1969-04-30 — Page 23

育教儒華:買三第張六第 日四十月三年西屣変 WAH KIU YAT PO

數學會考題預習

PR1969

CITY HALL

物理 科 (廿六)

·佳永燁、

PHYSICS (26)

49.(a) (1) Fleming's left hand motor rule

Foc:a.left handy:fs

(1) the Forefinger represents the field:

(2) the second finger. represente the

current t

then(3) the thumb represents the motion of Tam

wire carrying the current

Thumb-motion.

in the field.

Factore in increasing the amf of the dynamo zw.

(1) Increase the number of turns"

(2) Increase the field strength, and

(3) Increasing the speed of rotation, Heat required -2000 175 cal.

- 2000 x 75 x 4.10 30ulem or

watt-eo.

z75 x 4.28

3600

0.174 kw¬hr.

Electrical energy used « IVt joules

220

- 2000 x 75 x 4.18

2000 x 75 x 4.18

5.x 220

570.88c.

current

secona finger.

Left hand.

In the case here, the motion of the wire 16 out of the paper.

The force on the wire can he strengthened by: (1) using stronger magnets, and

(ii) passing heavier current.

The aniform field of the magnet superimposed on the field caused by the current carrying wire produces a resultant field. It is noticed the lines of this resultant field are closer together on one side of the wire than on the other, and thus they will exert a resultant sideways push on the wire.

(111) The conductor XY is in parallel with tha

linea of force of the magnetic field

The superimpose of one field with the other does not give rise to any apparent nation of the wire at all.

(b) Construction of a Simple U.G. Lynano

LOAD

FIELD MAGNET

A dynamo or generator is a machine for

converting mechanical energy into electrical energy. A cofl of wire is rotated in a magnetic field and there is a change of flu linkage which, in consequence, induces ent. The rotating part, called the armature, consiste of a coil of wire wound on a soft iron core, mounted on a shaft and free to rotate. The two ende of the wize in the armature are connected: to two metallic commutator. The commutator

conelets:of a brase or copper ring, aplit into two semi-circular segments which are insulated from the main shaft and from each other. Carbon brushes are tightly pressed on the commutator. They deliver the induced current to the outeide. circuit.

When the argature is forced to rotate, an. induged current passes through the carduis. When the left pole of the armature. approaches the north pole of the field magnet, the induced current will flow in such a direction as to produce an opposing north pole (benz's. Jaw). Current flowe: from the armature cut : through the commutator to brush B. Hence Ba

positive. When the armature makes a 180 degrees turn, this same pole approaches the south pole of the field magnet. The induced current will now flow in the.

opposite direction so as to produce an opposite south pole. Curtant, therefore, enters the brush. B. The brushes are so placed that they lose contact with one segment of the commutator and make contact with the other at the instant when the current changes direction in the armatüre. One of the brushes is, therefore, always positive, and the other is always megitive. Although. the: current flows in one direction in the external circuit, it is pulsating rather than constant, as in the case of battery.

9 min-

Cost of electricity in the heating

40.174 % 15 cents

: ane

50.

2,61 cents

It takes ye mines in heating and the cost is 2.61 centa.

10000)

(1) The induced emf is in such a direction a8

to oppose the ineerting N-pale. That is,

the end of the coll near to the N-pole of the magnet is N..

(11) Faraday's Law of electromagnetic induction Whenever there is a change in the magnetic flux linked. with the circuit an electromot- ive force is induced, the strength of which is proportional to the rate of change of the flux linked with the circuit,

(iii) Theatrength of the induded current depends

8

(1) the number of turns in the coil; (2) the strength of the magnet; and

the apeed with which the magnet je plunged into the coil.

Therefore, the faster the magnet is pushed into the coil, the heavier is the atrength of the induoed current. The

pointer of the galvanometer deflects ave

(b) (1) Induation coll

The induction coil is a device that changes a low-voltage DC. into a high... voltage which is mainly D.C

The essential parts of the induction coil.

a primary coil, consisting of ture of heavy wire wound around soft iron cor®!

(2) a sdcondary. coil, coneleting a fev

thousand turns of fine wire wound around the pritary and insulated from

(3) a care of soft-iron rode insulated

from each other;

(4) an automatio interrupter, the purpose of which is to open and close the primary circuit automatically; and (5) a condenser connected across the

interrupter,

At each maker of the current in the

Way circuit, the magnetic field builda:

and cuts the secondary, induding a high roltage in the coil. At each "break", the

sagnetic field collapses and outs.the: secondary again. Because the "break" is more rapid than the make", a much higher voltage is induoed on the former, The jmeny turne in the econdary, which result

in a high voltage (about 20,000 volts), elso give at a very high resistance. Therefore, very little current can be drawm from it, buta very high voltage. may: ba obtained form the secondary winding of an induction coil,

The purpose of a transformer is to increase or decrease the voltage on an alternating current. A simple transformer consists of a primary coil and a secondary coil, wound on a laminated iron core. If the, prinary coil is connected to a storage battery, a steady current flows in the primary coil, giving no change of flux Linkage, There will be no electromotive force induced in the secondary coil. The transformer does not work in this case,

power transmission

When current is transmitted through

a wire, energy is lost in the form of heat The loss is proportional to the square of the current and to the resistance. To

minimise this energy loss, either the redistance of the cable as reduced or the current 18 decreased. To achieve the former. the cable diameter is made as large as possible. The cost of large-diameter wire would be excessive. What is actually done for economical transmission e slectric power is to step up the voltage it the powerhouse. This will

三期星 日十三月四年九六九一圈公年八十五國民蘸中

內有學

两個不同的方向;

教

香港之大學教育中之一問題

一個四百萬人口的睿意,只有四五千多米糸的大一院的建立,與實殴的行飲指出,其背桄世有其品 ̧照我的預測,常帶備之大富教育美社會需要,必很多,但是我們只要的先进化一大方向 1

「配合香被素高的大學生的人數。基看海城的,如具槍的同題,可以一一加以著「

「俄籍人民之文化與知盡之水準,其中之間題,仍

師

交相

或

此

承

的

的有

置

五

數大

是

无

>我剑方

的學心然育的,其起

然

知越由這些流席,中文大學之未來之發股,便材 詹家送予弟,就唱之所亞、郭合、崇族之學院、;而中文大學,亦涵軍香港大學合併而亦不必在

從這些才情,大家亦陷當知道中交大昂辦理一代渎了人冊會的不要的大學。中文大學是燕蒸於 ]以爲榮?今更向此步步高陞,這明法中文大學之「方向,應當而且必然亦自然的要向更遠合以華人,求適合廣大華人杜爾的需要」之一方向走時, 現亦有上斤耐吉家送之子弟就讀,艾堂不皆可目,在。所以照我個人的黑豆,中文大學未來我圓的,這些事,而基普中文大逐風出他可,無校,都商 戰社宙文化人士高一導。而原只有若干中下展壮大學,以節省運費的開支,期限院固不必存在而中國語文之必需成爲一法宗的官方語文,亦

(精完 ·害者毅教授, 「之困難,淦在香港大學之上,不過同時大家亦顯此香港華人社會自身所生長出之爲蒸、麻谷、新之廣大華人社會,多至於可用於其他地區之華人,之翼切的認識。 有的地位,就不太是否好,至我們這些歌林之小設一十次部,不必另成立一大學,就有人在國人志跟外國大學任教岩之必需經習以外國文有教 回教中學已動工興建 凡有點五节法定的大離生,亦可惜永居於天之隔,在巾文大學成立之先,亦有人主張只在香港大學要求學生學好英文,以便聽他的課,此有如中國,只本其主觀的特殊立事討論,是斷然不是的。 .. 太學的畢葉生亦能得。在一四百萬居民的都市,學合併篇一大串的理由,以節米經費的開支上如有學習能力的,亦極學習以中文漢澡,而不愿先,只糠身在此山中一,我們只在學校內部,各人 在祀亡居的領導地位,所能內之封於榮街,中文一翻版時,則人們亦可持此以作爲此應與香港大,正業,而非敦外國文字與文學的,如又是年光市之用说。教育是毗會的大事,「不凡廬山美面目 大學亦有亦能。則以前贵港大學之吊業生,所能曬,如果它只基者液大學第二,或香港大學之另一街宙的人民。阿我的意思,而是外成的致命來此期望香世有一编教育學術區教育問題的專門刊物 其一方向,是使中文大學成爲名副其實的老一對這些要求負責。而當其爆開其成身學院,興業一文彬庫西方的低語,以中文加以講解,然後學生水不帷裔服大家。大亲烈要求計進一步之研究的 一院者,同時亦加中文大學,中文大學天的耍一之香港大學與大地學校的教師,亦應當用中天向上 是香港一切知識分子所共同爭取的···元

有像香港的。此中之原因,固然很多,但隨是顯

·瀝源的情形,這在世界其他任何文明都市,都少臨時的刺激,作消極性的反應,道於半很少有補 承認的學校以外的學校的教師互相比較,其待遇发的利塔的打算,用心思,依然的反感, 之內部之教射,互棋比較,就是此類敦蚶與政府一致,則落到實際,入結是依習慣的道路,其現

·都是太少的。又無論是政府承認的大學專上學校 #本觀念與現想的方向,如不能在大地上 |生:四五十矮法定的敦說的位置,無論如何說本的觀念與理想,觀念與想是一人的行爲之

·必須要能至少用於香齦的方向,亦要許多其實的理由,與對營運街

節理二

待遇進位,更似天然其他惠上華校之教師與一輛去與現在主張將崇蕊、獅、新亞分爲一第一制

走向且点

守

外頭根

的

與思

其

的你

據

與

無中校#

上海能是!交大學非著一邪會的需要方向走,向此一方向光時,我过隊 所首當江意料,而逐步加以實現的最切近的教育

有的,非

新天寢

鳖大吉犬

,協助興建,陌該校各項所需,將趙惠二百萬元 被乐說,其前途亦是遠大的。我們到書院真中,大的M,就會並依政府乐谋- 博大裏產校之散,有產業,無坛等,由信號齊站公司承建,勒斯城 光线由女大學的前途是海大的。此外香海其他大宮想,就得於我們大家共同努力的,當然除此以教英文中學,已開始興建,使地位與創業知 茲校爲不羋利性質:故得香港政府撥地

《戰,本港中國致惊梦仙器做多時之四 主建築費超過二百萬元 十一個月後工程可完成.

第

包八

·然此一方向去發展,則中文米學將是一愈來偉 我們之潢整做☆萦之大素团育之弑棘之一種表;而能米及到社會的中下新之中,以揭商整 一立,所開啟香洪大學教育的新頁」之方向去發莨,其他为基模之園方畢業生,將比就讀,有的是一整成少數的法足大學,當香港社會之上階領導者

IA

uecrease the current

(IV

constant), and

consequently the energy loss in the trans¬ mission. Steppin up voltages, transformers are used. This convenience of stepping up and down of voltages cannot be shared, by D.. transmission.

Alternating currens 18 used more extensively than direct current mainly because of the convenience of transmitting power by the use of transformers.

·TEST PAPERS

una rinciple of arunimeqey, APPLY. The

prancapie to explain way a piece of ETON BARKE in water but the ocean liner made of tons of iron will float:100

(b) A core of de

gm. Calow 25 gm, pero.c. weighs 5.0

the force required to aubmerge the cork in water, (ii) the mass of brase (density 8.5 gm, perc.c. ) which, fitted to the cork, will make the arrangement just Bink in water.k

A bydrometer consisting of a bult of volume 5 c.c. surmounted by a cylindrical stem of diameter 5 mm. sinks in water to a depth of 2 cm. above the bulb. To what depth of 2 ok. above the bulb. To what depth will it sink in a liquid of epecific gravity 0.957

(a) Detine coefficżent or static friotion, and

describe how you would determine the coefficient of static friction between two surfaces. (b) A train weighing 400 tons maintains a constant speed of 60 miles per hour over one mile of Level track. If the fractional forces amount of 15 lb.wt, for each ton, how much work is done by the engine moving over this mile, and what horge-power does it develop? Asawning the friction provides the only resistance to motion. (1 horse-power 550 ft.-lb per seo,) Calculate the horse-power developed be. a Locomotive in hauling a train weighing. 500- tons up. an incline of l in 100 at a speed of 45 miles per hour"

1. (a) State the condition needed for parallel for

to keep a body in equilibrium.. (b) A uniform rod 1 meter long, of see 500 gm.. is suspended in a horizontal position by two vertical spring-balances at points 10 om..

ron each end. Masses of 1 and 2 kgm. are hung on the rod at points respectively 20 cm. from one end and 25 on. from the reading of the spring-halances?

other, What are the (2) A 30

60 triangular set-square or negligible masa 18 suspended freely from the right-angled corner and weights are hung at the other two corners. What is the ratio of the masses of the weights if the hypotenuse of the set- square Bets horizontally?