1968-03-20 — Page 17

20 MAR 1968

CITY HATE-$21K

1968

自二廿月二年申伐歷

WAH KIU YAT PO #

塞

三期星

英文中學會考試題預習專欄

物理科(廿一)

·

陸永熾-

Physics (21)

40. Faraday's laws of Electrolvais

Law 1. The mes or any supētance liberated during,

electrolysia is proportional to the

quantity of electricity passed through the electrolyte

Law II When the same quantity or electricity

passes through various electrolytes, the masses of the different substances

liberated are proportional to their chemical equivalents,

To veriry the first law

A copper volta-meter is set up as in the following } diagram.

39. Soluti00

A Dry Cell ·

A dry cell (Plaab lamp battery) la a

modification of Leclench's call. The poleware

carbon (positive) and zinc (negative); the electrolyte is sa) -ammoniac paste lammonium chloride NBG), which contains ammonium ions (NH"), chloride ions (C1"), hydrogen too (8") and hydroxyl ions (01). Round the carbon pole is packed a mixture of carbon and manganaae dioxide to act as depolariser. The content is prevented from drying up by sealing the top of the call with pitch.

Brass cap (ive

contact)

Pitch top seal

Insulating carda

Covering

-Ammonium chloride

Jelly

Mixture of

powdered Carbon and Manganese Dioxide

Carbon rod (-vej

Cardboard washer

-ve contact made

here

When the carbos and sine polas are connected Los direit the current flows from the carbon to the zinc pole and back through the electrolyte. The current in the electrolyte consists of a drift of ions, the positive onas moving with the

current, the negative ones against ft, so that

electrolysis occurs at the polas.

約

External Current

discharged

-ionizz

Electrochemical actions in a Dry Cell.

Neither of the negativus ions is discharged at the zine; but instead zine ionizse, The atom losing two electrons each and becsaa zino ions in the electrolyte. The hydrogen icts are discharged at the carbon. Hydrogen is set fres, and oxidised. by the depolariser.

Kavants ke

The contents are very light Tp packed, This rendera, the cell portable and a low internal wasistente.

PLBACVENTA 29-

The depolarising antion or tae menganess diakide is slow, so that if a large current is Miken from the cell the manganese dioxide cannot >xidise the hydrogen as it is formed. For this reason, this cell is unsuitable for maintainina steady curçant ovar ș, long parlod

Factors determine its internet resistance

1

Carbon powder mixed with mnganese dioxide to; increase the conductivity.

2. The Semi-solid state of the slectrolyte and

depolariser closely packs the sell and reduce the internal resistance.

40, 4r

6.5hman

www.

40 hors

e.m.f. of gach cell -

B

- 1.5 volta

Total e.m.f. of battery - E

• (4 x 1.5) volta

roh

Internal resistance of each cell - Total internal resistance of a colla 45 ohmg Combined external résistance - &

6 2 4

ohms

6.4

慧

• 2.4 ohma

Main current, 1 -

R

(2.4% br) 1 - 6

24

(1)

Since the current in the 4-ohm resistor is 0.6 amperes,te aureat, in the 6-ohm one 15

5 x 0.67 amperes.

That is the min aurrant. 1 18 (0.6 - 0.4) ampere,

Subatitute this in (1)

2.4

Resistance la 0.9 ohm and

resistor is 0.4

Rheostat

Key

E

A double dnode (copper) is used to enewe thicker deposition on both sides of the cathode (copper). They are dipped into a strong solution, of copper sulphate solution slightly acidified with sulphuric acid.

The cathode is thoroughly cleaned, and weighed and then placed a position and connect as in diagram. A suitable current is led to pass for half an hour. During this tim constant tah is kept on the ammeter and the rheostat adjusted from time to time to tanta in a steady current, At the end of half an hour, the thode is removed, washed, dried and reweighed Several experiments are carried out with different currents and time durations. The resulta are recorded and compared. It is founa that

rasa of deposition current x time

Kracaution.

-

constant.

(1) Cleaning and weighing should be done very

estefully. Ang mistakes done baze will inquem

(2) Da “double anoda to

Sa large mass of

ros obtained when the

deposition and conde ambler poroentage nathode is weighed.

(3) Too'small a current will deposit very little

copper, higher per centage error occurs

(4) Too large a current will give a powdery

deposit and will be washed away.

(5) Current flowing aust be maintained constant

through out the experim

giment.

1

By Law II, the masses deposited is proportional to their chemical equivalents and

Let

y be the chemical equivalent of Couder

atomic weight of copper

J

valency of copper

PUBLIC LIBY

the chemical

108

aj be the mass of copper deposit

#2 be the mass of silver deposit

Therefore,

108

63

108 x 2

63

• 3.4

Ans. The mase of silver deposited is 3.4 gita

Questions for next week:

41. With the help of a diagram, describe the structure

of a simple elsatria motor and axʊlain ita action.

A motor is connected to a 240-volt supply by # cable of total resistance 0.16 ohm, the current taken being 15 amp. Find (a) the power lost in the cable, (b) the potential difference between the terminals of the motor, and (6) the cost of running the motor for 5 hours at 20g par unit,

12. State the factors which determine the resistaNOS

of a uniform metal wire and indicate how they do go.

Give a labelled diagres of a metro bridge and describe how it ia used to measure a resistance of about 10 chat. The theory of the asthod is not required,

Resistura of 6 chime and. 8 önas are. corriented, in parale) to a battery of ́s.n.f. -12 -vojta and" of internal resistance 6 1th our at it con rat n

What

日十二月三年八六九一靨公年七十五画基

1968中文中學會考試題預習專欄 英文科 ( - )

·何玉柱•

*EX

1.

Answera to Exercise EVIT+

differs from; In

-

2. consists of

3.

made an attack on; at dawn

b.

takes after("looks like)

5. compare

with 4 between themitme

ta

things)

b.

tell black from white (distinguish; or,

Light from darkness

horrified at the sight or ...

8.

arrived at the station

9.

arrivað in Parie

(Nota arrive et a place but in a big coun-

try or city)

10.

parted from....

with..TE

in my heart

11.

look after........(-take care of)

12.

heard from him(=50) letter from nim)

13.

is worthy of....

14.

filled with

15.

full of

16.

make use of

17.

with

18. prevented .....from

19.

20.

21.

22.

23.

take

took care or, attended to....

busy with our work; with sasel seastly)

in need of, borrow (something) from (someone) failed in

for instancei-for example). familiar with; In fact, similar to

argues with

25. called on (someone ››

at (some placel.: at home

(Note: call on person, at place):

26.

27.

28.

Cy

approve of

concentrated on; paid attention t

agree with, on;agree to

(Note: agree with persons...agree on plans!"

agree to actions)

angry with, angry at ·

(Note: to be angry with a person orätning.

but et actions, deeda)

30.

Beware of (be aware of))

31.

at work (-working)

32,

on duty

33. cure perso0/Fof{{0}

disease)

34.

35.

36.

37.

38.

in charge of responsible for, succeede in; boast of

prefer

good at

to

hard for ......... get used "to.............«? nice of you; for me (or, to me) made of, in Italy, by..... (Note:

be made of some material, in a placeï by some agent--person. But if during the process of making, the thing must have.A008) chemical change, we use from.

e.gi Wine is made from grapes."

Exercise XVIII

Use proper preppaltiopa ip the following passm,

ageuz

The day tood

The ALSSKad earthquake, 'as. I' line waiting to purchase grocerias;

I chatted 32 'the man behind me.' I asked him where he had been

the time 5 of the earthquake. 62. typical Alaskan spirit, he replied, "I was standing corner watching all the streets go by.

2 Many attempte have been made

to find out the effects road safety."

3.

4**

5.

4)

the past

21 alcohol 3} several reasons this is

疊

a complicated problem. It is hard to establisn that the accused driver had been drinking. Even when there is no douby that, there still remains the awkward task that the accident was

é

proving

7 fact due the

alcoholic atate of the driver.

I was startled

side

the middle

•

whe great nola, Uus- 3) the night the day #yesterday. I jumped 2 my bed and rushed 6 the verandah to see what had happened. My :

my greatest ashonlah- want, found that the cottages just opposite

2) my house were

fire, I felt great sympathy 1:” them as the people who lived

there were people who lived

hand

"

2) mouth, the loss their homes now would be as great a blow as could be. the verande hI could see the scene clearly. Everybody was doing his best to put 5).

the fire

16) amall buckets and hoses, However, their efforts seemed 17) vain. I could even see anxiety 18) people's facea. Fortunately fire engines came t

and

no time

a few minutes, the fire was con- trolled and extinguished, The most fortunate. thing as though about ten cottages were burnt down, there was no casualty.

The human body consists V. three main parta: the head, the trunk, and the limbs. The bony framework 2) the head (called skull) is covered ___) hair,

age, the hair often turns grey or white.

Inside the skull is the brain, the centre

the nervous system. The face consista the forehead, the eyes, the nose, the ears, the mouth, etc.

The eye is the organ

sight. We sed

2) our eyes. The eyeballs are covered ́3/ the eyelids, and they are further protected the eyelashes. We must take care not to strain our eyes

time

rest.

too much reading.

7 time we must give them a

the help a telescope- we can see distant things more clearly.

2) seans

1) a microscope we can see very small things which are invisible (2) the naked eye...

6. The mouth is really the opening

Lips--upper and lowere

the

which the

breath passen when we speak, and

·which we take, g) our food when we sat and