Text 6. Inductance

When a current flows through a conductor it sets up a magnetic field in the neighbourhood of the conductor. This is negligible in its effects in a number of cases, but there are also many cases where this magnetic field exerts a profound effect upon the circuit. The mag­netic field created by the current is represented by lines of magne­tic flux, these lines consisting of closed loops which are interlinked with the electric circuit, itself necessarily a closed circuit. If the current is steady, the magnetic flux is constant and produces no effect upon the circuit, but if the current changes then the strength of the magnetic field also changes. If the current increases, the total number of lines of magnetic flux is increased, so that the total num­ber of flux-linkages is also increased. It is, however, a fundamental law that whenever the number of flux-linkages changes, an e.m.f. is induced in the circuit linked with the flux. This e.m.f. is proportio­nal to the rate of change of linkages, and one volt is induced when the linkages change at the rate of 10⁸ linkages per second.

Unit of Inductance. — The unit of inductance is the henry, and a circuit is said to possess an inductance of 1 henry if 1 volt is induced when the current changes at the rate of 1 ampere per second. The symbol for inductance is L, so that a circuit possesses an inductance of L henries if L volts are induced due to a rate of change of current of 1 ampere per second.

This induced e.m.f. always acts in such a direction as to oppose the change of current in the circuit, and also the magnetic flux linked with it. Thus, if the current is rising, inductance tends to oppose its growth, and if the current is falling inductance tends to oppose its decay. Examp­les of this effect are found in the field circuit of an ordinary generator or motor, the field circuit being highly inductive. When switched into circuit the current does not immediately rise to its full value, but grows relatively slowly, while when the switch is opened, the current tends to continue as evinced by the spark at the opening contacts.

Mutual Inductance. — When inductance is due to flux linking with the same circuit that carries the current, it is called self-induc­tance, in order to distinguish it from inductance due to flux linking with the circuit due to current in a neighbouring circuit, which is called mutual inductance. In the latter case, two circuits are said to possess mutual inductance if a change of current in one circuit caus­es an e.m.f. to be induced in the other circuit.

Two circuits are said to possess a mutual inductance of L henries if L volts are induced in one circuit due to a rate of change of current of 1 ampere per second in the other circuit. The circuit in which the current is changing is called the primary circuit, while the circuit in which the e.m.f. is induced is called the secondary circuit. When two circuits possess mutual inductance, either circuit can be employed as the primary, the value of the mutual inductance being the same, no matter which circuit is the primary and which is the secondary.

Importance of Inductance in A. C. Circuits. — Inductance is a property of a circuit, just as is resistance, and is therefore possessed by d.c. as well as a.c. circuits. In d.c. circuits, however, its effects are not apparent when the current is steady, and are only noticeable when the current is started or stopped, or when it changes in value. The reason for this is that the induced e.m.f. resulting from induc­tance is due to thfc rate at which the current, and therefore the flux, is changing. If the current does not change, there is no induced e.m.f. In a.c. circuits, on the other hand, the current is always changing, and therefore the e.m.f. of self-induction is always present. This modifies the value of the current and has far-reaching effects.

Notes

inductance - індуктивність

negligible - незначний

to exert - напружувати (силу)

flux - постійний рух

e.m.f.= electromotive force - електрорушійна сила

henry - одиниця самоіндукції

to evince - виявляти, показувати

circuit - цикл; кругообіг

apparent - очевидний, явний

to modify - видозмінювати

far-reaching - далекосяжний

Comprehension questions:

1. What is the unit and the symbol of inductance?

2. How do we distinguish self-inductance and inductance?

3. What properties of inductance did you learn from the text?