Английский язык для моряков / Англ Труханова

QUESTIONS

1.What is the role of magnetism in the field of electricity?

2.What kinds of magnet do you know?

3.What is а permanent magnet?

4.What examples of permanent magnets can you give?

5.What is electromagnetism? ·

6.What happens to а wire when electricity flows through it?

7.What is the principle of electromagnetic method of producing elec­ tricity?

8.Where is electromagnetic method of producing electricity used?

3. ELECTRIC CURRENTS AND THEIR PROPERTIES

Electric charges in motion fonn electric current. Electric current is equal to the quantity of electric charges passing а given point in unit time. Conduction is the пате given to а movement or flow ofcharges. The charges may Ье either positive or negative. In electrical circuits the flow of electric charges is considered to Ье а flow of electrons (nega­ tive charges) along а conductor or through а conducting medium. But charges may also Ье ions when the conduction takes place in gaseous or liquid conductors in which the ions are moЬile.

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То make electrons move through а metal conductor, there exists

а driving force called electromotive force (e.m.f.), which moves elec­ tric charges from one point in the circuit to another. When the e.m.f. is applied·to the·ends ofthe wire, th free electrons move in one direction.

The greater the number Ьfparticipating electrons, the greater is the flow ofcurrent.

As it is inipossiЫe to see electric current, its existence is proved Ьу its effects. А current сап heat а conductor, it сап have а chemical action when passing through а solution, or it сап produce а magnetic effect. The currents сап Ье measured Ьу observing their heating, chemi­ cal or magnetic effects.

There exist several kinds of current: direct current, altemating

current, pulsating current.

Direct current (d.c.) is an electric current that flows in one di­ rection only and doesn't change its direction and magnitude at а con­ stant resistance ofа circuit.

The sources ofdirect current may Ье either galvanic elements, or thermoelements, or d.c. generators and accumulators. Besides, direct current may Ье obtained Ьу converting altemating current into direct one with the help ofrectifiers.

Almost all current calculations in d.c. circuits are based on Ohm's law. According to this law, any circuit section, connecting two points of this circuit and not having electromotive force, contains current

l=-R'

where U - voltage or potential difference between these two points,

R - resistance ofthe circuit section connecting them. Ohm's law сап Ье applied to the whole circuit in case there exists only one source ofsupply in it:

where Е - electromotive force ofd.c. source ofsupply; R - resistance of the entire circuit, including intemal resistance ofthesource ofsupply.

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Alternating current (а.с.) is an electric current that changes both its direction and magnitude at fixed intervals of time. The strength ofthe current in alternating circuits, unlike that in direct ones, depends not only on the resistance ofthe entire circuit and the voltage applied to it, but on the inductance and capacitance ofthe circuit sections as well.

Pulsating current is an electric current, alternative in its magni­ tude and staЫe in its direction. lt is produced when conversion ofan alternating current into а direct current in current rectifiers, in anode cir­ cuits ofelectronic lamps etc. takes place.

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QUESTIONS

1.What forms electric current?

2.What is electric current equal to?

3.What charges flow in electrical circuits?

4.What is conduction?

5.What charges flow in gaseous and liquid conductors?

6.What is electromotive force?

7.What happens to free electrons when the electromotive force is ap-

plied to the ends of the wire? 8. What kind of current is called direct current? 9. What are the sources of direct current?

10.How can alternating current Ье converted into direct one?

11.How can Ohm's law Ье applied to а circuit section? То an entire circuit?

12.What is alternating current characterized Ьу?

13.What does the strength of the current in а.с. circuits depend on?

14.When is pulsating current produced?

4.BASIC UNITS OF ELECTRICIТY AND MEASURING DEVICES

There exist three basic electrical units in any electrical circuit:

ampere, ohrn and volt. The ampere is an electrical unit for measuring the strength of

electric current in а circuit. One ampere represents the amount of cur­ rent generated Ьу 1 volt acting through the resistance of l ohm..For mea­

юring the intensity of an electric current in amperes а device ·called ilmmeter is used.

The ohm is an electrical unit for measuring the resistance or op­ )osition to the flow ofcurrent. AII substances show different resistanc­s to the flow ofelectricity through them. With the increase in the tem­ )erature the resistance of all metals increases while the resistance of ;arbon, insulating materials, (electrolytic) solution decreases. One ohm :epresents such а resistance that а one-volt addition to the potential pro-

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duces 1 ampere of current. The device used for measuring resistance is called ohmmeter. The most commonly used device for measuring in­ sulation resistance is megohmmeter or "megger". lt is usually em­

ployed for continuity, ground and short-circuit testing in general electri­ cal power work.

The volt is an electrical unit measuring the extemal force applied to а circuit to overcome the opposition to the flow of current. This force is called voltage and is also referred to as electromotive force or elec­ tric pressure. The electromotive force that causes а current of 1 ampere

to flow through а resistance of 1 ohm equals 1 volt. The device used for measuring voltage is called voltmeter.

One more important unit of electrical measurement is the watt - the unit of power: the power of 1 ampere of current pushed Ьу one volt

of electromotive force. Devices used for measuring delivery of electric energy in watts are called wattmeter and watthourmeter.

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4.What is resistance measured in?

5.What is the difference between the resistance of metals and that of carbon?

6.What does I ohm represent?

7.What is ohmmeter used for?

8.What device is used for measuring insulation resistance?

9.What testing is "megger" employed for?

lО. How is the external force applied to а circuit to overcome the resistance measured?

ll. What does l volt equal?

12. How is the power of electric current measured?

EXERCISES

I. Checkyourselfin your vocabulary.

What do уои са//:

1. the property of the molecules of iron to store energy in а field?

2.the study of electricity at rest?

3.the study of electricity in motion?

4.а piece of iron or steel which being magnetized retains its magne­ tism?

5.the method of producing electricity based оп operation of rotating generators in which electricity is produced Ьу conductors moving through а magnetic field?

6.а movement of charges?

7.а driving force making electrons move trough а metal conductor?

8.an electI"ic current that doesn't change its direction and magnitude?

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9.an electric current that changes both its direction and magnitude at fixed intervals of time?

1О. an electric current alternative in its magnitude and staЫe in its direction?

11.an electric unit intended for measuring the strength of electric current in а circuit?

12.а device used for measuring resistance?

13.а device used for measuring insulation resistance?

14.an extemal force applied to а circuit to overcome the opposition to the flow of current?

15.а device used for measuring voltage?

16.devices intended for measuring delivery of electric energy in watts?

II.Make sure уои 've grasped the contents of the texts given

above:

1.There was developed the idea that similar kinds of electricity:

а) attract each other; Ь) repel one another;

с) attract small pieces of iron.

2.Electrodynamics studies: а) electricity at rest;

Ь) random movement of free electrons; с) electricity in motion.

3.Examples of static electricity are:

а) charges on condenser plates;

Ь) magnetic lines of force around the wire; с) ions moving in liquid conductors.

4.Each of permanent magnets has: а) а north pole only;

Ь) а south pole only;

с) both а north and а south poles.

5.Permanent magnets are used for producing the magnetic field nec­ essary for operation of:

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а) ship's e\ectric propulsion plants; Ь) small e\ectrical motors;

с) generators of al\ sizes.

6.In e\ectrica\ circuits the flow of e\ectric charges is а flow of: а) electrons;

Ь) positive charges; с) ions.

7.When the e.m.f. is app\ied to the ends of the wire:

а) positive charges move in опе direction; Ь) free electrons move in random manner; с) free e\ectrons move in опе direction.

8.According to Ohm's law app\ied to the entire circuit the strength of the current is:

а) inversely proportional to the e.m.f. апd directly proportional to the resistance of the circuit;

Ь) directly proportional to the e.m.f. апd inversely proportional to

the resistance of the circuit;

с) inversely proportional to the e.m.f. and to the resistance of the circuit.

9. The strength of the current in а.с. circuits depends оп:

а) the resistance of the circuit and the voltage applied to it оп\у; Ь) the resistance of the circuit and the inductance. and capacitance of the circuit section;

с) the resistance of the circuit, the voltage applied to it, the inductance and capacitance of. the circuit sections.

1 О. With the increase in the temperature the resistance of а\1 meta\s: а) decreases;

Ь) increases;

с) remains unchangeaЬ\e.

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IJL Mindyour Grammar.

Put the verbs in brackets in the correctform - Present or Past

Simple Active.

1.The study of the nature of electricity (to begin) in the 18th century.

2.RubЬing glass with silk (to produce) static electricity.

3.Electromotive force (to make) electrons move through а conductor.

4.The experiment held yesterday (to prove) that these substances (to

show) different resistances.

5. If you want to measure insulation resistance, you (to use) "megger".

.6. Voltage (to overcome) the opposition to the flow of current

7.Many scientists (to investigate) electric phenomena in the 19th cen­ tury.

. 8. Electrons (to move) under the influence of e.m.f.

9. Ohm's law (to give) the possibllity to measure electric current in а circuit.

1О. After some experiments the scientists (to define) the law of measuring three basic electrical units.

11.Charges in motion (to give rise) to а magnetic field.

12.Michael Faraday (to discover) magnetic induction.

13.When the current (to stop), the magnetic field also (to disappear).

14.Loudspeakers and electric motors (to Ье) examples of application of magnetism.

15.The principal utilization of electricity (to increase) rapidly with the development of telegraph in 1844, electric motors in 1887 etc.

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UNIT 2.

SHIP'S DIRECT CURRENT ELECTRIC МACHINES

1. ТНЕ CONSTRUCTION AND ТНЕ PRINCIPLE OF OPERATION

D.C. machines converting mechanical energy into electrical one are referred to as d.c. generators. Those, оп the other hand, which convert electrical energy into mechanical one, are spoken of as d.c. motors. О.с. machines are reversiЫe and, therefore, identical in construction. Тhеу are composed oftwo main parts-thestationary part which is cal\ed the frame provided with the main po\es and the commutating poles, and the rotating part which is said to Ье the arrnature, its windings and com­ mutator being positioned оп it.

The frame is made of steel in the form of а closed magnetic con­ ductor. The main poles for the purposes of decreasing the losses are assemЬ\ed of sheet stee\ laminations. Mounted оп them, the field wind­ ings are built of copper iso\ated conductors. The main poles are in­ tended to produce the main magnetic flux.

The commutating poles are also made of stee\ and arranged mid­ way between the main poles. Their windings as well as those of the main po\es are built of copper conductors. The commutating poles with the windings are designed to ensure non-sparking operation of an e\ec­ tric machine.

The armature makes up а cylindrica\ core made of sheet steel lam­ inations, а two-layer winding being fixed in their slots. The former is ,uilt of sections made of isolated copper conductors.

The commutator is constructed of separate copper bars isolated :rom each other and from the frame. The section leads of the armature

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