a. To understand how mechanical motion is produced by magnetic repulsion,
study the actions in foldout 1, in the back of this lesson. Notice the lines of
force in the top picture. They are moving from the north pole to the south pole
and are traveling in almost straight lines. In fact, the lines would be straight
if the ends of the magnets were flat instead of curved. The magnetic lines of
force moving between the north pole and south pole of any magnet always take the
easiest path or route. The easiest path between the two poles is usually a
straight line, because a straight line is also the shortest path.
b. In B of the foldout the lines of force around a current carrying
conductor (wire) are illustrated. The + symbol on the end of the wire means the
current is flowing away from you as you view the wire. With the current flowing in
that direction, the lines of force in the magnetic field around the wire are moving
counterclockwise (note the arrows on the lines of force). If the current is
flowing toward you as you view the wire (notice the dot in the center of the wire
in D of the foldout), the lines of force would be moving clockwise. In other
words, the polarity would be reversed.
c. If a current carrying wire is placed in a magnetic field as in C of the
foldout, notice what happens to the lines of force that are moving from the north
pole to the south pole of the magnet. The lines of force traveling from north to
south bend down in this case because they are pushed downward by the
counterclockwise rotation of the lines of force around the current carrying wire.
Because the lines of force from the north to south pole pieces of the magnet try to
straighten out, they force the current carrying wire up (note the arrow). In D of
the foldout, the current is moving in the opposite direction in the wire and the
magnet's lines of force push down on this wire. Both C and D of the foldout are
good examples of magnetic repulsion (like poles pushing away from each other).
d. In the starter motor, like the generator, increasing the strength of the
pole shoes will increase the number of lines of force. Likewise, increasing the
current flow through the wire will increase the strength of the magnetic field
around the wire. When these magnetic forces oppose each other, as in views C and D
of the foldout, they try to push each other away. The opposing forces can be very
great if the wire is carrying enough current to make the magnetic field very strong.
e. Now, let's bend a wire to form a loop and place the loop in a magnetic
field (fig 1). Nothing happens until we send current through the loop. If we send
current flowing through the loop in the direction shown, the magnet's lines of
force push up on the right side of the loop and down on the left side. This
produces the torque to rotate the entire loop counterclockwise (to the left).
Actually, the loop would probably move only 1/4 of a revolution (90) because it
would be out of the magnetic field of the magnet. The loop would then be straight
up and down instead of straight across as shown.
f. To get continuous rotation we need a magnetic field large enough to
contain the loop. We would also need commutator bars and brushes like we had in
the generator.
OS 010, 6-P2
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