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Analyzing Magnetic Fields with Solenoids
Running head: ANALYZING MAGNETIC FIELDS WITH SOLENOIDS
Analyzing Magnetic Fields with Solenoids in Introductory Physics
James Kennicutt
Physics 690
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Analyzing Magnetic Fields with Solenoids
Abstract
Student difficulty understanding electricity and magnetism is a common problem in many
physics courses. Constructing simple solenoids out of a D-Cell battery, copper wire, a
nail, and a straw can help students understand this difficult topic. This experiment is
designed to help students visualize magnetic fields created by moving electrons in a
current carrying wire. Students also learn about the intensity of magnetic fields when an
iron core is introduced into the solenoid.
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Analyzing Magnetic Fields with Solenoids
Analyzing Magnetic Fields with Solenoids in Introductory Physics
James Kennicutt Dept. of Physics, SUNY-Buffalo State College, 1300 Elmwood Ave,
Buffalo, NY 14222 <[email protected]>
Abstract:
Student difficulty understanding electricity and magnetism is a common problem in many
physics courses. Constructing simple solenoids out of a D-Cell battery, copper wire, a
nail, and a straw can help students understand this difficult topic. This experiment is
designed to help students visualize magnetic fields created by moving electrons in a
current carrying wire. Students also learn about the intensity of magnetic fields when an
iron core is introduced into the solenoid.
Acknowledgement: This manuscript partially fulfilled requirements for PHY690:
Master's Project at SUNY- Buffalo State College, advised by Dr. Dan MacIsaac.
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Analyzing Magnetic Fields with Solenoids
One area in which physics students tend to struggle is magnetic fields and the
effects of electromagnetism. In order to help students understand this topic, it is
important to provide them the opportunity to gain visual and kinesthetic experience with
electromagnetism (REF1). Allowing students to construct three-dimensional models of a
solenoid, or a wire wrapped in a coil, can be beneficial for helping students with
visualizing magnetic fields (REF3). The activity of constructing these models may help
students to understand this topic and is also a low cost and simple experiment to conduct.
This will also facilitate the exploration of the interactions of solenoids with different
materials, the effects of different designs on magnetism, and the calculation of magnetism
based on permeability.
In order for students to gain the most knowledge from this activity, they should
already have a basic understanding of magnetic fields surrounding permanent magnets
and current-carrying wires. For help understanding this, they could have completed an
activity using a compass and iron fillings to view various magnetic fields. This activity
should help students become comfortable using the Right Hand Rule, predicting the
magnetic field around permanent magnets and current carrying wires, and understand
basic magnetic fields created by moving electrons. Then, they should be prepared to
construct a solenoid and predict the magnetic fields around them.
One of the most beneficial parts of this activity is that the materials required to
complete it are both inexpensive and easily acquired. You will need a length of straw,
some copper wire (about 3 feet per person), an iron nail (about 9 cm long), D-Cell
battery, compass, and a few paper clips. These materials can be purchased at a variety of
stores on the Internet or locally, and can cost as little as $10.00 for a class of 25 students.
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Analyzing Magnetic Fields with Solenoids
One thing to keep in mind while running this experiment is that you are essentially
shorting the battery. This will cause a large amount of current to flow through the wire,
which can heat the wire up in a short amount of time. To decrease the chances of a
student burning themselves, remind them to give the circuit a break every 30 seconds.
The D-Cell battery in this experiment will in essence be shorted causing the wire
to become warm. It is advantageous to allow the battery to rest every 30 seconds to allow
the cell to cool down and to ensure you do not to burn yourself on the warm wires.
Constructing one of the solenoids is very straightforward, so students can
construct their own solenoids to learn about the design and gain a better idea of the
magnetic field it creates. Before the activity, be sure that both ends of the copper wire are
stripped if you are using a coated wire and cut the straw to match the length of the nail.
You can then have your students wrap the wire around the straw with the nail placed
inside for support. In order to discuss the influence of the number of loops on the
magnetic field, you may wish to ask your students to note the number of loops they wrap
the wire around the straw. The wire should not be wrapped too tightly around the straw
because you want them to be able to insert and remove the iron nail so that the solenoid
can have either an open core (no nail) or an iron core (nail inside). There should be a few
inches of unwrapped wire at both ends of the solenoid so that there is enough room to
press the stripped sections of wire to the battery terminals (insert pictures). Now that the
solenoid is complete, connect the two ends to a D-Cell battery to the stripped sections of
wire to send a current through the wire and create a magnetic field around the solenoid.
You may find it beneficial to have your students explore different designs for
their solenoids in order to observe the effect that these changes could have on the
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Analyzing Magnetic Fields with Solenoids
magnetic field. Any alterations, even as simple as wrapping a different number of loops
around their straws, can produce interesting results that they can then compare to their
partner’s solenoid. Students should find that some solenoid designs create a larger
magnetic field than other designs. For instance, a solenoid with twice as many wraps of
coil should produce a field twice as strong, just as a solenoid with a third as many coils
should produce a field a third as strong. Some other factors you could change include
allowing the students to wrap the solenoids less tightly, wrap the solenoid in different
directions around the straw, or possibly put loops of wire on top of one another, while
always keeping in mind the number of loops they wrap around the straw. Other
interesting extensions to this activity may include using bent nails and circular pieces of
iron that may create interesting magnetic fields.
When your students have all constructed their individual solenoids there are a
variety of tasks you could have them complete to explore different interactions with a
compass and paper clip as an open or iron core solenoid. You could begin these tasks by
instructing them to connect the D-Cell batteries to the open core solenoids and observe
the interactions between their solenoids, a compass, and paper clips. You could discuss
these interactions and ask them to create a sketch of the solenoid including the direction
of current flow and polarity of the solenoid’s ends. Also, by assuming the permeability of
the open core solenoid, you could calculate the strength of the magnetic field created (see
questions). Then, if you asked your students to put the nail back into the middle of the
solenoid, they could see how it interacts with the compass and paper clips as an iron core
solenoid. With these iron core solenoids, you could use the permeability of iron to
estimate the new magnetic field strength inside the solenoid. Discussing these
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Analyzing Magnetic Fields with Solenoids
interactions and performing these calculations may help students connect the visual
examples of solenoids to physics applications.
After students have completed their own solenoids and observed how different
designs create different effects, they should be able to use a commercial solenoid and
understand how they are constructed and how they work. If you have access to a ring
flinger or some other apparatus that creates a strong magnetic field you can shock and
impress your students by demonstrating how strong a solenoids’ magnetic field can get.
A strong follow up to this experiment would help to show students how solenoids play a
major role in many electronic devices in our lives.
To further supplement this activity, you may also find it beneficial to introduce
students to the simulations found on The University of Colorado at Boulder’s Phet
website. Specifically the simulations Generator 2.02 and Faraday’s Law 2.00 (REF2).
Both applets have detailed and interesting solenoid diagrams that would allow your
students to see the magnetic field created by the solenoid as well as how a magnet can
create a current through a solenoid. These applets may be helpful for students to
understand the interactions between magnetism and current flow in their solenoids.
Constructing solenoids is a simple and low-cost activity that allows students to
see first-hand how the different properties of a solenoid affect the magnetic field
surrounding it. With the construction of simple solenoids and the use of these
supplemental diagrams, you can provide your students a visualization of the topic while
exploring some of the properties and calculations associated with magnetic fields.
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Analyzing Magnetic Fields with Solenoids
The activity described above can be taught through whiteboarding (REF4) and
the following questions can be used to help guide the lesson (REF3).
Q1. What should the magnetic field
A1. The magnetic field is concentrated into nearly
for your solenoid look like? (Hint: use uniform field in the center of a long solenoid. The
right hand rule).
field outside is weak and divergent.
Insert diagram
Q2. Sketch, label and confirm the
A2. …
polarity of the B field using a small
compass.
Q3. Using the number of turns/meter
A3. B= µnI
(n) and assuming a current of 1.0
µ =4π*10-7 Tm/A
amps, what is the strength of the B
n = 545.45n
field (in Teslas) in the center of your
I = 1.0 Amps
solenoid?
B=(4π*10-7 Tm/A)(545.45n)(1.0A) = 6.86*10-4 T
Q4. How does this strength compare
A4. The Earth's magnetic field strength is 10-12 T, so
to the Earth's magnetic field?
the field strength of our solenoid is roughly eight
orders of magnitude stronger.
Q5. Can you pick up paper clips or
A5. One cannot pick up paper clips, but the compass
staples with your solenoid? Does
does move.
your solenoid deflect the compass
needle?
Q6. Replace the nail or bolt making
A6. Two paper clips can be picked up and the magnet
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Analyzing Magnetic Fields with Solenoids
the device an iron core solenoid.
greatly affects the motion of the compass needle. It is
Now how many paper clips can you
much stronger than the one without an iron core.
pickup? How well does the
electromagnet deflect the compass
needle? Compare with the solenoid
without the iron core.
Q7. miron= 2000 mo. What does this
A7. The strength is 2000 times greater because the
mean about your electromagnet? 3b.
iron core causes the current to be centrally located.
How much stronger is the field
strength and why?
Q8. Comment on how an
A8. Electromagnets are used for many things such
electromagnet might be used.
as; moving items, trains. MRI tests, etc. Another
Identify a device that contains an
example is a doorbell, which uses an electromagnet
electromagnet and discuss how the
in the form of a solenoid that pulls on a spring-
magnet is used.
loaded striker. When the doorbell button is pressed,
the electromagnet is switched on, and the striker is
pulled back. It strikes one bell, and then--as the
person at the door releases the button--it strikes the
other. (1--Ding! 2--Dong!) Try holding the button
down and then releasing it slowly as you ring the
doorbell next time and observe this effect.
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Analyzing Magnetic Fields with Solenoids
References:
REF1 A. Arons, Teaching Introductory Physics, (John Wiley & Sons, New York, 1997)
REF2 M. Frankel, Physics Simulations, Retrieved July 2, 2009, from PHet Interactive
Simulations. Web site:
http://phet.colorado.edu/simulations/index.php?cat=Electricity_Magnets_and_Circuits
REF3 D. MacIsaac, Solenoids and Electromagnets. Retrieved March 24, 2009, from
Physics Education at Buffalo State College, Web site:
http://physicsed.buffalostate.edu/SeatExpts/EandM/solenoid/index.htm
REF4 D. MacIsaac, K. Falconer, “Whiteboarding in the Classroom,” Manuscript in
preparation, available from the authors, (2004).
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