Unit 13. Electromagnets and their uses

It is easily seen that in a solenoid there is a complete field around each turn. These fields are somewhat modified by the adjacent turns, there being many straight lines coming out from the sides of the coil. Should the coil be wound on a core of soft iron of high permeability, the core would absorb these straight lines and give a concentrated field from one end of the core to the other. We then have an electromagnet. We use soft iron not only because of its having a high permeability, but because its low retentivity allows very little residual magnetism when the current is turned off. The strength of the magnet increases with the number of amperes flowing and also with the number of turns. The product of amperes and turns is called ampere turns. The strength of an electromagnet with a given core is known to be proportional to the number of ampere turns, the strength of the field depending on the shape of the core. If the poles were brought together into a U shape, the field would become stronger. Of course, the two legs of the U must have opposite poles.

The most obvious use of the electromagnet is in lifting iron weights. They are often capable of holding pieces of iron or steel weighing thousands of pounds; they do not slip, as do hooks and ropes; and they can be operated by the throw of a switch at a distance. We know small, powerful electromagnets to be used by doctors to remove steel particles from the eye. Among the common applications of the electromagnet are the electrical bell, the telephone, the telegraph, radio loud-speaker, circuit breakers, relay for remote control of machines, electrical measuring instruments, motors, and generators.

EXERCISES I. Give antonyms for:

to enter, conductor, to finish, retentivity, to turn off, to reduce, to separate, to forbid, powerless.

2. Give examples on the use of the following words and expressions:

by... is meant, in order to, the former, the latter, to be familiar, therefore, no longer.

2. State what part of speech the words in italics belong to and trans­late the sentences:

1. Our engineers perfect different devices. 2. These devices are perfect in action. 3. Sometimes a small amount of power is quite sufficient for establishing communication over vast distances. 4. The amount of energy consumed by this motor amounts to that of the engine mentioned above. 5. The subject treated above had to do with some innovations achieved in radio engineering. 6. The apparatus used is subject to sudden changes of temperature. 7. This object has a good reflection surface. 8. Specialists object to this material being used as a filament. 9. Note that the density of the lines of force is the greatest at the poles. 10. The direction of the magnetic field at any point P is arbitrarily chosen as the direction in which a small magnetic needle point would point when placed at P without disturbing appreciably the existing conditions. 11. No line of force crosses another. 12. A freely suspended bar magnet lines itself up parallel with the lines of force existing between the north and south magnetic poles of the earth.

2. Observe the different ways of expressing condition in the fol­lowing sentences and translate the sentences with the Subjunctive mood:

1. Providing all the requirements were met, the efficiency of the apparatus would be increased. 2. If a natural magnet were suspended by a string from its centre so that it were free to turn, the magnet would be found to turn until the axis through its poles is lying north and south. 3. Had the conductors been moved up and down so as to cut the line of flux of the magnetic field between the poles of the magnets, an e.m.f.would have been generated at the two ends of the conductors. 4. The electric current will flow through a conductor such as a copper wire, if the two ends of the wire are maintained at a difference of potential.

2. Analyse the different functions of should and translate the following sentences:

1. In order to make the necessary observation the c'urrent should be passed for about half an hour. 2. We were sure we should be able to overcome all the difficulties in our research. 3. Should the temperature decrease, the velocity of electrons will decrease too. 4. Were the filament heated, we should get the electron emission. 5. It is essential that we should use iron as one of the principal magnetic substances in the case in question.

2. Translate the following sentences and observe the different ways of expressing obligation:

1. Where small amounts of power are to be transferred from the generator to the load the condition of maximum power is usually desirable.

2. One ought to know that the electric cell is a device for the 'transformation of chemical energy into electrical. 3. The electrons in two- element tubes have to go enormous distances before they arrive at the plate with their burden of electricity. 4. It should be pointed out that amplifier tubes act as amplifiers when operated with alternating current on their anodes. 5. When a condenser has an appreciable leakage its resistance must be considered to be in parallel with its e. m. f.

2. Be ready to talk about electromagnets and their uses.

3. Translate into English:

Якщо провідник, що утворює замкнений ланцюг (closed circuit), рухається в магнітнім полі так, що він перетинає силові лінії, електрони в провіднику будуть рухатися в напрямку, який залежить (1) від напрямку потоку магнітного поля й (2) від напрямку руху провідника через поле.

Величина е.р. с. у провіднику буде залежати від числа ліній, пересічних за одиницю часу. Те ж саме буде спостерігатися, коли провідник нерухливий, а силові лінії перетинають (cut through) провідник.

2. Supplementary reading.

TEXT 7

MAGNETIC EFFECT OF AN ELECTRIC CURRENT

Spinning electrons have a profound effect on magnetism, it is understandable that a current, which consists of a stream of electrons all moving in the same direction, should exert a magnetic field.

This can be illustrated by sending a current through a wire which passes through a sheet of paper on which there are some iron filings. Again, tapping the paper will show the lines of force, which in this case are circular and concentric with the wire. During the passage of the current, the electrons all spin in the same direction, but when the current ceases to flow the direction of the spin returns to a random distribution and consequently the magnetic field dies.

If the piece of wire carrying the current is formed into a coil, the circular lines of force round the wire combine and produce a magnetic field which is similar to the field produced by a bar magnet. This arrangement, which is called an electromagnet, is illustrated in the picture presented by the teacher.

If a soft iron core is now inserted through the coil of wire, and an arrangement is made for switching the current on and off rapidly, the core will become alternately magnetized and demagnetized so that it will alternately attract and release a light piece of iron. If the piece of iron is pivoted at one end and fitted with a spring it can be made to vibrate in the changing magnetic field. This is the principle of the electromagnetic bell.

The trembler itself switches the current on and oft by making and breaking the circuit from the cell to the coil.

This principle of obtaining mechanical energy from electrical energy is used in the electric motor.