АкушерствоАнатомияАнестезиологияВакцинопрофилактикаВалеологияВетеринарияГигиенаЗаболеванияИммунологияКардиологияНеврологияНефрологияОнкологияОториноларингологияОфтальмологияПаразитологияПедиатрияПервая помощьПсихиатрияПульмонологияРеанимацияРевматологияСтоматологияТерапияТоксикологияТравматологияУрологияФармакологияФармацевтикаФизиотерапияФтизиатрияХирургияЭндокринологияЭпидемиология

SECTION TWO

Pronunciation Drill 1

Practise the sound [o:]:

also, always, already, small, almost, although, thought,

all, important, four, ball, orbit, force, call, brought,

portion, saw, ordinary, orderly, alternating, audio, source, cause, fall

Pronunciation Drill 2

While reading these words pay attention to the stress

process carrier emitter sufficiently

method barrier electrode inadequate

metal energy enable available

unit quantity important majority

TEXT 3

ELECTRON EMISSION

1. The electron tube depends for its action on a stream of
electrons that act as current carriers. To produce this stream
of electrons a special metal electrode (cathode) is present in
every tube. But at ordinary room temperatures the free elec­
trons in the cathode cannot leave its surface because of cer­
tain restraining forces that act as a barrier. These attractive
surface forces tend to keep the electrons within the cathode
substance, except for a small portion that happens to have
sufficient kinetic energy (energy of motion) to break through
the barrier. The majority of electrons move too slowly for
this to happen.

2. To escape from the surface of the material the electrons
must perform a certain amount of work to overcome the re­
straining surface forces. To do this work the electrons must
have sufficient energy imparted to them from some external
source of energy, since their own kinetic energy is inadequate.
There are four principal methods of obtaining electron emis­
sion from the surface of the material: thermionic emission,
photoelectric emission, field emission and secondary emis-

. sion.

3. Thermionic emission. It is the most important and one
\ most commonly used in electron tubes. In this method the

metal is heated, resulting in increased thermal or kinetic energy of the unbound electrons. Thus, a greater number of electrons will attain sufficient speed and energy to escape from the surface of the emitter. The number of electrons re­leased per unit area of an emitting surface is related to the absolute temperature of the cathode and a quantity of the

work an electron must perform when escaping from the emitting surface.

4. The thermionic emission is obtained by heating the
cathode electrically. This may be produced in two ways:
1. by using the electrons emitted from the heating spiral for
the conduction of current (direct heating) or 2. by arranging
the heating spiral in a nickel cylinder coated with barium
oxide which emits the electrons (indirect heating). Normally,
the method of indirect heating is used.

5. Photoelectric emission. In this process the energy of
the light radiation falling upon the metal surface is transferred
to the free electrons within the metal and speeds them up
sufficiently to enable them to leave the surface.

6. Field or cold-cathode emission. The application of a
strong electric field (i.e. a high positive voltage outside the
cathode surface) will literally pull the electrons out of the
material surface, because of the attraction of the positive field.
The stronger the field, the greater the field emission from
the cold emitter surface.

7. Secondary emission. When high-speed electrons sud­
denly strike a metallic surface they give up their kinetic
energy to the electrons and atoms which they strike. Some
of the bombarding electrons collide directly with free elec­
trons on the metal surface and may knock them out from the
surface. The electrons freed in this way are known as second­
ary emission electrons, since the primary electrons from
some other source must be available to bombard the second­
ary electron-emitting surface.

2600 EXERCISES

1. Review questions:

1. What does the action of the electron tube depend on?

2. What is present in every tube to produce the stream of
electrons? 3. At what temperatures free electrons cannot
leave their surface of the cathode? 4. What forces tend to
keep the electrons within the cathode substance? 5. What
must the electrons do to escape? 6. What must the electrons
have to overcome the restraining surface forces? 7. How many
methods are there for obtaining electron emission? 8. What
are they? 9. What imparts the external energy to the elec­
trons in thermionic emission? 10. What energy is used for
producing free electrons in photoelectric emission? 11. What
is field emission? 12. How is secondary emission obtained?
13. What emission is the most commonly used in electronics?

II. Make up an abstract of the text basing on the answers to the above questions.

III. Translate the international words without a dictionary.
cathode, emitter, material, cylinder, portion, energy,

radiation, temperature, thermal, adequate, absolute, special, emission, electron, normally

IV. Define to what parts of speech these words belong and
translate them:

realize, equalize, electrify, classify, originate, strength-гп, widen, increasingly, widely, likewise, otherwise, for-ivard, towards, upward, outward, downward

V. Translate these antonyms and memorize them:

1. be present (v), be absent

2. primary (adj), secondary

3. relative (adj), absolute

4. outside (adj), inside

5. majority (n) minority

6. common (adj), special

7. external (adj), internal

8. slow (adj), quick, rapid

9. free (adj), bound
10. strong (adj), weak

VI. Translate these words and word combinations and learn them:

because of, since, except for, a number of, the same, within, in this way, suddenly, sufficiently, literally

Test I

Find the correct answer out of the three given to each question:

1. Which of the following devices depends for its action on a
stream of electrons in vacuum:

a gas tube, an electron tube, a transistor

2. Which of the following forces keep the electrons within the
emitter substance:

internal forces, external forces, attractive surface forces

3. Which of the following kinds of emission depends on in­creased thermal energy of electrons: thermionic emission, secondary emission, field emission

4. Which of the following kinds of emission depends on astrong field:

thermionic emission, field emission, photoelectric emis­sion

5. Which of the methods of emission is the most important and widely used:

field emission, thermionic emission, photoelectric emis­sion

Test 2

Find Russian equivalents for the English words and word

combinations (see p. 147):

1. a number of (а) буквально

2. except for (b) тот же самый

3. literally (с) так как; с тех пор как; с

4. suddenly (d) из-за

5. within (e) достаточно

6. in this way (f) внезапно

7. since (g) за исключением

8. the same (h) ряд

9. because of (i) таким образом

10. sufficiently (j) внутри; в; в пределах

Test 3

Find an antonym (a), (b), (c) or (d) to the word in bold type:

1. The primary question — (a) principal; (b) difficult;
(c) secondary; (d) new

2. A slow motion of electrons — (a) similar; (b) rapid;

(c) steady; (d) continuous

3. The internal forces — (a) attractive; (b) strong; (c)
restraining; (d) external

4. A small portion — (a) great; (b) similar; (c) external;

(d) common

5. A high speed — (a) decreased; (b) low; (c) adequate;
(d) sufficient

6. The strong attractive forces — (a) slight; (b) equal;

(c) unequal; (d) weak

7. Outside the tube — (a) near; (b) in front of; (c) in­
side; (d) around

8. Cold water - - (a) clean; (b) hot; (c) boiled;

(d) mineral

9. To heat the liquid — (a) boil; (b) change; (c) evapo­
rate; (d) cool

10. The majority of the electrons — (a) a great number of; (b) weight; (c) minority; (d) amount

11. Common measuring devices — (a) various; (b) neces-
sary; (c) special; (d) new

12. The flow is rapidly determined — (a) often; (b) com­
monly; (c) frequently; (d) slowly

13. Many particles — (a) light; (b) few; (c) free; (d) heavy

14. To be present in this tube — (a) be included; (b) be
inclosed; (c) be changed; (d) be absent

15. Free particles — (a) small; (b) bound; (c) loose; (d)
tiny

16. Absolute temperature — (a) high; (b) low; (c) relative;
(d) the same

17. More interested in — (a) usually; (b) always; (c) nev­
er; (d) less

18. Negative charges — (a) the same; (b) unlike; (c) dif­
ferent; (d) positive

19. Large particles — (a) a lot of; (b) various; (c) tiny;
(d) great

20. In a downward direction — (a) upward; (b) outward;
(c) straight; (d) right

Test 4

Finish each sentence choosing one of the three variants (a),

(b) or (c) based on the text from Section II:

1. The electron tube depends for its action on...

(a) restraining forces; (b) a stream of electrons; (с) а magnetic field.

2. A special metal electrode is present in every tube to pro­
duce...

(a) a magnetic field; (b) a stream of positive charges:

(c) a stream of electrons.

3. At ordinary room temperatures the "free" electrons in
the metallic cathode cannot leave its surface because of...

(a) attractive forces acting as a barrier; (b) forces of the external magnetic field; (c) thermal energy of the atoms.

4. A small portion of electrons has sufficient kinetic energy
to break through...

(a) the surface of the tube; (b) the plate substance; (c) the surface barrier.

5. To escape from the surface of the cathode the electrons
must have...

(a) attractive internal force; (b) sufficient energy from some external energy source; (c) low speed.

6. The energy for electron emission comes from...

(a) internal sources; (b) external sources; (c) exter­nal and internal sources.

7. There are...

(a) two principal methods of obtaining electron emis­sion; (b) three principal methods of obtaining electron emission; (c) four principal methods of obtaining electron emission.

8. The cathode metal is heated in...

(a) photoelectric emission; (b) cold-cathode emis­sion; (c) thermionic emission.

9. In thermionic emission the cathode is made of...

(a) metal; (b) semiconductor; (c) an insulator.

10. In thermionic emission the number of released electrons
depends on...

(a) resistance; (b) cooling; (c) temperature.

11. In photoelectric emission the energy of the light falls...

(a) upon the surface of the non-conducting material;

(b) into the glass envelope filled with the gas; (c) upon
the surface of the metal.

12. In photoelectric emission the energy of the light radia­
tion is transferred to...

(a) free electrons; (b) bound particles; (c) positive charges.

13. In photoelectric emission electrons to which the energy
of the light radiation is transferred are...

(a) on the surface of the metal; (b) in the space about the cathode; (c) within the metal.

14. Field emission is...

(a) hot-cathode emission; (b) photoelectric emission;

(c) cold-cathode emission.

15. Electrons escape from the cathode surface because of...

(a) the attraction of the positive field; (b) the attrac­tion of the negative field; (c) the cooling of the cathode's metal.

16. When high-speed electrons suddenly strike a metallic
surface they give up their kinetic energy to...

(a) electrons; (b) positive particles; (c) positive charges.

17. Some of the bombarding electrons collide directly with...

(a) positively charged particles; (b) uncharged par­ticles; (c) free electrons.

18. From the surface the bombarding electrons may knock
out...

(a) uncharged particles; (b) free electrons; (c) positive

charges.

19. The electrons freed by bombarding are known as...

(a) secondary emission electrons; (b) thermionic emis­sion electrons; (c) photoelectric emission electrons.

20. The most important and the most commonly used meth­od of emission is...

(a) secondary emission; (b) field emission; (c) thermi­onic emission.

(See keys to the tests on p. 147.)

After reading the following text be ready to answer these ques­tions:

1. How many methods are there for heating cathodes? 2. Where to is the electric current applied in the direct meth­od? 3. Where to is the electric current applied in the indi­rect method? 4. What materials are used for cathodes? 5. What current can be used for heating them?

TEXT

1. Cathodes are heated electrically either directly or in­
directly. In the direct method, the electric current is applied
directly to a wire, called filament, that also serves as an elec­
tron emitter.

2. In the indirect method the electric current is applied
to a separate heater element, located inside a cylinder that
is coated with the emitting material. The cathode is thus
heated indirectly through heat transfer (трансформатор на­
кала) from the heater element. Alternating or direct current
can be used for both methods of heating.

3. The most commonly used materials for cathodes are
tungsten (вольфрам), heated to temperatures between 2000°
and 2800° C, thoriated tungsten (торированный вольфрам)
operated at about 1700° С and oxide-coated cathodes opera­
ted at about 800° to 1000° C. Most tubes are indirectly
heated.

Дата добавления: 2015-09-27 | Просмотры: 1441 | Нарушение авторских прав