育教濟經頁三第張五第 BAJACERASI WAH KIU YAT PO
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四期星
日六月五年六七九一曆公年五十六國民藏中
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牌車新發月三 個百六千一逾
僑經濟
起來七牌
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十三内沙
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分示
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力夜機工
三六百爐餘港江版 四獾 十百 五 兼十百十七成頭。”
罗僑榮
1976中學會考試題預習專欄
物理科(廿五),何輝明
PHYSICS (25)
Solution for last week
1.(a) Lenz's law states that the
direction of an induced current
is such as to oppose the cause producing it. Faraday law of electromagnetic induction states that the induced electromotive force is proportional to the rate of change of magnetic flux. through the circuit.
(b) (1)
By the right-
hand grip rule,
A is a south pole.
Direction of current in coil XI
When the switch is just closed, there is a change in flux linking the coil YA
Therefore by Faraday's law of electromagnetic induction, a current is induced in the coil Y. By Lenz's law, the direction of induced current in Ry should be from m to n
south pole should produced at B to oppose the increasing change in magnetic flux Linking coil Y.)
(iii) Since the magnetic flux
linking coil Y remains unchanged, the induced current in the coil Y is reduced to zero soon after the switch S 18. closed. (By Faraday's law of electromagnetic induction)
3
When the resistance Ry is increasing, the current in coil X is decreasing and so is the magnetic flux produced in coil X.ESP
law, a
the magnetic flux linking coil Y 18 1/2
By Lenz's north pole should be induced at B so as to oppose the decreasing change in the magnetic flux linking coil Y.
the direction of the induced current in the resistor Ry is from n to me
(v) When the coil Y is moving
towards the coil X, the flux linking the coil Y is increasing..
By Lenz's law, a south pole should be induced at B so as to oppose the
increasing change of magnetic flux..
Therefore, the induced current through the resistor Ry is from m to a
(vi) When the velocity of the
coil Y towards the coil X is increasing, the direction of the induced current in the coil Y remains unchanged as in (v) (By Lenz's law); But the magnitude of the induced current in the coil Y la increasing since the rate of change of magnetic flux linking the coll Y is increasing (By Faraday's law of electromagnetic induction)
2.(a)(1) At constant temperature,
resistance of a
ig
affected by its length and cross-sectional area.
2 conducting wi
(ii) At constant temperature,
the resistance of a
conducting wire is pro-
portional to its length
and inversely proportional
to its cross-sectional
area.
ie. R =
A
where P is the propor
tional constant.
known as resisti- vity and
of the wïré
f. (6)
(ii)Suppose we have a
proton at D instead:
of the deuterium. What is its acceleration?
(in terms of a)
(iii)Suppose we have an
alpha particle at D,
what is its acceleration (in terms of a)?
個數新八牌本七版
增
9. The purpose of step 2 18:
a. To clear the slide of any dust,
one end.
b. To push all the blood to o
c. To spread the blood evenly along
the slide.
prevent clotting of the blood.
d. To P
e. To prevent it from drying up.
a. To prevent the needle from rust-
ing.
生物科(廿五)
10 The purpose of step 6 ia:
卓唐慕借
Ris the resistance
#25
BIOLOGY
is the length of
the wire
is the cross-
sectional area of the wire
(b) Lat be the resistivity of
the material making the wire. Assume that the initial · length and initial, cross- sectional area of the con- ducting wire are (, and respectively.
From (a) (11),
the original resistance of the wire
R12 = § (La)
(1)
After pulling, the resistance
of the wire ( 2 = ala
volume of the material making the wire is constant
• La^2 = loto
where A, is the cross-
sectional area of the wire after pulling.
the resistance of the wire after pulling
= n2p ( <°)
Compare (1)
(2)
and (2), we know that the resistance of wire is increased by times.
Question for next
week..
1. The symbols X and Y in the table
below represent unidentified nuclei:
A..
30265
and
and
30165
and
and 31
37x66
and
65
30465
and 30r65
Which pairs are isotopes?
1) Which do X and Y have the
same number of neutrons?
(iii)In which pair might X change
into Y by absorbing a neutron?
(iv) In which pair might I change
into Y by emitting an particle?
(v)
In which pair might X change into I by emitting a particle?
A proton, P., is a particle
carrying a single positive charge,
༡
A deuterium H is a charged hydrogen atom carrying a single positive charge, and having twice the mass of a proton.
A deuterium, D, is situated between two plates, P and Q. which are suddenly charged by joining to a battery. P becomes positive, and Q negative. The deuterium starts to accelerate with an acceleration à
R
(1) Which way does the
deuterium go?
the
Revision Test 4: MC on experiments:
The following appartus was set up shown in diagram and left
sun for several hours
precautions must be carried out if your purpose is to show the effect of the absence of light in photosynthesis?
I. Destarch the plant before the
start of the experiment- -II. Expose the plant to light for
sometime before the start of
the experiment“
ÍII. Use light-proof, to cover the
leaf partially.
IV. Use transparent paper to co-
ver the leaf partially. a. I only
CT II & IV only. b. I & III only d. IV only. e. None of the above is necessary
After the end of a certain a leaf was tested eriment, shown below.
Alcohol
rodine
·Solution
The colour in the leaf was noted and it was seen to have changed. This colour change can beat be explained by:
the presence of sugar formed in photosynthesis.
b. the presence of starch because of the role of chlorophyll in photo- synthesis,
the removal of chlorophyll
the alcohol::
the role of sunlight in phot theads..
the control for the experiment to show the effect of the absence
light in photosynthesis”
8. A blood slide is be prepared.
and stained with Wright's stain The steps in making such a sli are shown in the following set of diagrams.
b. To mix the alcohol"
To make it easier to
finger.
ick the
d. To sterilize the needle. e. To prevent the alcohol from
evaporating.
11.The purpose of step 4 is: a. To squeeze the blood out. b. To prevent clotting.
c. To prevent, the blood cells from
plasmolyzing.
d. To prevent the cells from shrink
ing..
e. To sterilize the akin.
12.The most numerous component of the
blood is seen to be:
a. Red blood cells.
b. White blood cells.
c. Blood platelets.
d. plasmagenes phagocytes
Answers explained:
6. B Since the only way we can show
that photosynthesis has taken place is to show that starch was made, we must ensure that at the start of the experiment there is no starch in the leaf and to do so we must destarch the plant by keeping it in the dark for, at least 24 hours. As the purpose
of the exp. is to show the effect
of the absence of light we should not let light fall on a certain portion of the leaf and so we must use light-proof paper to cover the leaf partially.
The
7:
B
alcohol removes the chloro- phyll so that the effect of the iodine solution would be more
visible. The colour changed to
deep blue-black in the area which was formerly green. This shows that chlorophyll is neces- sary for photosynthesis to take place.
8. A The finger is wiped with alco
hol to sterilize it, then a needle is usedwhich was dipped in alcohol to sterilize it), the finger is pricked to puncture the skin, a drop of blood is then deposited at one end of a clean slide, s thin film of the e blood is smeared over the slide with the aid of another slide, the stain is added
and left for a minute or wo, after that, water is added for
2-3 minutes and then the slide
is rinsed off.
cotton woul
alcohol
hol
10:D
The correct sequence of steps in making the slide is:
a. 4,6,1,5,2 and then 3.
b. 4,6,1,5,3 and then 2.
c. 4,6,1,3,5 and then 2,
d. 4,1,6,3,2 and then 5. e. 4,6,2,3,5 and then 2
9. C A thin layer of cells can
easily be studied if they are spread out and not clumped.
This is important in avoiding an infection. 11.E If you do not do this, an
infection may result.
12 A A large number of red blood-
cells are visible since there are about 5 million of them per cubic
millimeter.