TITLE: Magnetic Fields and Bermuda Triangles
AUTHOR: Alfons N. Bouchier; Los Alamos High School,
Los Alamos, NM
GRADE LEVEL/SUBJECT: 8-12
OVERVIEW: The magnetic field around a permanent magnet, like the
gravitational field around a massive object, is not only invisible,
but hard for students to comprehend. With no concrete experience to
draw from, they tend to ignore this basic concept, or at best,
memorize facts about it.
PURPOSE: This activity shows how to map a magnetic field, and to
find how a bar magnet's field combines with the Earth's magnetic
field to form a complex resultant field.
OBJECTIVES: Students will be able to:
1. Use a compass to determine the direction of a magnetic field at
any point in space.
2. Explain how two magnetic fields combine.
3. Predict the location of areas where two fields can cancel each
other to leave no resultant field. ( "Bermuda Triangles" )
RESOURCES/MATERIALS: Weak to moderate strength bar magnets
correctly labeled, very small compasses (sold as 'flux path
compasses' ), butcher paper.
ACTIVITIES AND PROCEDURES:
1. Ask students to write down several things that they already
know about force fields. After several minutes, ask for volunteers
to share ideas. Work up interest in the topic.
2. Demonstrate the familiar field around a bar magnet with iron
filings. Ask them how this is affected by Earth's magnetic field.
Point out that with a more sensitive detector they can see how the
two combine.
3. Have each student team :
a. tape a large square of butcher paper down to their table,
large enough that a compass can lose a bar magnet's effect. Lay a
bar magnet on the paper and trace around it. Label N and S.
b. lay a small compass on the paper, touching the bar magnet
anywhere. Place a dot outside the compass case, in line with the
end of the needle away from the magnet. Move the compass in the
direction the needle points until it is just beyond the dot you just
made. Place a second dot where the needle points now. Continue to
move the compass and make dots until you reach another point on
the bar magnet or the edge of the paper. Connect the dots with a
smooth curve. Repeat for other lines, spread out more or less evenly
over the area.
c. determine and label a direction for each line. The N-seeking
tip of a compass will point to the N pole of the Earth or the S end of
a bar magnet. Place arrowheads on each line.
d. find and label areas where each of the two fields dominate so
much that they appear to be the only factor. Find and label an area
where the two fields cancel, and explain their evidence for this
cancelling.
4. Post the field maps. Have each team explain which side of their
map faced geomagnetic N, and how the geomagnetic lines were
"warped" by the bar magnet. Discuss any crossed field lines - is this
really possible?
TYING IT ALL TOGETHER:
1. Make and discuss overhead transparencies of the geomagnetic
field as seen from space, stretched by the solar wind.
2. Ask for similarities to gravitational and electrical fields, as
well as differences.
3. For that special group that really needs some extra attention, be
sure to tape an extra magnet to the bottom of their table before
class. Ask them to hypothesize why theirs is the only field map with
an extra series of curves.....
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John Kurilecjmk@ofcn.org