Page 56 - Basic Electrical Engineering

The four fingers bend in the direction of current through the coil. The
direction in which the thumb points is the direction of flux produced. In Fig.
1.4 (b), we have shown the cross-sectional view of the same coil. For the

direction of current flow through the coil, cross-sections have been shown by
putting cross and dot convention. The upper side of the coil turns 1, 2, 3, 4, 5

will indicate that current is entering while they will come out from the other
side as shown in the bottom conductor cross-sections. By applying the cork

screw rule also we can determine the direction of the resultant magnetic field
and show the positions of North and South poles formed. If the direction of

current flow through the coil is reversed, the direction of the magnetic lines
of force will be opposite, and hence the positions of North and South poles
will change.

If we apply some alternating voltage across the coil as shown in Fig. 1.5,
the polarity of power supply will change in every half cycle of the applied

voltage. If a sinusoidal ac supply is provided, both the magnitude as well as
the direction of current flow will change. As a result, the magnitude of the

magnetic field produced will change starting from zero value reaching its
maximum value, then getting reduced again to zero, and then becoming

negative. The direction of flux produced will change in every half cycle of
current flow. Such a magnetic field whose magnitude as also its direction
changes is called a pulsating alternating magnetic field. In case of dc supply,

the magnetic field produced will be of constant magnitude and fixed polarity.

Figure 1.5 AC supply to a coil produces an alternating magnetic field of varying magnitude