Forming Inductors

Electronic Principles
Page Navigation
21

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.