S APR 1967
六廿月二年添丁麿院
育敎僑華
英文中學會考試題預習專欄
物理科
( 廿三 )
•
梁明。
PHYSICS (23)
H.M. Leung
Solution.
43. (a)
20 12
41%
552
Resistance of 2012 and SIzin paralisi. 1/(1/20
- 4 Q
+
2/5) 2
Resistance of the complete circuit
• 422 + 4.52
- 82
When 80 watta are being dissipated in the 5. ooil:
(i) From
EI
* E-IR
ป
80 Watta
R
5
2 80
I -
- 16
aap.
I4
(ii) To find the total current through the
circuit, we find the voltage at AB first,
80 From E
- 20 volts.
W
-
-
current through the 2012 resistance - 20/20
- 1.
total current
(iii) The supply voltage
-
4
amp.
號
5
amp.
= total current : total registance
* 5 x 8 volts.
→ 40
volts.
Answer: The resistance of the complete cir-
cuit is 8 ohms,
(i) The current in the 5 ohma
co 1
3日 4 amperes; (ii) The total current through the circuit 18 5 amperes.
(iii) The supply voltage is 40 volta,
粒
WAH KIU YAT PO
報 日 儲
日期
盘
Fower of the fire
-
working
From the formula W = EI
已海宁四年六六况一六千五剑民中
ฟ
Current of the circuity £ t
With the lid and lamp removed from the'
Dalorimeter, connect the lamp and at the
variable resistance so that the ammeter reads
1.5 amperes. Let the lamp run for a little
while so that the current becomes steady.
Place the lamp on one side to cool to room temperature. Now run in from a burette en- ough water to cower the bulb to ite buck. Record the volume of water.
Switch on the lamp, rapidly make any silgat adjustments of the variable resistance to ob tain a current of 1.5 amperes and at the same time, start the stop water. Allow the current to flow for 7 minutes exactly, stirring at regular intervals. At the end, read the tem-
thuế,. obtaining the rise in temper-,
perature,
ature.
Remove the lid," lamp and all water from too, bulb. Allow to cool to room temperature and then set the rheostat to give a current of / 2 amperes. Repeat the experiment as before using exactly the same volume of water for, the same time of 7 minutes. Find the rise 14 temperature.
If we neglect neat losses, the heat given out by the lamp equals the heat gained by the water and the calorimeter.
In these two experimente," the heat given" out by the lamp in each experiment will have been proportional to
the temperature rise in each experiment.
Let t be the temperature rise in experiment,
to experiment,
the firat)
be the temperature rise in the
then it will be found that, within the limite of experimental error,
-+1
t.
(1.5)2 (2,0)
Thus, no neat produced by a ourrent of electricity flowing through a metallic con- luctor in a given time le proportional to the
quare of the current also the heat produced 8 proportional to the resistance of the com- luotor and d the time, Ire combing these into
single statement, aptain Jorte
Law.
IR
Where 日 18 505
1 - B/R
2000
amp..
250
From 'mst where 2.-1/
- at
m(60 - 10),
50m
Also Q
watt x time
J
2000 x 7 x 60°x00
1.2
cala
- 12,000,000
50m
ical.
☐ - 240,000
- 240
12,000,000
'kg.
[Answer: The current taken by the fire-work- ing is 8 amperes. The mass_of_water 1母 240 kg.
2100 wat沓能
(c) Water equivalent of the cylinder 2000 gu
mase of water e.
28,000 gm temperature of water before heating-10°c temperature of water after heating -70°c power of electricity. price of electricity per unit' =20 centa the current_of_the heating coil - W/E.
2100/200. - 10.5 (ii) the time required_to_beat_the_water_to,
.70°C
*From
(i)
elso Q
Q=009 t
-
amp.
(2000 + 28,000}x(70–10) 30,000 x 60
1,800,000
2100xT/4.2
ca.
cal..
where T is the time in second)
1,800,000 × 4.2
-
2100
*ȧill cost of heating the wate 2100 x 1 x 20
cost
•
42.
:seconds)
nour..
cents
-Outer Calorimeter
stirrer
lagging
The apparatus is set up as shown above. A
12 volt 24-watt car bulb is mounted with its Cholder on a piece of wood which serves as
ja lid to the inner and outer calorizeters. Two holes in the lid allow the stirree and thermometer to reach the water in the innar
calorimeter. An ammeter reading to 3 aaperes :: a variable resistance rated at 12 volts 3
amperes, together with a stop watch complete i
the equipment.
Heat produced in calories,
R the resistance in ohms
J. the electrical equivalent of heat
) W-20 watts, E-12 volt-
From W=ET
(ii) Cost of running the lamp for 100apure
at a cost of 30 cents' per unit.
Sost
100x20x30
1000
- 60
ONG ROY
KD
oente
Answer(1) The current of the heating coil
is 10.5 amperes. (ii) The time
quired to heat the water to 70°C
一
ia one hour. (iii) The cost of heat; ing the water to 70°C is 42 cents.
Toples for revision LIAM
1. The magnetic effect of current.
2. The behaviour of a flat coil carrying a current en a magnetic field. The moving coil galvanome-
ter, The_D_C..._motor.
Describe a simple form of moving-coll gal=] vanometer, and explain its action, (b) Explain how a shunt is used to adapʊ an.
ordinary galvanometer to measure currents.
The maximum current that an ammeter of 0.12 resistance can read ie 5 amp. Determine what
length of copper wire
De connecLER 物 parallel with the ammeter so that it can be employed to. measure a maximum current of 125 amp. (specific resistance of copper
1.3 x 10~6 per_cm cube, Diameter of wirel
0.31 mm.} Draw diagṛams snowing the lines of magnetic) force in a horizontal plans associated with, a current flowing in: (i) a long straight, vertical wire;(11) a solenoid with its axis) horizontal. In each diagram show the direct, ion of the current and in (11) state a rule, which giyes the polarity of the coil. (b) Describe_the principle and
CIBRARIES
44.(a)
Answer: (1) The resistance of the lamp is 7.2 ohms (11) The cost of running the lamp_for_100_hours is 60 cents.
IfTone"joule of work is done when one lomb of electricity peases between two point then the potential difference between these two points is one volt.r
coue
Temperature or water before heating-10c Temperature of water after heating m6003
Time of heating.
Potential differenge
PU
- 7
-250;
action of a Dadd
motor.
snis End
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日夜班,郎日開
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一)幼稚園高低斑男女生
(二)小鹕 Pl:P.6男女生
本校招考下列新生
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學司及官排名號,該校
英文部、刘雅初极九十名、其他各設五至十名
·中一新生八五五名 小一新生一八〇
本校下年度招收下列新生 一九六七至六八年度新生入學考試握告
利瑪竇書院
有此事。
草在一九三七年時巴
-
西及比較上不窝的成
一辦理,在於一般人 祈福像者。根本上,
一定頻費甚多,超 粛作爲敎育工乆井不 交- 利用電
隊全安通交校學
「隊一增區龍九
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中文部
中一新生四〇五名、小一新生九一名。 幼稚初級九十名,其他各段五至十名
我快方式 凡亲節,交 脂保要退路的秩序! 交通的熱忱,然後方 通常餓,蔥應有维持
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本即可辣照,成人说 游少失事的發生,所以一批良好交通的技
申請;凡欲申附上列率額者屆時請到本校下列各校 本校將於四月十二日(星期三)開始辦理新生入學
. 處佤取有關表格
“三校-桀水均大埔頂七十號°C電話- 八一批) 第二校“尖沙咀山林里十八號(僅路-大七○HOK). - 第一校-大坑東四三九〇號地段八嚆-二七):
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