買二第張六第
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物理科(二)
PHYSICS (21)
39.(8) The ability or a material to retain its magnetism
is called retentivity, Steel has retentivity and therefore is used for permanent magnets.
The material used for an electromagnet anould nave a poor retentivity. It should, however, be easily magnetised. Soft iron processes such ability and is used in making electromagnets. The ease with a materiaI can concentrate magnetic lines of force is called its permeabili Soft iron is very permeable, and is used to act
signet keepers in forming a close chain.
The magnet in a loudspeaker is a permanent magnet, it should retain its magnetism for a long period of time. Steel is suggested for that. purpose.
Telephone earpiace must have a springy magnetic alloy diagram of poor retentivity and high permeability,
(b) Haking magnets
Magnetization by stroking:
A magnetic substance can be made into a magnet by stroking it another magnet. Thus stroking the bar from left to right with north pole of a magnet will leave the bar magnetized, with a south pole at its right end. The atoms of the magnetized bar lined up and produced, two polás.
Magnetisation by Electricity:
The best method of making magnets 18 to the magnatio effect of an electric current. strong magnetic field is set up by using a noid with 500 or more turns of insulated copper wire connected in series with a 6- or 12- volt electric souras. The bar to be
agnetised is put into the solenoid and the current is switched on and off. It is unnecessary to leave the current on for any Length of time, as the bar with not will not become magnetised any more strongly and the noil, may be damaged by
The polarity or the magnet depends on the direction of flow of the current. In holding the solenoid with your fingers in dirsation to the current flow, the thumb when stretch out points to the North point.
[c] The three similar bars are put together and ther
released. One of them will fall off. Obviously, this is a non-magnatic bar. The magnet attracts the magnetic bar as shown.
九六九
報日僑茶
三期星日六廿月三年九六九一萜公年八十五婴民蓥中育教僑锻
DIY EUROPA
(廿一
劉士俊
Test 21
are not arrangea in any definite order, só that there magnetic poles neutralize each other. In magnetized bar, large numbers of atoms have their north poles pointing in the same direction. Thus the ends of the bar has large numbers of atoms whose magnetic poles are neutralized. The" unneutralized magnetic poles form a strong field outside the bar,
i) The strength of a magnet cannot be increased,
beyond a certain limit, magnetic saturation. This occurs when all the molecular magnet's have been drawn into straight lines along the length of the bar.
Heating supplies extra energy to the vibrating molecular magnets. The orderly arrangement of the atoms is destroyed,
The molecular magnets tend to form a close chain. Therefore, magnets can be kept with söft iron as keepers in a close chain arrangement.
(iv) than a magnet is broken into two, the
neutralized poles are set free. The two separate parts become individual magnets. Each has N- and 5- poles at the ends, (r) Arrange the molecular magnets with all north
poles in one direction, the polew will neutralize one another, leaving no free pole in the ring.
40.(a) (1) Negatively charged.
111) Positively charged:
Like charges repel sach other,
英文科
In each of the following sentences put a tick (vag against the word which beat completes the meaning.
****
I,
(a) Between you and
there is no secret
their
your.
(b) It was
his hin ho her
whom I met
she
(c) The servant has bought a new hat. So it ia
(a) He and
she her hera them
are close friends,
You know better than
wine
Fou
Unlike charges attract,
(f) You know my name but I don't know
yours
them
(8) Her pencil is
their them theire
Jour
ara blunt.
The polarity can be checked with a charged gold-leaf electroscope.
There are 1onized air surrounding us. The lonized air will contain both megatively and positively charged, freely moving ions, ionized air will discharge any charges already present. Warning discharges the moisture surrounding the apparatus making demonstrations easier. When a charged rod is brough near to a neutral body, a cork, some of the charges (same charge) in the cork are repelled to the far end. end nearest to the charged rod bas oppose charges. The attraction between the rod and the nearer side
is greater than the repulsion, hence the two bödiss attract.
When contact is made, electrons pass from the rod into the cork. Charges on the near and are neutralized. The rod and the cork have like charges only. Like charges repel. This explains why a neutral body is first attracted and then repelled.
No charge remain on A for both cases. The inducing charge on A exactly neutralise the
site induced change, die d
When charge A is lowered deep inside the can, the divergence of the lear increases. On touching C, divergence remins constant. The "induced opposite charges inside the can are
neutralised but the induced charges cutan ko s
the can remain there..
(11) Before touching
after touching
لما
them
Let
they thefr theirs
go wi
she
(1) Nobody except
her hera they
Was absent.
who
whom
whose
what
are you apsakingi
Put a tick (...) against the word which best completes the meaning of the sentence,
(a) That boy is so
·Care careful carelessLY careless
he never does
ything right.
(b) of the four widows, the fattest is
Well
wealth
wealthy.
wealthiest.
The weather to-daj
hottest hotter
hot heatTM
than yesterday,
(a) Iron is
useful used useless
(未完轉入第六張第三買)
Leaving trie, non~; magnet bar alone, the other two is arranged as a letter 1. If they attack one another the horizontal bar is magnetic while the vertical one is a magnet. If not, the horizontal bar is a magnet, the other is magnetic but not a
T
magnet has its atoms arranged so that large numbers of a tome have their north poles pointing in one direction. Thus the ends have large numbers of atoms whose magnetic poles are nuc neutralized. The middle of the magnet has no magnetic strength at all. The poles are neutralized;
d) Theory of Magnetian
It is assumed that each individual atom of magnetic substance is, in effect, a tiry magnet with a north point and a south pole. (Most recent theory indicates that groups of atoms are magnetic unit known as domains). The magnetism is due to the notion of the electrons in the atoms. The atoms of an unmagnetized bar
(111) No charge left at A
The inducing and induced opposite charges exactly neutralise each other,
Questions for next week
Describe and explain how a gold-leat electroscops my be charged positively by induction.
Describe an electrophorus and explain, now it works, What is the source of energy of the spark which may be obtained from it?
(c) A charge is placed on: (1) a spherical conductor;
(11) a pour-shaped conductor, Draw a sketch
showing the wariation of the density of the charge all over each surface. How would you show by experiment that your answer is correctf: When the centre of a horizonta1. windmill is pivoted on an insulated pointer and connected to the high voltage terminal of a Van de Graaff generator, the windmill spina round when the generator is working. Knowing that the ends of the metal have pointed ends, explain why this happens, (d) Explain the following: A bird sitting on a
high-voltage wire of the National Grid, system is unaffected, but a fatal accident my be caused when a person unknowingly touches the live mine terminal which is at a relatively low voltage.
(a) In terms of electron theory, explain sach
the following -
(1) Matter is electridal in nature, (11) A negatively charged body can attract.
neutral body
(111) A positively charged body will be
discharged if a glowing body is brought
near it
(iv) The passage or a current of electricity-
(v) The difference between a conductor and an
insulator
(vi) What happens when a tody becomes charged
positively?
42.(a) What is meant by the e,n,f, of a cell? (b) State Ohm's law and defins resistance.
Darive an expression for the resistance of two conductors connected: (1) in seriøs j (11) in parallel.
What values of resistance could you obtain You were supplied with three coils sach of Tesistance 1 ohm?
battery consisting of 6 cells, each of auf, 2.0 volts and internal resistance 0.1 ohm connected to a 9-ohm cell. Find the current in the coil, If now a 6-ohm coil is connected in parallel with the one of 9 ohme, Find the new value of the current in the latter, What would be the reading of a high-resistance voltnster, connected to the terminals of the
in each of the abèrve circuits?:
The diagram represents an operating circuit. The current, measured at point A is 4 amp. (1) Find the resistance of the circuit between B and C. (1) Find the current at C. (111) Find the voltage drop between A and B. (iv) Find the current in the 4-ohm resistor,
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