Read the text and ask 6-8 questions. An important aspect of the operation of a coil is its property to oppose any change in current through it

Inductance

An important aspect of the operation of a coil is its property to oppose any change in current through it. This property is called inductance. To explain it, the coil will first be straightened out and its functioning as a straight piece of wire will be discussed.

When a current of electrons starts to flow along any conductor, a magnetic field starts to expand from the center of the wire. These lines of force move outward, through the conducting material itself, and then continue into the air. As the lines of force sweep outward through the conductor, they induce an e.m.f. in the conductor itself.

Because of its opposing direction it is called a counter e.m.f., or a back e.m.f. The direction of this self-induced counter e.m.f. can be verified by using the left-hand generator rule.

The effect of this backward pressure built up in the conductor is to oppose the establishment of a maximum value of current. It must be understood that this is a temporary condition. When the current eventually reaches a steady value in the conductor, the lines of force will no longer be expanding or moving and there will no longer be any counter e.m.f. produced. At the instant when current begins to flow, the lines of force are expanding at the greatest rate and the greatest value of counter e.m.f. will be developed. At the starting instant the counter e.m.f. has a value just short of the applied, or source voltage. The current value is small at the start, of current flow. However, as the lines of force move outward, the number of lines of force cutting the conductor per second becomes progressively smaller and the counter e.m.f. becomes progressively less. After a period of time the lines of force expand to their greatest extent, the counter e.m.f. ceases to be generated, and the only e.m.f. in the circuit is that of the source. Maximum current can flow in the wire of circuit, since the inductance is no longer reacting against the source voltage. When a varying d-c is flowing, as the source voltage increases, the counter e.m.f. opposes the increase. As the source voltage decreases, the self- induced e.m.f. opposes the decrease. An alternating current is in a constant state of change and a continual opposing or reacting results. From this behavior comes the definition: Inductance is the property of any circuit which opposes any change in current. The unit of measurement of inductance is the henry, defined as the amount of inductance required to produce an average counter e.m.f. of one volt when average current change of one ampere per second is under way in the circuit. Inductance is represented by the symbol L in electrical problems, and henrys by h.

3. Are the following sentences True or False?

1. When a current of electrons starts to flow along any conductor, a magnetic field stays permanent.

2. This induced voltage is always in a direction opposite to the direction of current flow.

3. The current value is big at the start of current flow.

4. The unit of measurement of inductance is the watt.

5. Inductance is represented by the symbol L.

6. Inductance is the property of any circuit which opposes any change in current.

7. As a source voltage decreases, the self-induced e.m.f. opposes the increase.