# Faraday's Law Magnet through a Coil

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```					2007                                                                Alabama Science in Motion

Electromagnetic Induction–Magnet and a Coil
Equipment:
ITEM                              QTY     ITEM                                  QTY
GLX                                 1     Large Base and Support Rod             1
Voltage Probe                       1     Three Finger Clamp                     1
Cylindrical Magnet (set of two)     1     90° Clamp                              1
Coil                                1     Pad or Styrofoam Cup                   1
Tape                                1
Safety Precaution
      Keep the magnets away from the GLX, watches and calculators.
Question:
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?
Background
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.
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               Fig. 1: Electromagnetic
magnetic field. A second is how fast the magnetic field           induction
changes. Another factor is the number of turns (loops) of
wire that are in the coil.

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2007                                                                  Alabama Science in Motion

Preview
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
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

Fig. 3: Equipment setup
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2007                                                                  Alabama Science in Motion

the top position when the coil number of turns(3200) is up.
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 (    ) on the GLX to start recording data.

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

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2007                                                               Alabama Science in Motion

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.

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.     Repeat measurements from step 3 for each of your data runs.

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2007                                                                     Alabama Science in Motion

Electromagnetic Induction–Magnet and a Coil
Name ________________________________ 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.
Student sketches will vary.

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.
See sample data provided. Title should be Voltage vs                        Sample run
Time.

Data Table
(Voltage will vary with release height and strength of magnet for the 3200 turn coil.)

Run       Pole     Voltage,   Voltage,            Observations
peak 1     peak 2

1       North    -3.63      +5.43      Released from just above coil
2       South    +3.76      -5.47      Released from just above coil
3       North    -4.46      +5.94      Released from 1in above coil
4       North    -5.67      +6.95      Released from 2 in above coil
5       North    -7.29      +8.26      Released from 5 in above coil
(Ext 4)   North    -3.48      +6.46      Two coils stacked in series.
Released from just above coil

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)?
The two peaks are due to the two poles of the magnet. One pole pushes and the other
pulls on the charges of the conductor as it falls through the coil. As a result, the
charges move one direction and then the other.

2.     How does changing which pole of the magnet enters the coil first (North/South)
change the shape of the Voltage versus Time graph?

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2007                                                                  Alabama Science in Motion

Provided the red lead for the voltage sensor is in the top position with the coil turns #
(3200) up and the North pole enters the coil first, then the North pole gives a negative
peak first. If the South Pole enters first, then the first peak is positive.
Changing which pole enters first determines which peak occurs first.
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.
The second peak should always be greater than the first due to the greater rate of
change in the magnetic field caused by the falling magnet. Acceleration of the magnet
causes the magetic field to change faster for the second pole entering the coil.
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?
Just as in (3) above, the faster the magnet is moving, the higher the peak. With more
time to accelerate for the higher release points, the faster the magnet is moving when it
passes through the coil. The faster the magnet moves, the greater the change in
magnetic field around the coil, therefore the greater the induced voltage in the coil.
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)
   Combine two magnets in various arrangements.
   Move the coil while the magnet position remains fixed.
   Move both the coil and the magnet at the same time.
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).
If you reverse the voltage sensor leads, the direction of the peaks would reverse for
each pole of the magnet. North would go from negative to positive. South would go
from positive to negative.

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2007                                                                      Alabama Science in Motion

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 ‘south-
south’.
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
    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

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