1971-08-02 — Page 23

REFERENC. LIBRARY

1971

育教備曲黃三第張六第 日二十月六年其辛服夏 WAH KIU YAT PO

報日僑華 CITY HALL

-期星 日二月八年一七九一公年十六國民中

Indirectly heated Cathode

小學畢業同學

暑期進修專欄

智慧出版社主編

LEX眼中學同學暑期進修專欄 臺通英文書主

物理進修

THE ELECTRONS & THEIR EMISSION*

Electrons orbit round their nucleus, carrying, with

them their fundamental charge of - 1.6 x 10 coulombs. They are light particles. Each electron

Its mass weighs only 1/1640 of that of a proton. is 9,1 x 10-18 m. Electrons are common in all substances. They can emit from materials in var ious ways. The cheff ways of emission are:

(1) Thermionic Emission;

(2) Photoelectric Emission; ana (3) Secondary Emission.

Thermionic Emission

The emission of electrons from hot bodies is known as thermionic emission. Atoms in a metal are re- gularly spaced, forming regular lattice. Each atom nas a number of free electrons orbitting round it. On gaining enough energy, some of chese electrons. break through the boundary to boil into the space.. The space will soon be crowded with these space charges

The energy required by an electron to boil from" metal depends on the work function of the metal. An electron may escape from a metal if it pose es an energy greater than the work function.

Proelectric Emission

Light gives energy to the electrons in the surfaces of some metals, enabling the free electrons te preak through the boundary and escape. This 1s called photo-electric emission..

Zinc emits electrons when exposed to X-rays or ultraviolet rays.

Sodium gives emission of electrons when exposed to all colours of light except orange and

proportional.

The number of photoelectrons emitted per second is

the intensity of the incident ray oFlight The photoelectrons, as they are called, escaped with a range of velocities from zero to a maximum, velocity which increases in relation to the frequency of the incident ray. The velocity 15 Independent of the intensity of light Emission will not occur no matter how strong the intensity 15. If the incident ray has a frequency below a threshold frequency.

Einstein's Photoclectric Equation

To liberate an electron from the surface of a metal, a quantity of energy, called the work. Junction W, which is a characteristic of the metal, is required to be supplied to the metal. A photon of energy "h" causes an electron to be emitted if hf is greater than where n is Planck's constant which 5-6,6 x 10-34 joules sec and is the frequency of light incident. The excess energy (hf - ) appears as kinetic energy of the emitted electron which escapes with

vejocity having a value which depends on

inf - W = 3my?

max

This is

Photoelectric Equation.a.

If the photon has only just enough energy to 11. berate an electron, the electron gains no more

inetic energy It's velocity is zero

There is a minimum Frequency, the threshold freq- bency F given by hf, -W, below which no photo- electric assion is possible.

Secondar; Emission

Rapidly moving Electrons will lose their kinetic

This bombardment energy on striking, a metal.

agitates the surface atom thus causing the ers trons

to escape from the metal This is known as secondary emission of electrons.

Dode Vacuum Tube

Construction,

glass envelope

Plate (anode)

Cathode

Heater

terminal pins

Schemati symbol

thermionic diode is a special vacuum tube with: a glass envelope enclosing the metal electrodes in a vacuum. The cathode when heated by filament produces a thermionic emission of electrons. The electrons are accelerated by an electric field. between the electrodes. They are collected in the anode, or the plate. The anode is at higher positive potential. As there are two electrodes only, these tubes are known as Di-ode.

Cathode

There are two types or cachode!

(1) Directly heated cathode, and

(2) Indirectly heated cathode.

Both will have thermionic emission of electrons on being heated and they receive thermal energy exceeding the work function of the metal.

ก

Filament

Directly neater,

Cathode

Generally, the indirectly heared cathode is prefer- red. The filament is a tungsten-alloy which heats the oxide-coated cathode. The cathode is cylin drical so as to render radiation effectively from the heater. There is no electrical contact between

noce and the heater.

The directly heated filament-cathode ofter made of tungsten. Pure tungsten produces thermionic

The direct-: emission when it glows white-hot.. heated type is use in power rubes for large: amount of thermionic emission.

late or all

The plate surrounds the cathode, and usually made of iron, nickel, or molybdenu large plate area is used to conduct larger value of plate current. Vacuum

Air is exhausted from the envelope for The purpose of:-

(1) Preventing or stowing Cown the oxidî-

zing action of the Filament (2) enabling the cathode to emit more e

electrons in vacuum and (3) enabling electrons only to travel

in vacuum, and thereby clearing off

"The Fons in a vacinem

The vacuum rube diodes are being superseded by semiconductor devices in the modern electronic. Equipment, but many apparatus containing tubes exist and there are certain tasks which at pre- sent t can only be performed in tubes. Charateristics of a diode tube

The current flowing in a diode circuft depends chiefly on the anode voltage.

3/2 where is a constant I and V

are the plate or anode current and voltage respectively.

Diode voltampere

characteristics for

different cathode temperatures.

Anode curre

Anode voltage.

829

725

630°

Note the following important characteristics evident from the curves above

1) The plate (anode) current is practically O without any anode voltage Although there 15 thermionic emission, without anode volt ge electrons carinot De attracted to the anode

(2) The current in the circuit flows in one dir

estion only. The plate itself canno, ema c electrons: the negative plate voltage also repels the space-charge electrons back to the cathode. As a result, the electron cur rent canno, flow in the reverse direction (3) Increasing the amount of positive plate

voltage can increase the amount of place -current. "Na

(4) At a high enough value of positive plate

voltage, however, the current cannot be creased any more, because of saruration. The saturation value of current means tna. more electron's cannot be supplied by the space charge, unless the cathode temperary is increased For more emissane

Diode Rectification

The fact that the diode allows current to flow in only one direction means that its useful as a rectifier to change alternating cur rent to direct current Diodes used in such a way is known as rectifiers

Rectifiers have a low resistance for the current to flow in one direction, known as the forward direction, and the voltage applied then is forward-biased. Rectifiers with reverse- biased boltage offer high resistance to allow the current to flow. They conduct, in fact, in un- "directional·

Forvard-blas Current flowing

D

Half-wave Rectification

Reverse-bias Frent not flowing

AD

Electron current

Conventional:

In half-wàve rectum Lacult, the w souree is connected in series with the recur fler (the diode) and a load for the d a

AAQ

source

Diode rectifier

output.

Pulsating Output

(未完轉入第四張第四頁)

英文數學

CHAPTER FOUR

MULTIPLICATIÓN

means "multiply'

Multiply eight hundred and ninety-four by sis

SOLUTION: The required product

894 x.6=

ANSWER! The required product-18-5364

There were four books. Each had 8 pictures, How many pictures were

there altogether in these books? SOLUTION

The required number of pictures - 4 x 8 ANSWER The required number of pictures is 32.

Example

A woman works everyday in the week except Sunday. How many days "does

she work in eighteen weeks?

SOLUTION: The required number of days

- 18 x (7 - 1) - 18 x 6 ■ 108

ANSWER: She works in 108 days.".

family drinks 3 pints of milk everyday, "How man

drink in 8 weeks?

required number of gallons of milk *SOLUTION The

- 3 x 8 x 7. (pints) - 168 (pints) ■ 21 (gallons) ANSWER: They drink 21 gallons of milk in 8 weeks.

gallons do they

1 catty of pork costs $3.20. How much would 3 cattion 8 tacle cost? SOLUTION: The required total cost $3.20 ANSHER: The required total cost is $11.20

Example 6

Find the total age of six boyé. If the average age of the first thres In 18, the average age of the next two is 15 and the sixth one is 17 years old

SOLUTION:

The required total age (3 x 18+ 2 x 15 +7) years

91 years, ANSWER: The required total age is 91 years.

NAME

CLASS

·EXERCISE.

MARKS

DATE

(1) From 85 times 47 take 85 times 46

A(2) Multiply 1 sq. yd. 4 sq. ft. by 10,

(3) A lorry can carry 123 bags of coal. How many bags

of coal can nine lorries carry?

(4) How many marbles are needed so that 36 children

can each be given 12 marbles?'

(5) A tea-room has 23 tables with room for 5 people

at each table. How many people are there when every seat is taken?:

people

6) How many dozen oranges are there. In 14 boxes,

each containing 504 oranges?.....

(6).

_dozen.

gallona,

(8).

:(7) How many gallons of milk are required to give

384 children half a pint each?

(8) A, truck weighs 2 tons 18 lb. and holds 2 tõns

:784 Ibs. of cargoes. What is the total weight of

eight such loaded trucks?

(9) A bar of chocolate weighs two oz. How many pounds

and ounces will five dozen bars of chocolate weigh? (10) Find the product when 2 hrs. 3 min, and 45 sec. is

-multiplied by 12..

(11) Fill in the missing figures:

(a).

3. 2. la

13:0

46 70

。 ་

1

2 '6 3.1

(12) 12 lb, of ten A at $3.50 per 1b, are mixed with

7 lb. of tea B at $5.20 per 1b. What is the total cost" of the mixture?

((12); $)

(13) The price of pork is $5.00 per catty and fish $6.40.

per catty, A woman buys 3 catties of pork and 6 catties. of fish. What will be the total cost?..

(13) $

(14) At a car park, the parking fee is 60g per hour, A man

left his car there at 8.00 a.m. on Monday and collected it. at 6.00 p.m. on Wednesday of the same week, How nuch had be to pay?

(14)

(15) The charge for each unit of electricity for lighting-

is 28g and for each unit for power is 122. The rent for the two meters is $1.00 per month. In a month, if:

man has used 50 units for lighting and 100 units for power, how much has he to pay altogether?

XFHC155 3.

(15)

(3) 8342 - 5759 - 2583. (1) 9667 - 9979 - 3668 (6) 3 picula 77 catties 6. taels

(2) 3539

· {75) — 1189 1812 (5) 654; 5 sq. ft. 64 sq. in. "(8) 1. hr. 25. min.: I sec.:

gallon 7. pints (10) 2 hr, 45 min. (11) 2 16. 9 oz. (12) 47 yd.. 1. ft. 10 in.

(13) 257% (14) & cheung 9-chek & chưên (15) 3 ton 9 cut. 5 15.

12 pur. 95 16. (17)-3 picula 78 eatties 15 taels (18) 8 tons 6 cit. 10 1b, 15

第四章:乘法

:林,若气本诠8日,開4本共有圖豐書下世?!

某婚人每辧星期日休息,問從18個星助內,黎婚人共工作楚天? 長無能日飲牛奶3品悅,48期共多少?

国”、“街西1503元2魚,間買38需实若干?

有人,投督3人之平均年齡年18岁,另2人之平均年路爲1

兼愛四、

*若第六人爲17歲,四

(1) 8574735462

(2) 1704147787

(3)一

不可放123袋,問9確實車載案若于?

(4)下绿竹小孩可分得波子12枚,分餘36位小孩,我需城子若干位?

(5)茶共有片23張,而展證可容5人,問補至時共有客人若干

若您有5044,14椒共有橼若干?

(7)若链小孩確得關牛品悅,間存小號84名,並親牛奶多少加魚! 我的一句話2018錄,並可淑貨2年7848年守蔴貨物的駕卓共板多少

C97 朱站一塊2安士,問60塊共靂沿十士?

(10)求1212小時3分5秒之間

(1)

012)

泥去數字,填在空格內。

链匆13元5角之A12磅 每65元2角之B混合,問混合茯茶價共值多少? (13) 猪肉每斤15元,魚每斤條6元4年,某婚人買餐肉3斤及低6斤,阋共值多少?

143,有一車場每小時收費6角,某人於星期一上午8時將車停入後,於同一個內之星期三下午

等取,問他應付欸多少

(15)按每28分,力證每泠12分,兩電鏻租金每月共1元。在一個月內,某人用燈電50

成分獎100座∗問褒共付多少?