Magnets

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

Objectives:

     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

Strategy:

     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