買四第張五第
日三初三月年未丁磨
WAH KIU YAT PO
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日二十月四年七六九一八年六十五國民中
育效
育教僑華
英文中學會考試題預習專欄益
英文中學會考試題預習專欄
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化學科
二廿四
0
王錦釗。
CHEMISTRY,
N
物理科
( 廿四 )
梁海明·
PHYSICS. (24) H.M. Leung!
-phosphor bronze
Solution.
45. (a)
Concave
Suspension wire.
Mirror
coil on
former
retangular
"Magnetic Field of a Solenoid.
De Tule governing the polarity of the coils Grasp the solenoid along its length with the right hand so that the fingers point in) the direction of the current. The extended thumb will indicate the end of the coil which is the north pole.
Rotation
-Commutator Brush
41.(a)A solution of ammonium nitrite will decompose on
gentle heating, with the evolution of a gas nitrogen.
NH¿NU2 = 2Hɔ0 * Na
(bjwhen solid ammonium sulphate is heated, it will
sublime, the compound undergoing dissociation at the same time.
NH2 50 2NH H250
Ic)When solid ammonium dichromate is heated, it decom-'
poses rapidly with the evolution of heat and sparke are given off At the same time, the solid swelle up yielding a huge volume of green ash, chromium sesquioxide,
4.2.
TEST
(NH1), Cr2 ©2 = N2 ↑ Cr2 0, + kHzÛ
OBSERVATION
1/Jubstance "
A gas 19 evolved M is heated which turns red with caustic litmus blue
soda
INFERENCE
The gas gaven of 18 Bamonia since it is the › only common alkaline
gas.
M 18 an ammonium salt, because when an ammonius salt 19 heated with an alkalı, ammonia gas 19 1 given off
NH
OH NH.ON
WH21 • H2O
The above
Soft iron
igure snows
-Spiral connection.
the important pointa of a moving coil (Reflecting) Galvanometer. The electrical principles involved are that when the current passes through the coil and the permanent magnet make the coil turn- ing. The spiral, spring is a fine phosphor- bronze suspension wire and a spiral connec- tion of the same metal. This type of sus- pension enables the coil to be turned by a very small current. The torsion of the sus - pension wire supplies the force which re- turns the coil to its original position when the current ceas es to flow. The small con- cave mirror mounted at the top of the coil reflects a beam of light from a lamp placed near 1 metre away. The reflection gives a spot of light on a transparent scale placed near the lamp. The lamp and its focusing device are fitted with cross wiree so that, when the distances and focus have been ad- justed, a sharp image of the cross wires within a spot of light is thrown woan the scale.
a centre
zer
The transparent scale nas and is calibrated in centimetres and dille- metres. The spot of light, with its cross- wires shadows is brought to the zero position by moving the torsion head to which is at- tached the suspension wire Burrent travela into the coil through the suspension wire and the spiral connection. So the reflectea bean turns through twice the angle through which he mirror turns, the device provides the equivalent of weightless pointer 2 met res long.
(b) When currents
01 several amperes have to
be measured, a low resistance which by-passes the greater part of the current is placed in parallel with a millimeter, and then only
a small known fraction of the total current passes through the meter itself. A resistan- ce used in this way is called a shunt.
.Rs
Rm
ONGK
Let R be the resistance of the meter, R" be the resistance of the shunt, Let Nbe the number of times the maximum
ourrent required is greater than the max- imum current which qan be read by the meter Now, 1/N of the total current will flow through the meter and 1- 1/N (N-1)/N wil1 flow through the shunt.
Voltage drop across shunt = voltage drop I across meter; that is,
N-1
ROX
Rx
២.
R
R
m
N
(c) Applying the formulas found in (b).
R - 0:1
03
N
-.25/5 = 3
R 你
*
0.1
0.05 32
0.031
Cross-section of the wire 201
-)2π cm2 Specific resistance of copper 1.3 x 102
per cm.outs.
length of copper1®0.01552πc
1.3 x 10
29.04
am
Answer: The length of copper wire must be
29:04 cm.
16.(a)(1) Diagram showing the lines or magnetio
force in a horizontal plane associated an en
current flowing in a long straight,. vortical wire-
Currant flowing
downward
Gurrent flowingi
upward.
Cell
The above figure illustrates the construct- ion of a simple D.C. motor. It consists
a rectangular coil of wire mounted on a
Spindle so that it can rotate between the curved pole pieces of a U-shaped permanent magnet. The two ends of the coil are solder- ed respectivele vsako halves of a co- per split ri
Two carb
lightly
springs
circuit
coil r
Suppo
positi
on
cu the din
ing's 1
the side upward f
force. Ther
causes the com direction
ed to press
by means of cted in
it the
ntal
switched
Win
Flem-
that
an
Award
uple which clockwise
it reaches the vertical position. In this position the brushes touch she space between the two halves of the commutator and the current is cut off. The coil does not come to rest, since its mo- mentum carries it past the vertical, and w when this has occurred the two commutator halves automatically change contact from one bruch to the other. This reverses the current through the coil, and consequently also reverses the directions of the forces on the sides of the coil. Side ab is now on the right hand side with a downward fores
(11)Aqueous
solution of M+ silver
nitrate solution
A while ppt. 18 obtained which
is insoluble in nitric acid
The precipitate formen
is silver chloride
which is insoluble in nitric acid.
C1 Ag -
AgC1.
M is therefore a chloride)
Hance la ammonium chloride.
cant licks
Q. 5. (a)and(b)Put some lead nitrate crystals in a hard-glass.
fitted with a rubber stopper and a delivery tube which is connected to one arm of an empty U-tube.The U-Tube | is immersed in a freezing mixture. The other arm of the U-tube is connected to another delivery tube Which leads to a trougn of water.
$IBRA
on it, and side cd the left hand side with an upward force. The coil thus continues' to rotate in a clockwise direction for so long as the current is passing.
Topice for revision this week:
1. Ohm's Law; resistance; the ohm. Resistivityi 2. Ammeters, Voltmeters. Ammeter ahunta.
Questionaì
47. (a) Descrive a metnoa or finding the internal resistance of a cell. Explain how the resul
le calculated and give a diagram of the circuit, (b) Two Daniell cells, of the same internal re-,
sistance and each having an e.m.f. of 1.1 volts, were joined in series with an ammeter and an unknown resistance. The ammeter read-
ng was 0.16 amp. When the oells were re- placed by an accumulator (e.n.f. 2.2 volts) of negligible resistance the ammeter then read 0.2 amp. Calculates(1) the internal re- sistance of each of the Daniel1 cells, and (11) the current in the original circuit 11. one cell had been used, (o) What length of 0.2 in-water
m
wire will have the same resistance as 100 ya of 0.1 in-diameter copper, ware? For aluminu resistivity = 1.05 miorohm-inch #zerohms per inch cube), and for copper resistivity - 0.7 microhm-inch,
148.(a) A moving-coil galvanometer gives a full-
scale deflection with a current of 0.015 amp; when it is shunted with a wire of resis tance 0.505 it may be used as an ammeter with a full-scale reading denoting 1.5 amp. What is the resistance on the galvanometer, and what resistance must be connected in series with the instrument, the shunt having been removed) in order to convert it into a voltmetes reading up to 15 volts?
How would you convert a 1006ugalvanometer reading up to 120 into a voltmeter readin up to 2.4 volta?
A
The lead nitrate is then heated. The substance Becomposes giving off a dark brown gas. nitrogen dioxide, and a colourless gas, oxygen
2b (NO), - 2Pb0+ #NO. 1- + 0.1
The nitrogen-dioxide is separated from the oxygen by passing the mixture of the two gases into the -tube where the nitrogen dioxide condenses as e ellow liquid, dinitrogen tetroxide. (N204). If impurities such as dinitrogen trioxide are present- the colour of the liquid will probably be green
The oxygen does not condense as a liquid in ure U-tube, but passes through into the trough of water, where it is collected by displacement of water.
Make an aqueous solution of lead nitrate and ado dilute sulphuric acid. A white precipitate of leau' sulphate is formed, which ia separated from the
Lead sulphate remains in mixture by filtration:
the filter.
Pb (NOx), +
H2 30
- Fb 504*
• + 2:NO
Make an aqueous solution of lead nitrate. Hang a zine rod in this solution, and a 'lead tree', which is a case of lead crystals, is formed as a result of the displacing action of zine on lead.
co www3/2 + 2
(NOx) Pb
1)Action of nitrogen dioxide on
water--
Jitrogen dioxide is a mixed acid anhydride, When it dissolves in cold water a mixture of two acids. nitrous and nitric acida. 18 formed.
32.
H2O
HNO, HNO
+
nitrous acid
nitrio anta
Unless the liquid is ice-cold some of the nitrous acid decomposes. For example, if much water is present, nitric acid and nitric oxide are formed,
=
3 MNO HNO2 + 2NO + H2O BRI
11)Aotion of nitrogen dioxide on moist sulphur dioxide
Nitrogen dioxide will oxidise moist sulphur dioxide to sulphuric acid
NO 2
д
- NO
QUESTIONS FOR NEXT WEEK (25)
Q.1 (a) How would prepare and collect a pure specimen of
nitrogen peroxide in the laboratory? Illustrate your answer with a labelled diagram & the equation (b) Name the products which are formed when nitrogan
peroxide reacts with (1) cold water, (11) warm water, (111) damp sulphur dioxide, "(iv) buranı magnesium.
Give equations for all the reactions you descride',
Q.2.Describe how you would perform the brown ring' test
on a sample of sodium nitrate and explain the chemica) reactions involved.
Q. 3. How would you prepare orystalline specimens of sodim. altrate and lead nitrate from a mixture of sodium carbonate and Lead carbonate? *
Compare the Botion of beats ous-chiene two nitrates;
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