再四第張五第日八十月三年丑
南僑樂
1973英文中學會考試題預習專欄
物理科 (廿七)
Physics (27)
2. Cathode rays.
It was discovered by ittorft
that the cathode of a vacuum tube emitted some kind of rays which casted a shadow of the obstacle. placed in front of the cathode on a fluoresc ent screen. He called these rays the cathode rays, Later, experiments found that cathode rays consist of streams of negatively charged particles. The charge to mass ratio (1/m) of cathode rays was determined and found nearly identical to the ratio of electrons. Hence cathode rays are identified as electron streams.
Properties of cathode rays-
m
(1) Deflected in an electric field * in a direction opposite to the
field.
(2) Deflected in a magnetic field
in a direction determined by Fleming's left hand rule.
3. Application of cathode rays.
(A) X-ray tube
high voltage
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The inputs to XX & YY are from external source.
If the input to YY is simu- soidal & the input to XX is simply a horizontal sweep, the resulting
wave form observed on the goreen
is as shown in fig 3
Cathode ray tube are commonly found in all sorts of electronic
devices such as television, cathode ray oscillograph (CRO) etc.
4. Radio activity
There are three kinds of radiations emitted by radioactive substances. The radiations are X-particles, B-particles and
-rays
The substance which decays by emitting an α-particle is said to undergo an -decay. An
α-particle is a helium nucleus. It must be noted that what causes a radioactive decay of a substance is the nucleus of the atom. This în an α-decay, the nucleus of the radioactive atom has its masa number decreased by 4 & its atomic number decreased by 2. The nucleus which undergoes a decay is called the parent nucleus while the product nucleus is called the daughter nucleus. IF
2X is the parent mucleus and Y is the daughter, then an ✅ decay can be represented by the equation:
-particles
aluminium plates
of a few cm thick..
SETOTIIBA,
sheets of
Stopping
日十三月四年三七九一座公年二十六國民華中育教僑華
are therefore multiplied. When
velocity of light.
dectector cloud chamber Geiger Muller tubes.
material.
paper of a
few cm thick.
ionizing
100,000
100
penetrating
power
1.00
velocity
about 1% of velocity of
light
about 10% of
Remod
Remark: The ionizing power of the radiations listed above is a ratio That is the ionizing power of an.
particle is about 100,000 times as great as that of a-ray photon whereas the ionizing power of an electron is about 100 times as great.
5. Detec
(A) Wilson's cloud chamber.
Source holder
W Conciation
the electrons produced reach the
anode, they constitute the anode
ourrent which is then suitably
amplified and the number of
radiations incident on the Geiger: Muller tube can be counted on a scalar.
6. Half-life
The half life of radiactive substance is defined as the time for half of the atoms initially present to decay.
e.g. The half-life of radon is 1620 yrs. This means that if we have 1 kg of radon. Then in
Tin 1620 yrs. We have kg left.
a further 1620 yrs, we have 1 kg left and so on.
Half life of radioactive substance
s ranges from millionth of a second to million of years.
Solution to ex. 6.
In
58
the number of
neutrons is 58 29 29.
60
In zyNi
1s 60 - 29
neutron number
When a nucleus absorbs a neutron, the mass number of the nucleus is increased by Its atomic number is unchanged. This can be seen from the equation
If a proton inside a nucleus absorbs an electron, mass number of the nucleus is unchanged whereas the atomic number is decreased by 1. This can be represented by
cooling system
X-may's
These
Cathode rays are accelerated towards a metal target usually copper embedded in platinum, by means of a very high p.d. between the cathode and the target. highly energetic electrons are able to mock the K-, L- or M- shell electrons of the target atoms. Vacancies are therefore formed in the inner shells. Electrons in the higher orbits will jump back to fill the holes. In doing so, the extra energies are emitted as X-ray photons. 1.8. photons of very short wave lengths.
Remark:
(1) About 1% of the incident
cathode rays lose their energy in producing X-rays. 99% of the cathode rays lose their energies by converting into heat. Thus a tremendous amount of heat is developed. If the target is not cooled suitably, it may be melted soon after the X-ray tube is suited on. The target is generally cooled electrically or is made hollow so that a continuous stream of oil
passes the target and carries away the heat developed.
Properties of X-rays.
(1) Very short wavelengths.
(About
-10-8
ém.)
(2) Very large penetration power.
(Continued to cathode ray tube.
(B) Cathode-ray tube
AAA
As shown in the figure, C is the cathode.
G is an electrode
with a hole which is maintained. at a potential negative relative to the cathode so that it controls the number of electrons passing through it. Aq, An and Az are electrodes comprising the so-call- ed electron lens. They are all negative with respect to the cath- ode
By adjusting the potentials OF Ap
the cathode rays
• A2 & A3 from the cathode can be made to converge at a point on the screen. The plates labelled XX and YY are also electrodes. The XX electrod- es imposes a horizontal movement of the beam while the YY impose a vertical movement of the beam.
In a B-decay, the parent loses one electron through the reaction
+ B
The atomic number of daughter is Z + 1 because the atomic number of the 6-particle can be taken as-1.
Indeed the reaction described above is only one of the three possible modes of 6-decay. Another 2 modes require an acquair -tance of elementary particles and more advanced electron theory which is beyond the scope.
An nucleus is said to undergo
decay if it emits a photon.
The reaction is
The parent and daughter are the same nucleus. But the parent is in an excited state while the daughter is in a stable state or at least is more stable than the parent.
The properties of the 3 types of radiations are tabulated as follows.
QJEZ
undeflected
deflected
undeflected
deflected
about 30 cm || -rays
thickness of
lead.
particles
-rays.
Source:
Stroncium 90
Cobalt 60.
charge!
neutral
1.
mass
2 proton a
1 electron
2 neutron
·100,000
In a
deflected
velocity of
electric In an
field
deflected
- particles
+2e Americium 241
magnetic
field
light
large
Range in about 4 cm.
air
piston
As shown in the figure, the piston is almost frictionless. The chamber contains a dust-free. supersaturated water vapour. When the piston is suddenly lower- ed, the expansion of the chamber volume causes the temperature to fall. Water vapour therefore condenses. The condensation takes place preferably on the positively charged ions produced by the radiations. By illuminating the chamber by light, the droplets. reveal the path of the radiation for particles, the tracks in cloud chamber are short and straight lines. For electrons, the tracks are irregular. For -rays, the tracks are short. and irregular
tracks
B-trucks
8tracks
The tracks are straight. because X-particles are heavy particles and they have very short range in air. The electron-tracka are irregular because when an electron collides with a water. molecule, it is deflected in another direction and so on. The Y-tracks are indeed produced by electrons But these electrom result from the ionization of the water molecule produced by
-rays. These electrons have. only moderate energies. Hence their tracks are short and irregular.
(B) Geiger Muller tube
radiation
mica windows
to amplifier
an
When a radiation is incident on the Geiger-Muller tube, it produces ionization of the atoma inside the tube. Fach ionization produces 2 charged particles electron and a positively charged. particle.
The electrons travel towards the anode and the positive particle travel towards the cathod On their way to the electrod -es, the charged particles collide with other atoms in the tube and produce secondary ionizations. The number of charged particles
1
protons and 2 neutrons inside a nucleus combine to form a brolium nucleus and then leaves the nucleus. This is an
X-decay. The reaction can be represented by
The mass number decreased by 4.
& the atomic number decreased
Suppose the p.d. between the cathode and the anode is 400 V and an electron beam having a mass or 10-5
kg and a charge of 18.x
coulomb is emitted
10 thode.
by the
a. What is the K.E. of the beam
on reach the anode.
What is the velocity of the electrons at the anode.
If each electron has a charge
of 1.6 x 10.
-19 coulomb,
find
the number of electrons reach ing the anode?
If the electron beam takes 0.01 sec. to reach the anode, find the anode current?
prode
beam of electrons is emitted by the cathode. If an elect ric field is imposed as shown, how will the beam be deflected If the beam is to travel in a straight line by imposing a magnetic field, what is the direction of the applied magnetic field?
In fig 8, X means the direct- ion of the applied magnetic. field is into the paper. What is the direction of deflection of the particles, emitted from the -source?
Lsource
X X X X 2 x x x x
XXXX
What
4. Visible light, X-rays and
gamma rays are em waves. is the chief difference between them? What is the production mechanism of each type of the ém waves?
Page 20Page 21
★教僑華頁一第張六第 日八廿月三年丑癸
1913英文中學會考試題預習專欄
聚道英文書院主編
歷史科 (廿七)
History (27)
Multiple Choice General Revisi
European History
The World Wars:
31. Arrange the following in Chronological order:
I the Dreikaiserbund (Germany
Austria, Russia)
IT the Anglo-Japanese Alliance III. the Dual Alliance (France,
Russia)
IV. the Triple Entente (France,
Britain, Russia).
V. the Triple Alliance (Italy,
Germany, Austria)
II, III, IV
I, III, II, IV
I, II, III, IV
V. II, IV:
III, II, IV
What were Eismarck's aims in creating the Drei kaiserbund in 18727
to isolate France & to. keep European peace for internal German reforms.
B. to prevent revolutions C. to prevent absolutism amo
its members
D. to replace the Treat
Frankfurt
of
Why did Italy join Germany Austria in 1882 Triple Alliance?
A. because France obtained Tunis
B. to gain prestige
C. because of her fear of isolat
-ion
D. to check Russia
E. to gain more colonies
34. Which of the following was not a reason for the France-Bussi-
alliance 1893?
A. their fear of growing German
ambitions.
3. Russia borrowed money from France to develop Siberia.
0. the Heligoland Treaty of 1890 between Germany & Britain.
Germany's new aggressive
policy after the fall of Bismarck.
France helped Russia
the Black Sea Treaty
break
The following are all results of the Dual Alliance except
P. Britain became the only
isolated power,
A new balance of power in
1973英文中學會考試題預習享譽。
地理科
(廿七)
Economic Geography (27)
Multiple Choice Revision:-
1. The world's largest producer
of coffee is
A U.S.A.
B. Java.
C. Ghana
D. Cuba
E. Brazil.
Cotton plants are easily.
destroyed by a kind of pest known as “
A, silkworm
Bitse tse flies
O. mosquito
D. boll weevil
E
larvae.
The conditions necessary for. the growing of wheat are as follows:-
I. a tropical region
II. moderate rainfall of
TII
20′′ - 40′′
extensive and undulating lowland
IV. a warm summer temperature
* of 21°0) 12.
V. Sandy & well-drained soil.
A. II, III & IV only
B. I, III & IV only
0. II, III, IV & V only
D. All of them
E. I, II & III only
The type of farming which is
commonly practised in countries such as China, India & Japan
A. commercial grain farming B. intensive subsistence
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European was formed.