Hoi Huynh Clemente Community Academy
1147 North Western Avenue
Chicago, Illinois 60647
(312) 534-4000


To experience another kind of force (beside gravity or electric charge ...)
which is always present and affecting us: magnetism.
To explain how magnets, magnetic poles and magnetic fields are related.
To formulate the force law for magnets: (F = k mm')
r2 Materials:

Bar and cow magnets, magnetic compasses, rulers, cereals and papers


Activity 1. Grind the cereal into very fine pieces. Use the cow magnet to
attract iron from the cereal.

Activity 2. On the projector, show the two poles of two magnets and how
they attract or repel each other.

Activity 3. Break a bar magnet into two halves. Turn a half magnet around
on the projector to show they repel each other; magnetic poles always occur in
pairs on each magnet. (Otherwise the north half and the south half poles should
always be attracted.)

Activity 4. Place a bar magnet on a large piece of paper. Place a compass
near the north of the magnet. When the needle comes to rest, make a dot or an
arrow on the paper to mark the direction in which the north pole of the compass
needle is pointing. Move the compass until the south pole of the needle points
to the mark. When the needle is again at rest, make another dot to mark again
the direction. Repeat this procedure and connect the points by a smooth curve.
Repeat the procedure to draw a few field lines on each side of the magnet.
Notice that each line begins at the north pole of the magnet and ends at the
south pole. Do any of the field lines cross? The rate at which the needle
quivers as it comes to a stop is proportional to the field strength. FOR THE
TEACHER: On the projector, cover a bar magnet with a transparency. Sprinkle
some iron filings on the transparency while gently tapping it. Compare the
pattern made by the filings and the field lines.

Activity 5. Place a magnetic compass on the middle of a metric ruler so
that the north and south of the needle is perpendicular to the ruler. Place a
bar magnet on each side of the compass on the ruler with the south-seeking poles
of the magnets pointing toward the compass. When the compass needle stops at
the old position again (pointing north), compare the distances from the compass
to the two magnets. If the distances are the same, then the magnets have the
same strengths (m=m'). If not, the north-seeking compass needle points toward
the stronger magnet (m not = m'). Move the stronger magnet along the ruler until
the forces of the magnets balance. The ratio of the square of the distances is
proportional to the strength of the poles.

Activity 6. Place two magnets on the overhead. Use Newton's Third Law to
"show" that the magnets pull (or push) equally on each other. Therefore, the
force must be proportional to each magnet's strength (m, m'). Show that the
pull or push (F) is larger when the distance (r) between the magnets is less,
thus the force (F) is inversely proportional to the distance (r). It is not
obvious that the force is inversely proportional to the square of the distance ,
but the similarity to the Gravitational Force (Fg) and the Electric Force (Fe)
should lead to acceptance of the square of the distance between the magnets in
the formula:
F = k mm' r2

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