ELECTRONIC PRINCIPLES - OD1647 - LESSON 1/TASK 1
The direction of this induced voltage may be determined by
applying the LEFTHAND RULE FOR GENERATORS. This rule is
applied to a portion of conductor 2 that is "lifted" and
enlarged for this purpose in figure 12, view A, on the previous
page. This rule states that if you point the thumb of your left
hand in the direction of relative motion of the conductor and
your index finger in the direction of the magnetic field, your
middle finger, extended as shown, will now indicate the
direction of the induced current, which will generate the
induced voltage (cemf) as shown.
In figure 12, view B, the same section of conductor 2 is shown
after the switch has been opened. The flux field is collapsing.
Applying the lefthand rule in this case shows that the reversal
of flux MOVEMENT has caused a reversal in the direction of the
induced voltage. The induced voltage is now in the same
direction as the battery voltage. The most important thing to
note is that the selfinduced voltage opposes BOTH changes in
current. That is, when the switch is closed, this voltage
delays the initial buildup of current by opposing the battery
voltage. When the switch is opened, it keeps the current
flowing in the same direction by aiding the battery voltage.
From the above explanation, it can be seen that when current is
building up, it produces a growing magnetic field. This field
induces an emf in the direction opposite to the actual flow of
current. This induced emf opposes the growth of the current and
the growth of the magnetic field. If the increasing current had
not set up a magnetic field, there would have been no opposition
to its growth. The whole reaction, or opposition, is caused by
the creation or collapse of the magnetic field, the lines of
which, as they expand or contract, cut across the conductor and
develop the counter emf.
Since all circuits have conductors in them, we can assume that
all circuits have inductance. However, inductance has its
greatest effect only when there is a change in current.
Inductance does NOT oppose current, only a CHANGE in current.
Where current is constantly changing, as in an ac circuit,
inductance has more effect.
(1) Forming Inductors. To increase the property of
inductance, the conductor can be formed into a loop or coil. A
coil is also called an inductor.
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