Electromagnetic Induction and Inductors

E.m.f. induced in a coil and the Lenz's Law

In all O.L. physics text-books, you can find the description of experiments similar to those shown in Figure 2.1.

(a) (b)
Fig. 2.1

In Figure 2.1(a), no current is detected when there is no relative motion between the magnet and the coil. In Figure 2.1(b), an e.m.f. is induced in the coil when the magnet moves, or when there is a relative motion between the magnet and the coil, which results in a change of magnetic flux through the coil. The induced e.m.f. causes an induced current flowing through the circuit and their relation is given by :

Induced current = Induced e.m.f. / of the circuit

Lenz's Law states that the direction of the induced e.m.f., and so is the induced current if it exists, always tends to oppose the flux change producing it. In Figure 2.1(b), when the N-pole is approaching the coil, the induced current will flow in a direction such that a N-pole is induced at end A. A repulsion is set up between the two N-poles and the movement of the magnet is thus opposed.


Self-inductance

If the current passing through a coil changes, the magnetic field (a more appropriate term is the magnetic flux) associated with it also changes. An e.m.f. is induced in the coil itself and this type of electromagnetic induction is called self-induction. The coil is called an inductor and is said to possess a self-inductance or simply inductance, symbol L. The unit of L is Henry (H). In more rigorous treatment, 1 H is defined as the inductance to induce an e.m.f. of 1 V in an inductor when the current in it is changing at a rate of 1 A s-1.


Mutual Inductance

When two coils are close to each other, the current flowing in one coil will produce a magnetic flux linking the neighbouring coil. Hence a of this current will induce an e.m.f. in the neighbouring coil. This type of electromagnetic induction is called mutual induction and the two coils are said to have mutual inductance.


Energy stored in an inductor magnetic field

In building up a magnetic field in an inductor of inductance L, energy is supplied from the power source which provides a current I to the coil of the inductor. The energy, symbol W, is stored in the magnetic field and it is given by :

W = LI2 / 2

Example 2.1

What is the energy stored in the coil when A is passing through an inductor of inductance H ?

L = H,      I = A
W = LI2 / 2 = 2 / 2 = J


Different types of inductors

Almost all inductors are in the shape of a coil because maximum inductance per unit volume are obtained by the coil configuration. Some common types of inductors are listed in Table 2.1.

Inductors Circuit Symbol Characteristics and Usage
Large value iron cored inductors (chokes)
  1. control current in a.c. circuits, e.g. in fluorescent lamps
  2. filters for smoothing d.c.
Small value air cored coils used as radio frequency chokes
Dust iron cored coils used as radio tuning circuits
TABLE 2.1


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