It was noted previously in this section that an electric
current flowing through a conductor creates a magnetic field around the
conductor. In Fig. 2.9, the shaded circle represents a cross section of a
conductor with current flowing in toward the paper. The
current is flowing from negative to positive.
When the current flows as indicated, the magnetic field is
in a counterclockwise direction. This is easily determined by the use of the
left-hand rule, which is based upon the true direction of current flow. When a
wire is grasped in the left hand with the thumb pointing from negative to
positive, the magnetic field around the conductor is in the direction that the
fingers are pointing.
If a current-carrying wire is bent into a loop, the loop
assumes the properties of a magnet; that is, one side of the loop will be a
north pole and the other side will be a south pole. If a soft-iron core is
placed in the loop, the magnetic lines of force will traverse the iron core and
it becomes a magnet.
When a wire is made into a coil and connected to a source of
power, the fields of the separate turns join and thread through the entire coil
as shown in Fig. 2.10a. Figure 2.10b shows a cross section of the same coil.
Note that the lines of force produced by one turn of the coil combine with the
lines of force from the other turns and thread through the coil, thus giving
the coil a magnetic polarity.
The polarity of the coil is easily determined by the use of
the left-hand rule for coils: When a coil is grasped m the left hand with the
fingers pointing in the direction of current flow, that is, from negative to
positive, the thumb will point toward the north pole of the coil.
When a soft-iron core is placed in a coil, an electromagnet
is produced. Of course, the wire in the coil must be insulated so that there
can be no short circuit between the turns of the coil. A typical electromagnet
is made by winding many turns of insulated wire on a soft-iron core which has
been wrapped with an insulating material.
The turns of wire are placed as close together as possible
to help prevent magnetic lines of force from passing between the turns. Figure
2.11 is a cross-sectional drawing of an electromagnet. The strength of an
electromagnet is proportional to the product of the current passing through the
coil and the number of turns in the coil.
This value is usually expressed in ampere-turns. If a
current of 5 amp is flowing in a coil of an electromagnet and there are 300
turns of wire in the coil, the coil will have an mmf of 1,500 amp-turns. Since
the gilbert is also a measure of mmf and 1 amp-turn is equal to 1.26 gilberts,
the mmf may also be given as 1,890 gilberts.
The ultimate strength of the magnet also depends upon the permeability
of the core material.
The force exerted upon a magnetic material by an
electromagnet is inversely proportional to the square of the distance between
the pole of the magnet and the material. For example, if a magnet exerts a pull
of 1 Ib upon an iron bar when the bar is f in. from the magnet, then the pull
will only be & lb when the bar is 1 in. from the magnet.
For this reason, the design of electrical equipment using
electromagnetic actuation requires careful consideration of the distance
through which the magnetic force must act. This is especially important in voltage
regulators and relays.
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