頁二第張六第日九初月四年戌庚
WAH KILJAYAT PO
育僑華
郭口倫者
Closed chains in an.
unmagnetized bar
三期日三十月五年〇七九一圈公年九十五圃民華中 育教儒華
慶中文中學
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英文科 (廿八) 王淑方 •
-5-1970
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物理科
(廿八)
PHYSIOS (28)]
MAGNETISM
Substances say be classified. maij
magnetic
non-magnetio, and magnets
Magnetic substance is one capable of being magnetised; A non-magnetic substance cannot be magnetised, and a magnet possess magnetian.
There are two important classes of magnetic substances (1) para-ṇagietic substances which are attracted by
a magnetic field and tend to move towards the strongest parto of the field, a.g., iron, cobalt and nickel and
(2) diamagnetic substances, such as oismuth azu anti-
mony, which are repelled by a magnetic field and so send to move towards the weakest part of the field.
Properties of a mamat
1:
7.
4 permanent magnet will only attract para-mag- netic (ferromagnetic) substances
A magnet points to the N-S direction when set swing freely.
A
lisagot has two poles. A pole is where ite magnetic effect seems to be concentrated.
Poles always exist le paira, North and South Pales. Single pole cannot exist
Like, polas repel.
Unlike poles attract.
When a magnet is broken into two. also act as magnets,
two pieces
If a magnet is heated; ite magnetic STIGOT LOBOS, It can be remagnetized when cooled,
The strength of magnetism reaches & maximum a a point of saturation.
Distinction among magnet, magnetic and non-magnetic
bare of identical physical appearance
Three bars are put together
pree barg are put
as showm. The magnet and
the magnetic bars would
cling together while the non-#agnetic bar would fell off as magnete attract ferro- magnetic substances only.
Open chaine in a magnetizea, waze
Free poles appear at the ends.
Magnetic phenomena explained by Molecular Theory: ▲, When a magnet is broken into small portion, each
portion becomes a magnet with two poles, Free ende are always exist forming the N po and the and the south pole.
Saturation. There is a limit to the amount of saturation as all the dormains are aligned, that ie all the molecular magnets are in open chaïn position.
The phenomena of retenzivity and susceptibility, Soft iron 16 easily magnetised; it 18 very suse ceptible to magnetisation, but soon losen its magnetism when the magnetising field is removed, that is it has low retentivity. It is believed
that in sofft iron the molecules are loosely packed, and so are easily arraged and diearranged, Steel, on the other hand, has a low susceptibility and high retentavity as the molecules are tightly packed, and steel is useful for making paermanent magnets.
D. Heating magnete. When a magnet is heated or
violently knocked, the alignment of dormains has be" disarranged and loses its magnetism, Methods of magnetising
Single stroke method.
Solenoid. method
Double stroke method
كان السا
The strong magnetic field set up by the electric current lines up the domains of the speciman Mora effectively than any other method.
Note the direction of current flow and the direction of the magnetic field and the poles of the saget.
TEST: TWO
ESSON TWENTY-EIGHT
SECTION FIVE
TEST PAPERS
(ENGLISH PAPER II, continued)
Complete the following sentences by adding a, an, the, some,
some, any, IF NECESSARY
write the answer against its number given in the margin. If nothing needs to be added, write X-
I think he is(1) very good athlete who may come (2) first in (3) cross-country race that is going to be held tomorrow.
It is better not to tell anyone. because (4) people cannot keep a secret.
Ask him to bring us (5) chalk. There isn't (6) in this drawer. (d) (7) honesty of the clerk was never doubted for one moment.
He took up (8) swimming to get some exercise,
It is unnecessary to have (9), oral test every week,
(g) When he saw the ghost he was)
overcome by (10) fear. Complete the following questions replies as in the examples,
EXAMPLES:
(2)
(3)
[(90.
short
"Your son works very wafu, doesn't he?" "Yes, he does""" He has a deep out on bar. hasn't he?"
"No, he hasn
"It's a lovely day,
Yes,
"You won't forget to put some wo
before you go out,
"No,
(c) "They liked it,
"Yes,
(d) "If he hadn't had an accident, Joh might have.
been a wonderful pianist,
(a) "You want to ask my permission to go to the
ma,
"Yes,
T
Magnetic. SXAN
Magnetic length
role strength.
The bare that left behind are the magnet and the magnetic bär.
Arrange the two bars in T shape. If attraction occura The horizontal bar is the magnetic substance and the vertical bar is the magnet,, If attraction does not occur, the horizontal bar is the magnet. The two poles of a magnet are near the ex- tremitien The centre of the bar is neutral.
The straight line joining the poles fa called the magnetic axis.
The magnetic pozon are the pointə in a magneti through which pass the resultant of the forces of
attraction or repulsion on any other magnetic poles placed near
The 18tance measured from poir to pole ie called the magnetio length. It differs from the physical length of the magnet.
pole of unit strength 18 one which, when placed 1 co in air: from a similar pole, causes a mutual force of repulsion of one-dyne
Molecular theory of Magnetinu
A ferromagnetic substance is considered as one having When the substance is small molecular magnets in it.
no magnetized, these molecular magnets are in a form › of closed chains. here are no free poles and so external magnetic field is formed. The act of magn metizing causes the closed chains to open and rejoin as open-ended chains. The free poles at the ende construct an external field.
These molicular magnets are called magnetic domaene. Pure non-magnetic materials do not have the ability ito form donáine.
Tapping or hammering
Gently tap a bar of soft iron while it is paralle1 to la magnetic field or the Earth magnetic field. This agitates the domains so that they fall into lines
Methods of depametising
Hasting the specimen above a temperature called the Curie Point
Hammer the speciemen while it is now aligned in. any field
Mametic Inquction
The
then a permanent magnet is brought close to a dar or ungagnetised soft iron, the bar acquires the proper- ty of attracting other ferromagnetic material. soft iron hae become an induced magnet by the tempor
The effect. ary lining-up of some of its domains. effect explains why a previously unmagnetized bar can be attracted by a magnet: it may disappear when the baermanent magnet is removed.
Kagaste Held
A magnetic field is a region where
1
a force of repulsion is experienced voca north pole is placed there, or
a moving electric charge electrons for example) ozneri sacés à force."
Magnetic Miciu menda My
The force per unit north pole at a poins 19 a field de referred to as the magnetic field intensi sy 188 vector quantity.
Mametic lines of force
A magnetic line of force te an imaginary line dese cribed by a free N-pole placed in a magnetic field. It is used to represent graphically the field in- tensity distribution
Lines of force are
orce
(1) drawn from N to 5 poles
(2) repel one another:
(4)
never intercept
tend to contract.
Terrestrial Magnetism
The earth 18 assumed to be a large spherical magnet. displaying two poles, one near Hudson Ray in Canada. and
the other in Antarctica. The fact that these poles do not coincide with the geographic poles leads
which is to the concept of the angle of declination, the angle between the geographip meridian and the magnetic meridian, at a given location,
The fact that the earth is not flat gives rise to the concept that the compass needle dips at angle do horizontal except at the magnetic equator of the earth, his angle is called the angle of dip,
VII Complete each of the following sentences by
writing in the blank space the most suitable expression you can find from those given below The word or words in brackets will give you a guide.
(a) All their plans,
because of the ban on foreign travel. (came to nothing) (b) He has been
with influenza for
more than a week. (ill in bed).
(c) When he discovered that his opponent was stronger.
(withdrew) than he, he soon
He was
gun. (cheated)
ELECTRICITY.
Electrostatics
by the man who sold in that
未完轉入第六張第三頁
Electricity is an abstract concept incapable of de- finition in terms of anything simpler, Electric charge 18 therefore a basic concept to be added to those of length, mass and time discussed in mechan- 1c8. Charges in made evident by the attractive and repulsive force observed to exist between charged bodies. There are two, and only two types of elec- trostatic charges positive charges and negative charges,
Coulomb's Law of Electrostatice
Like charg tract
repel and
charges at-
the force of attraction or repulsion Detwei two charged bodies depends directly unpon the product of the charges, and inversely, up- on the square of the distance between them.
An atom in excess of electrons is eaid to be nega
tively charged.
An atom in deficiency or electrone 18 said to be positively charged.
Electrified by friction
When a body is rubbed, it gains or loses one or more conduction electron (cutermost orbital electrone or free, electrone). If a body gaine electrone, it ac- quirede a negative charge; if it loses electrone, it acquires a positive charge, because it will then, have an excess of protonę.
Conductor and insulator
conductor has free electrons that can move s981) within the body),
An
insulator has Id