10. Complete the following sentences:

1. 1. If I asked the teacher to explain grammar to me she... 2. If he had been to England last year, he... 3. If I asked my sister to bring me the dictionary, she …4. Iif she asked her friend to help her, he … 5. If they had been asked to carry the experiment out, they …

2. 1. … provided the weather were fine. 2. … unless it rains. 3. … if we are free on Sunday. 4. … provided we had had time yesterday. 5. … if you brought me the textbook.

11. Speak on the heating effect of an electric current.

12. Translate the following text.

Semiconduction and Solid State

In a solid, electrical current is carried by electrons. These are free to move along the solid when an external electrical field is applied to it. Conductivity can be shown to be a function of the density of free electrons and the ease with which they move. Whatever is the nature of these conducting electrons, they follow the laws of quantum mechanics when they pass from one power state to another, and they do so by emission or absorbtion of definite quantities of power.

LESSON 16

THE MAGNETIC EFFECT OF AN ELECTRIC CURRENT

The invention of the voltaic cell in 1800 gave electrical experimenters a source of a constant flow of current. Seven years later the Danish scientist and experimenter, Oersted, decided to establish the relation between a flow of current and a magnetic needle. It took him at least 13 years more to find out that a compass needle is deflected when brought near a wire through which the electric current is flowing. Before he made his important discovery Oersted had tried many times to place a current-carrying wire at right angle to the magnetic needle, nevertheless he could detect no deflection. At last, during a lecture and in the presence of his students, he adjusted by chance a wire parallel to the needle. Then both he and his class saw that when the current was turned on, the needle deflected almost at right angles towards the conductor. When the direction of the current was reversed, the direction of the needle pointed in was reversed too. With the current flowing from left to right, the north end of the needle moves away from us as seen in Fig. 10. Oersted also pointed out that when the wire was adjusted below the needle, the deflection

was reversed. Hence, the movement of the needle also depended on the position of the wire, above it or below it.

The above-mentioned phenomenon highly interested Am­pere. (That the unit of current is named after the famous French physicist and mathematician is probably known to everyone.)

Ampere heard of Oersted's achievements and he, in hi: turn, repeated the experiment and added a number of further valuable observations and state­ments. His contribution to "electrodynamics," as he him­self called the new science, began in 1820 under the influ­ence of Oersted's discovery and continued throughout the rest of his life. Everyone knows the rule thanks to which we can always find the direction of the magnetic effect of a current, i It is known as Ampere's rule. It was Ampere who estab­lished and proved that magnetic effects could be produced with­out any magnets, by means of electricity alone. He turned his attention to the behaviour of electric current in a single straight conductor and in a con­ductor that is formed into a coil, i.e. a solenoid.

When a wire conducting a current is formed into a coil of several turns, the amount of magnetism is greatly increased. It is not difficult to understand that the greater the number of turns of wire, the greater will be the m.m.f. (that is the mag­neto-motive force) produced within the coil by any constant amount of current flowing through it. In addition, when doubling the current, we double the magnetism generated in the coil. However, we must not forget that an electric charge at rest does not produce any magnetic effect at all.

VA solenoid has two poles which attract and repel the poles of other magnets. While suspended, it takes up a north and a south direction exactly like the compass needle. A core of iron becomes strongly magnetized if placed within the sole­noid while the current is flowing."

When winding a coil of wire on an iron core, we obtain what is called an electromagnet. It is, so to say, a temporary magnet provided by electricity. Its behaviour is very simple. The device is lifeless unless an electric current flows through the coil. However, the device comes to life provided the cur­rent flows. The iron core will act as a magnet as long as the current continues passing along the winding. One may ask: "What advantage does an electromagnet possess over an ordi­nary magnet since both can attract and repel magnetic mate­rials?" That electromagnets are controllable and reliable magnets is perhaps known even to a schoolboy. They become strong magnets when we want them to. They will lose their magnetic properties as soon as the current is turned off.