.
Place a protective pad or styrofoam cup underneath the coil to catch the bar
magnet after it falls through the coil.

Be careful to leave enough room under the coil so the magnet can fall completely
through the coil before it reaches the pad or cushion. Make sure the magnet does
not bounce back up towards the coil or it may be detected and alter your graph.
Record Data:

NOTE: The procedure is easier if one person handles the equipment and a
second person handles the GLX.

The voltage sensor can read a maximum of +/- 10 volts. If your graph looks
clipped off at +/- 10 volts, try releasing the magnet from closer to the coil.
1.
Hold the magnet just above the coil so the north end of the magnet is down and
will enter the coil first. (On some bar magnets, the north end is marked with a
stripe or notch.)
2.
Press Start (
3.
Drop the magnet through the center of the coil, and then press
to stop data
recording. Note: a rolled up piece of paper placed in the center of the coil will help
guide the magnet through the opening.
4.
Reverse the orientation of the magnet so the south end of the magnet will fall
through the coil first and repeat the data recording process.
5.
How do you think changing the release height will change the voltage graph? (See
question 4) Now explore releasing the magnet from two or three different heights
above the coil. Try heights from 1 to about 6 inches above the coil. As your record
each run, make sure you note in your table which pole of the magnet entered the
coil first for each run number displayed on the graph. Also, indicate the
approximate release height in the Comments column in your data table.
Revised 03/2008
) on the GLX to start recording data.
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Alabama Science in Motion
Electricity: Electromagnetic Induction
Analysis:
Use the GLX graphing tools to examine each run
of data for your graphs of Voltage versus Time.
Record the data as directed.
1.
To change the Graph screen to show a
specific run of data, press
to activate
the vertical axis menu. Press the arrow keys
(
) to move up and over to ‘Run #’ in the
upper right hand corner. With the Run #
highlighted, press
to open the menu.
Select the desired data run in the menu, and press
to activate your choice.
Tools Menu
Zoom Tool
2.
In the Graph screen, press
to open the ‘Tools’ menu and select ‘Zoom’. A
cross should appear on the graph as seen in the image above.
3.
Use the cross to draw a box around the two peaks. Use the arrow keys to
Position the cross at a corner of the region you wish to enlarge and press the
check to set the corner. Move the cross to the diagonal corner and press check to
complete the box.
4.
Now open the ‘Tools’ menu and select ‘Smart Tool’.
5.
Move the cursor to the first peak of voltage and record the value in the Data
Table. Move the cursor to the second peak of voltage and record its value in your
table. (you may want to pause and look at question #3)
6.
Revised 03/2008
Repeat measurements from step 3 for each of your data runs.
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Alabama Science in Motion
Electricity: Electromagnetic Induction
Student Data Sheet
Name ______________________________ Partner’s Name(s) ______________
Period _____________
Date ___________
Data:
A). On the left hand axis, sketch your prediction for the shape of the Voltage versus
Time graph. Include a title, units and labels for your axes.
B). On the right hand axis, sketch the graph for one of your data runs. Again, be sure
to provide an appropriate title and label the axis.
Data Table
Run
Pole
1
North
2
South
3
North
4
South
Voltage,
peak 1
Voltage,
peak 2
Observations/ Comments
5
6
7
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Alabama Science in Motion
Electricity: Electromagnetic Induction
Questions:
1.
For each run, why are there two peaks of voltage? Why do the two peaks point in
opposite directions (that is, why is one positive when the other is negative)?
2.
How does changing which pole of the magnet enters the coil first (North/South)
change the shape of the Voltage versus Time graph?
3.
In each run, how does the magnitude (amount) of the voltage of the second peak
compare to the magnitude of the voltage of the first peak? Is this relationship
consistent for all runs? Explain why you think this happens.
4.
What correlation exists between the peak voltages and the height from which you
release the magnet? How is this question related to question 3 above?
5.
In the Background section you were asked, “Can you think of another way to
change the strength of the magnetic field near a coil?” Answer this question in the
space below. If you have time, use the equipment to test your answer. (See
Extensions 1 and 2)
6.
What would happen if you reversed the banana leads on the coil and repeated
your first data run? If you have time, use the equipment to test your explanation.
See Extension (3).
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Alabama Science in Motion
Electricity: Electromagnetic Induction
Extension 1
In the original procedure, you varied the strength of the magnetic field around the coil by
dropping the magnet through the coil. Recall from the Background section that there is more
than one way to change the strength of the magnetic field relative to the coil. Were you able to
predict another way to vary the magnetic field strength as seen by the coil? Try the variations
that follow. See if any of these match your ideas on how to change the strength of the
magnetic field relative to the coil.
Repeat the procedure using a pair of bar magnets instead of just one bar magnet. You can
tape the magnets together as needed.
Double Bar Magnet
North–South Poles
1.
Tape two bar magnets together so each end has a ‘north’ and ‘south’ pole together.
2.
Repeat the process to record data.
North–North, South–South Poles
3.
Rearrange the two bar magnets so one end is ‘north-north’ and the other end is ‘southsouth’.
4.
Repeat the data recording process.

Record your results and answer the questions in the Lab Report section.
Extension 2
Did you consider moving the coil instead of moving the magnet? Try clamping the magnet in
place and holding the coil in your hand. DO NOT DROP THE COIL! Try recording the voltage
in the coil while moving the coil relative to the fixed position of the magnet. Be sure to try
different directions and speeds.

Record your observations for Extension 2 below the questions on the Student Response
section. Be sure to indicate direction and speed of motion when talking about the
resulting Voltage vs Time graph.
Extension 3
Review the question found in step 2 of the Experiment Setup section. Switch the orientation of
banana leads of the voltage sensor on the coil so that the red lead is in the lower position when
the coil number of turns (3200) is up. Predict how this change might alter the voltage vs time
graph? Sketch your prediction before testing your idea using the equipment.

To test your prediction, repeat your original first run with the north end of the magnet
down. Make sure to note under the comments column in your data table the change in
the leads for this run.
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