Matter and Motion
Magnetic Induction Lab
Introduction
In this lab you will conduct qualitative, quantitative and practical applications of the electromagnetic
induction.
Pre-lab Activity
A bar magnet is released above a circular loop of conducting wire as shown, and falls through the loop.
The loop is held fixed. Determine the following directions, as viewed when looking at the loop from
above.
side view
1) What is the direction of any induced current in the wire loop, while
the bar magnet is falling towards the loop from above? (as viewed when
looking at the loop from above)
S
S
N
Clockwise
Counterclockwise
No induced current
view from
above
Explain your reasoning clearly and compare with your lab partner.
side view
3) What is the direction of any induced current in the wire loop, while the bar
magnet is falling away from the loop from below? (as viewed when looking
at the loop from above)
Clockwise
Counterclockwise
No induced current
Explain your reasoning clearly and compare with your lab partner.
S
S
N
view from
above
4) The current in the loop set up a magnetic field and acts like another bar magnet. What is the orientation
of this bar magnet (N pole up or S pole up)? What effect would the equivalent magnetic moment of the
loop have on the falling magnet?
5) ) The current in the loop set up a magnetic field and acts like another bar magnet. What is the
orientation of this bar magnet (N pole up or S pole up)? What effect would the equivalent magnetic
moment of the loop have on the falling magnet?
Activity 1: Magnets Dropping Through Tubes – Observations
Equipment
• PVC tube
• Copper tube
• 1 thin cylinder magnet
1) In fall quarter, we observed the motion of a magnet falling through a vertically oriented PVC tube and
through a vertically oriented copper tube. Repeat this activity, paying close attention to the motion of the
magnet in both cases, noting similarities and differences. Make sure to catch the magnet either in your
hand or in the Styrofoam cup cushioned with paper towels.
2) Use a force analysis to explain qualitatively the motion of the magnet when falling through the PVC
tube. By this, I mean draw a free body diagram and sketch out qualitative the acceleration vs. time,
velocity vs. time, and position vs. time graphs.
3) Use an energy analysis to explain qualitatively the motion of the magnet when falling through the PVC
tube. By this, I mean draw before and after diagrams showing different types of energy, and jot down
work and energy relationships.
4) Modify your force analysis to discuss the motion of the magnet when falling through the copper tube.
By this I mean, discuss how you know that there must be another force or forces acting in this case, and
what you can say about the direction and magnitude of these other forces.
5) Modify your energy analysis to discuss the motion of the magnet when falling through the copper tube.
By this I mean, discuss how you know that there must be another form or forms of energy to keep track of
in this case, and what you can say about these other forms of energy.
6) Use your conclusions from the prelab questions to explain your observations regarding the magnet
falling through the copper tube (imagine the tube as consisting of lots of conducting loops either above or
below the falling magnet). Also, your explanation should account for the difference between the PVC and
copper.
Activity 2: Magnets Dropping Through Tubes – Quantitative Measurements
Equipment (available at front table, work in groups of 3)
• PVC tube
• 3 cylinder magnets
• 3200-turn coil
• Red and black banana plug lead to clip leads (you might need to use banana plug to banana plug
leads instead)
• LabQuest + power supply + USB connector
• Voltage sensor (connects to LabQuest)
• Styrofoam cup and paper towels (to catch falling magnets)
1) Consider a magnetic dipole (perhaps in the form of a cylindrical bar magnet) moving through a
horizontally oriented conducting loop, with the N pole end of the magnet facing down (see the figures on
the previous page). Remind yourself of what the magnetic field of a magnetic dipole looks like, by
sketching the field for a magnet oriented vertically with N pole pointing down.
2) Use your sketch of the field lines from a bar magnet to determine what the magnetic flux through the
conducting loop is, as the bar magnet begins above the loop, approaches the loop, goes through the loop,
emerges below the loop, and moves away from the loop. Sketch a qualitative graph of magnetic flux vs.
time.
3) Use your qualitative graph of magnetic flux vs. time to sketch a qualitative graph of emf vs. time.
dΦ B
Recall that ε = −
. Draw your two graphs so that the horizontal axes are lined up, so that you can
dt
identify times of important features as being at the same time (similar to what we do for position vs. time
and velocity vs. time graphs). Compare your graphs with your lab partner and show it to David.
5) Now, make the measurement directly. Hook up the 3200-turn coil to the LabQuest. Hold the coil so
that its opening is aligned vertically, with the side labeled 3200 facing up. Put the PVC tube through the
coil vertically; position the coil roughly ¾ of the way down from the top of the PVC tube. Place the
styrofoam cup with paper towels under the bottom of the PVC tube to provide a cushiony place for any
dropped magnet to land. Adjust the data collection in LoggerPro so you can take 1000 samples per second
as possible, and adjust the collection time down to 2 seconds (more than enough for dropping the magnet
down the PVC tube)
6) Hit Collect, drop the magnet with its N pole pointing down, and obtain a graph of potential (emf) vs.
time. You may need to play around with this some, working on timing and other experimental details.
With a sample rate of 1000/second, the computer might not be able to plot in real time.
7) Once you obtain a graph of potential vs. time, zoom in on the feature of interest until it
fills the screen as symmetrically as possible. Print this graph and copy it into your lab
notebook, and label it appropriately. Compare this graph to the graph you showed to your
instructor. How did you do?
8) Obtain a graph of potential vs. time, this time with the S pole facing down. Again, zoom
in on the graph and print it for you lab notebook. Does this look like you’d expect?
N
S
S
N
S
N
9) This time, drop two magnets, with oppositely oriented poles down the tube. In order to do
this, you’ll need some scheme to release the bottom magnet. I’ve indicated such a scheme in
the figure – please consult with me if you’re not sure how to interpret the figure. Obtain a
graph of potential vs. time for this situation, zoom in on the graph, print it for you lab
notebook. What do you think about this graph?
Actitivty 3: A Primitive Microphone
Build the following a primitive microphone. Place a small magnet part way
into a 2" x 4" block. Attach a small wire coil to Scotch tape and fix the tape taught between the too tacks
in the block so the coil is just above the exposed end of the magnet, but not touching it. Attach the ends of
the coil to an amplifier that is connected to a speaker. With the appropriate settings, you should be able to
speak with your mouth near the coil and have the sound amplified and
projected by this system. How does this "microphone" work?
Postlab Activity
In the Dilbert cartoon shown, they
claim that their method will generate
electricity.
a) In this situation, how do they think
that mechanical energy (spinning)
could be turned into electrical energy
(electricity)?
b) Let’s examine this situation a little more closely. Let’s say that the field lines from a horseshoe magnet
are as pictured, and that the coil of copper wire has the orientation shown. Also, say the copper wire spins
in the plane of the page as shown. What do you think about the claim made in the cartoon? Briefly
explain your answer.