Automotive Electrical and Electronic Systems Classroom Manual Fifth Edition Update by John F. Kershaw

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Magnetism 57
Figure 4-8. Right-hand rule for field direction; used
with the conventional flow theory.
Figure 4-9.
Current direction symbols.
with the thumb pointing in the direction of
conventional current flow (positive to negative),
and the finger tips will then point in the direction of
the magnetic lines of force, as shown in Figure 4-
8. For the rest of this chapter, the electron-flow theory
(negative to positive) and the left-hand rule
are used.
Field Interaction
The cylinders of flux surrounding current-carrying
conductors interact with other magnetic fields. In
the following illustrations, the cross symbol (+)
indicates current moving inward, or away from
you. It represents the tail of an arrow. The dot symbol
(.) represents an arrowhead and indicates current
moving outward, or toward you (Figure 4-9).
If two conductors carry current in opposite
directions, their magnetic fields are also in opposite
directions (according to the left-hand rule). If
they are placed side by side, Figure 4-10, the
opposing flux lines between the conductors create
a strong magnetic field. Current-carrying conductors
tend to move out of a strong field into a weak
field, so the conductors move away from each
other (Figure 4-11).
If the two conductors carry current in the same
direction, their fields are in the same direction. As
seen in Figure 4-12, the flux lines between the
two conductors cancel each other out, leaving a
very weak field. In Figure 4-13, the conductors
are drawn into this weak field; that is, they move
closer together.
Figure 4-10.
fields.
Figure 4-11.
fields.
Conductors with opposing magnetic
Conductors will move apart into weaker
Motor Principle
Electric motors, such as automobile starter
motors, use field interaction to change electrical
energy into mechanical energy (Figure 4-14). If
Figure 4-12.
fields.
Conductors with the same magnetic
two conductors carrying current in opposite
directions are placed between strong north and
south poles, the magnetic field of the conductor