二日二十月二年未丁赛
WAY KIN
英文中學會考試題預習 專欄
物理科 (十一)
·梁海明·
PHYSICS (21) H.M. Leung
Solution.
39.(e) e rub the ebonite with a rur so that the
ebonite carries a negative charge. By the method of induction, we can charge the gold- leaf electroscope so that it carries positive oharge. Then, we lower the body from top of the electroscope. If there is a divergence in the gold-leaf, the charge on the body ia positive. If the gold-leaf gradually ool.. lapses when the body is near to the cap of the electroscope, the body then might carry negative charge or be an uncharged body. To clarify this ambiguity, we first diecherge the electroscope, and bring the body again near it. If there is no change in the gold- leaf, the body is uncharged. If there is a divergence in the gold-leaf, the body.car- ries negative charge.
(b)
Brass cap Insulator
| Earthed metal case
Brass Rod
"Gold-leat
•Gold leaf Electroscope
Brass plate E
A rod is used charged with electricity of opposite sign to that required on the elso- troscope. The rod is brought near the cap so that the leaf diverges by the amount. desired, and electroscope is then moment- arily earthed by touching it with the finger, The rod is then removed, leaving the elec troscope charged
elack
Earth
charging a Gold-beef positively.
Fout break the earth connection. and then remove,
the mor
duction
break the
Charging a good leaf negatively by induction (12) A proof plane is pressed into contact with the outside surface of the hollow conductor at various places in turn and
then transferring the charge to the
electroscope..
The divergence. of the lead
PA
will give a rough measure" of the amount of the charge transferned. If the proof plane is pressed finto contact ruth the inside of the hollow conductor, it is found that there is no charge at all. (c)辦 The'electrophorue confistavof a circular
with
slab of insulating material, together he
e: brass disc on an insulating handle. ebonite slab is given a negative charge by, flicking it with a piece of fur, and then the brass disc 18 placed on top of ut. This results in positive and negative phargas becoming induced respectively on the lower; and upper surfaces of the brass disc. The braas dino is now earthed momentarily: by touching it with the finger so that the negative induced charge_is.repelledito. earth er
Insulating Handle
Brass disc
Electrons
tamaran
(ii) Brass disco
removed
"T) Charges induced. (11) Bracs die
temporarily earthed on brass dive.
Action of the electrophorus An insulated brass sphere is given a posit- ive charge from an electrophorus: It is thea: brought into contact with the outside of an insulated hollow conductor, the surface of the hollow conductor carries negative charges: by induction. When it is brought into con tact with the indide of the hollow conductor, the charge in the brass sphera is neutralised
40.(a)
by the hollow of two electrodes,
A primary cell
one positive, the other negative, and an electrolyte, usually made or a dilute s pauric acid. The positive electrode is a copper plate, and the negative electrode ie日 made of zinc plate: (i)Polarisations
în use
When a simple cell
is Tound that the current quickly falls to a very small value. This defect results from the formation of a layer of hydrogen bubbles an the copper plate and is called polarisation Ahe hydrogen layer weakens the current forw two reasons. First, the hydrogen layer sete up a "back e.n.f." in the cell in opposition to that due to the copper and zino; secondly, the gas partially insulates the plate, and therefore increases the internal rearstanc of the cell.
(11)Local motion: If the sine in the simple cell conelate of:
bubbles of hy- drogen will:
coming off the zinc. This
actign is caused, small impurit- ph.net up. tiny
Bubbles of the impurity and. eives in the acad
·ng zinc:alowly
In the Daniall cell polarisation dues not occur because copper, instead of hydrogeo deposited od to copper wan.In order to
ama iramated gino ie prevent local au
the positive pole is a carbon plate surrounded by a larssing mixtura o powdered carbon and man- Polarisatroo stoma is prevented by the
the:
de po-
日二十月三日七六九一零公布六十五市宝
We must use 3 celle"in"parallel to form a' group so that the currenia 6 amp.“We have to use 6 groups in series to obtain an emf 01_12V The following in the required diagram!
(c)
押
(11)(1) 36 volts at 2 amp. All the 18 bat-
teries should be connected in series.
(2): 18.volta at 4 amp.
Nine batteries are connected in ́series,
to form a group. Two groups are connected in
parallel.
事
車
A lead accumulator is a secondary cell, hence it can be recharged after it has rui down. Moreover, the internal resistance of a lead accumulator is very low, thus giving a large current with very little drop in terminal potential difference.
Topics for revision this week:
The chemical effect of electric current. Electrolytes and non-electrolytes. Faraday's bang. Electrochemical equivalent and the measurement
of current electrolytically. The coulomb: the ampere.
Questions. 41. (a)
State Faraday's Law of Electrolysis and ́ describe experiments to verify them. (b) Calculate the time required to deposit a layer of copper 0.1 mm thick and 10 sq.cm. in area, using a current of 2 amp. Take the density of copper as 9.0 gm per cio, and its electrochemical equivalent as 0.0003 gm pex coulomb.
A Daniell cell supplies a current of 1 amp.) for 1 hour. How much zinc is used and how fuch copper is deposited on the copper plate. during the time? (chemical equivalent of copper. 31.5, of zinc- 32,5; Ec.E of sɔpper] -0.000326 gm per coulomb.)
RIES
42.(a) It is requeired to deposit electrolytically
a layer of copper 2 mm, thick on a surface of area 30 sq.cn. The battery available has
. of 3.3Volts and an internal resis-1 tance of 2,0; the resistance of the plat- ing bath is 0 30. If the current to be Mused is 0.5 amp, find: (1) the value of the
series
ies resistance that must be included in the circuit; (ii) time taken to deposit the copper. (The density of copper is 8.9 gm per c... and the electrochemical equivalent of copper is 0.00033 g@ per coulomb.) (b) A current flowing in a silver voltamster.
deposite 1.32 gm. of silver on the cathode: in 10 minutes. An ammeter in series with the " voltameter raads 2.5 amp. Find the error of the ammeter. ( Assume the electrochemi cal equivalent of silver to be 0.0011 gm.
LIBRA
Kunese di Sie
watiapidined the hydro
manganese
Ioad, acadacele
per coulomb.)
End
此員
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兩間大學難適量供應 宜考虑協助專上院校
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一件不容易的事,無 讀幼稚綳 龔) 星期我們不必另一
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| 正日文專校
日七鏓逃正專職
四、新運概況 開始接受報名,招新生及神班生,於國來自勉,不可恸:懂 教育司註冊 創辧最早
四號至五七六號二樓開設九龍分校,號日,教育下一代而整 學生長途跋涉,在九龍旺角彌敦進五七,我希望大家煜「 至新界九龍該校就讀者。現戍校谌?今天燈節
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【六委員
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1計有:留學生考選委 魏大公、宋叙五、第 最近,故警院已提升繼聯委會若美 五、生活
• (E) |
主神九旺老
創班雞角。
新生分粥現
導正日文
學校
-旺角彌敦菹七四至七六號二樓
* 九龍分校 開新生及神诞生
: 即日開始報名
覦實被優,教法正拨,辉彤透澈,易明,錄音镪橉鸭
地
斑别招优班————四歲半至五歲半
高主教書院 驗 部招生
小學
報名日期:三月廿八日(星期二)上午八時半至十一時半 下午一時半至四時半,
一年級——五歲半至六歲半
天滨 音:
報名地點:香港羅便臣道二號本校
報名手機:携惴1出世紙 相片兩張報名費二元貼好 郵票及寫上申請人姓名及地址之封三個
201
校監 祁良神父