1969-04-09 — Page 15

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食教係華頁三第張四第6日三廿月二年西己歴夏 WA1-966 132+

報日僑筆

三期星6九四年九六九一骞公年八十五國民 鞠如

1969

مقالك

學會考試題預習

Parts

Acid accumulators

Alka litia

accúmalat ora

Positiva

物理科 ( 廿三 )

pols

陸永熾

Red Tead (Pob

Nickel bydroxide

育華僑華

PHYSICS (23)

Negative

pole

Litharge (602)

Ça dmdum

接第四張第二賽)

SOLUTION

43.(x) In a Simple Cell

(1) The positive pole (11) The negative pole (111) The electrolyte

copper plate

-zine rod

e solution of dilute sulphuric acid

(iv) The source of energy-chemical (potential)

nergy.

(1) When a simple cell is in use it is found that the current quickly falls to a very small value. This effect results from the formation of a layer of hydrogen bubbles. which is an insulator to current. It suffocates the action of the cell.

(1) The hydrogen bubbles blankets the positiv

copper plate and is called FOLARISATION. ((111)

Electrolyte Dilute sulphuric

acid

(a) When fully charged, the s.m.f. of a load acid

cell is 2.2 volts. The cell is regarded as

fully discharged when the e.m.f. drops to 1.5 volts.

(d) Re-charging two accumulators from a D.C. supply

ahargar

Caustic potash

九六九中学入学試題預 英文科 (廿三)

劉士俊

on

Now they are coming out

at of

the shop and

into

along

walking to

School in

the atrest.

an

to into

of

Copper) +

ElacṬrfyta }

hydragan. bubbles

The hydrogen layer weakens the current for

two reasons.

(3) It sets up

in opposition

and zinc.

"back e.m.f." in the cell

to

that due to the coppeI

(2) It hydrogen partially insulates the

copper plate and hence increases the internal resistance of the cell.

SIMPLE CELL

Brass cap (+va contact) Pitch top seal.

Insulating card covering Zing can (~76) ·

Rheostat to regulate) charging current-

(1) Direct current must be used in charging an accumulator since this is a process to revaras the chemical reaction which ham been playing in discharging the cell. (11) The rheostat is used to regulate the

charging current. Too high or too low current in charging the accumulators may -lead to harmful effects.

Persistently undercharging a cell may lead to sulphation with the indication of a yellowish white patch on the plate, and an increase in internal resistance.

(1) The conditions of an accumulator is judged

its specific gravity of the electrolyte. A hydrometer is used to measure the S.G. of it. When the 5.G. Le 1.25, it is fully charged. When the 3.G. is 1.18, it is fully discharged, b

(11) Corrosion is caused by the acid or acid

fumes attacking the connectors. Dirts or impurities may also cause corrosion. To prevent corrosion, the lugs and connectors must be kept clean and covered up by grease (111) A yoltmeter is used to measure the e.m.i

and hence the condition

s

(iv) I a call is sulphata or the cells,

Ammonium Chloride Jelly

100

Mixture of powdered

Carbon and Manganese

Dioxide

Carbon rod (+10)

Cardboard Washer

Eve contact made here

The commercial dry cell is a type of Leclanche cell. It consists of a zinc yeasel

vessel is which forms the negative terminal, The in contact with a moist paste of ammonium. chloride and glue which surrounds a muslin bag containing a carbon rod and a mixture of powdered carbon and manganese dioxide. The carbon rod is the positive, terminal. A layer or cardboard separates the bottom of the zinc vesse from the muslin bag, and the top of the vessel. 1s sealed with pitch, apart from a small vent to permit the escape of the gasos liberated in the cell.

*

When the external circuit is completed, positive zinc ions go into ammonium chloride paste. The Nh lona move to the carbon rod where they give up their positive change

The uncharged NH is unstable. It changes into JH, and hydrogen. The hydrogen thus liberated 1. >xidized by the manganese dioxide; water and phe de oxide (M2) are produced. The action san sa summarized by chemical equations;

20

2NH GÌ 2021.

2GHT

2NH

2NH

The wet type of Lechanche cell needs very little attention and is very well suited to intermittent use. The manganese dioxide in the porous por, is loosely packed, so the polarization is slow.-

In dry cELLJ wise caroon, and mingandyo dioxide mixture is very tightly packed. This gives a low internal resistance and a rapid depolarization. It is concise and portable.

In primary cells only small current is. produced as a result of non-reversible chemical changes. In contrast with primary cells, the secondary cells, also known as storage cells iscumulators can be recharged after use, Another advantage of a secondary call is that it. maa low internal resistance and hence can givə. 1 large current with very little drop in

terminal potential difference.

it has to empty out half of the electrolyte and fill with distilled water, Place on charge at a low rate until S.G, and voltage become normal.

6 accumulators

Let R be the external resistance, added, £ be the charging voltage and 1, be the charging current.

- IR 6 x 1.9

IR ---100 - 11,4

100

22,2 ohms.

The resistanės used is 22.2 ohms.

uestions for next week!

45.(a) Roferring to the diagram,

(1) The pole at I 18 A

pole.

(11) Connect the terminals of one dry cell to

the end of the coil to produce the pola indicated.

(111) If the soft iron core 18 removed, the

strength of the pole at:X will (increase decrease, remin the same). Give reason I u your answer.

(iv) If some of the turns of the coil are removed, the strength of the pole at will

(v) If a commercial lead storage cell is

substituted for the dry cell, the strength of the pole at X will

(b) In the accompanying

diagram, complete the circuit in such a way that the bells are in parallel and both ring when the button is pushed.

N

(e) complete the diagram in such a manner that the

bell will ring when either switch is closed.

tó. Can't you see them? I Came here, and look out into

off At

this window.

4. In the following passage there is no punctuation, Write the passage in the space given, putting lig all the punctuation, a

The giant said I can make myself into a

11on

I do not think you can said the oat

can and I will said the giant

Below are five short conversations. In each of them put a tick() against the group of words. which gives the best meaning.

"Bera's my new coat."

"How much

"How many,

"How long

did you pay for

"How far

"How do

(b) "He didn't like that picture.

"So did I." ( ) "Weither día I." "Either I did." "Neither I did." "Eighter did I.?

the servant?”

"There's the

He is gone." went."

"He is there." "He had gone. "He 80.

"He has broken his leg." "Send for the doctor." ( "Sned to the ambulance," "Send to his home." (-. "Send to the hospital. "Send him in."{{\)}

"How many pens have we bought?"

"At last (

"At least

at all (2

"At lesse "At once

Answers to Test

(a) off, (b) yet," (c) in, (d) up, (e) lately, (f)] holidays, (g) why, (h) on, (1) which, (1) because, (k) instead, (1) quiet, (a) too. (n) since, (o) but, (p) tallest, (q) late.

(a) Does she speak English very well? (b) Will be bring his camera with him? (c) Did an old beggar stand by the gater (d) Did the girl cut her hand with a knifer, (e) Did he easily lift the heavy wright? (a) 1. D. 2. B. 3.0, 4.CA, (6) 1, C. 2.

4. E. 5. D, (c) 1.C. 2. A, 3. B, 4.

2. B, 3. A, 4. B, S. D.

c), (d), (e), (8), (1).

A resistance coll or nominal value 2 obra 1s found to have an actual resistance of 2,12 ohms. What length of wire of resistance 20 ohms per matre must be connected in parallel with the coil in order that the combination shall be exactly 2 ohma?

(a) When a cell has ita terminala connected through

an ammeter to a resistance of 2 pham the ammeter indicates a current of 0.35 amp. Another resistance, of 3 ohms, is then joined in ceries with the original 2 ohma and the current is reduced to 0.2 amp. Calculate the B.M.F. and the internal resistance of the ce11.

A length of copper wire or mass 4.5 kom ons resistance of 14.7 ohms.

Calculate the length and diameter of the wire, Density of copper - 8.93 gm/c.c. Resistivity of copper - 1.8 x 10-6 one-cm (e) A galvanomster having a resistance of 200 onns gives a full-scale deflection for a current of 0.2 milliamp. Show, with the aid of diagrams, / how you would adapt the instrument for use as (1) an ammeter having a range of 0 to 1 amp.) (11) a voltmeter to read from 0 to 100 volts.

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四第張四第日三廿月二年西巴雅里 WAH KIU YAT PO