育教僑理
買三第張四第
日四廿月三年米丁春夏
育敎僑華
英文中學會考試題預習專欄
物理科
(廿七)·梁海明·
(PHYSICS (27)
H.M. Leung)
Solution,
(51.(a)
The figure illustrates the construction
of a simple form
of A.C. generator for the continuous production of e- Jectric current by electromagnetion: induotion. The machine has a reo- tangular coil of wize which is To-
tated in the mag-
netio field pe-
tween the poles
U-shaped permanen
R
Rotation
BRUSHES
S
magnet. The ends of the coil are connected w to two slip rings mounted on the coil spindle. Current may be obtained through the two carbon brushes. As the coil rotates ite sides cut lines of force, and therefore a currens ie induced in it, By Fleming's right-hand: rule, the current flows from back to Pront walong side A and from front to back along
side B.
As a starting point, condider the instant that the coil is in the vertical position. with its side A uppermost.
Ons Complete
~E.M.F. generated
Yo
Cycle
Nolutions
3/4
In this position the sides of the coll ar moving along the lines of force. No cutting of Jines is taking place, and therefore the e.m f. 18 zero. During the first quarter of a ratatton. the a.m.f, increases from carow
to a maximum where N KENDE SK-Ron-.
tal position. After'
again the second quarter of the rotation;
and once more becomes-zero when the co2 18:
in the vertical position with side B
most. During the second half of the rotation the.
anerated follows the same pattan
as
dirept
in the first half, except that the
of the e,m.f. is reversed.
If the: slip rings. and brushes of the aim) ple A.C. g nerator, are replaced by a single split ring with two diametrically opposed
enes the machine becomes converted into
genera
The brushea are an ranged so that when a coil 187 passing throug
position the two halves of the split zing ar duet on the int of ohangin oont noterom
brush to another” The solit] called.apom/ kator
gnet 10 iS otions Å oylindrical
lated copper wire is wound round
CITY
HALL
WAH、KIU YAT FO
a paper tube Successive layers of turns am separated by stri
calico. When the coil. ds findehed, its ends are connected to sensitive galvanometer. A magnet is then plunged into the coil and the galvanometer hddcld ix nogiced to give a momentary des flection, ahowing that a current had been induced in the coil. On removal of the magi net from the coil the galvanometer gives aw other deflection in the opposite direction.. This effect is called electromagnetio induc- tion. No current is induced by allowing the magnet to remain at rest inside or outside
magnet pulled yout
• of coll
期星
日三月五年七六九一聚公年六千五國民華中
(varying electric current is set up havLNg)
the same frequency as the Bound waves. A similar electric current is caused to pass t through the solenoids in the receiver. This alters the strength of the magnetism in the. U-shaped magnet and produces a
a corresponding variation in the pull of the diaphragm.
The transmitter of a telephone instrumenti icongrets essentially of two polished carbon
discs placed a short distance apart, the space between being filled with #mall carban
the discs granules. One
CARBON One of is fixed and the other connected to a metal diaphragm. When a person speaks into THIN the transmitter the djan (phragm is setsin motion
by the sound waves, and
INS ALUMINIUM DIAPHRAGMY
GRANULES
so the carbon granules are.cÁRBAN eubjected to vardable
Compression. Consequently the
BLOCK
¿TRANSFORMER TO BATTERY
Electromagnetics
Induction,
Faradey's law of'electromagnetio induotion/ Whenever t theres a change in the magnétic flux linked with a circuit an electromotive force is induced, the strength of which is proportional to the rate of change of the flux linked with the circuit.
Induced electropative force is another way of expressing the electromagnetic induction We think of the induced current as a result of the wire being cut by lines of force when the magnet moves. We aay that an electro- motive force 1s induced whenever there ag a change in the number of linea of force thread through, or linked with, the coil 52.(a) A
wgassuming the transformer is an iddal one,
we can calculate the ratio of number of turna in secondary to
that in primary,
Frome Secondary
primary e.ma
Number oi turng in seconde
aber of turns in、primary Secondary e.n.f.- 15 volts Ar Primary e.m.f.
200 volte A. C Number of turns. in secondary
Number of tuzna in pr mary
PRIMARY
SECONDARY
ephone regelver magnet formed by plac bar magnet across the
of two soft-iron baray
laced so that it ex
on a springy magn diaphragm. Two
are wound in oppo
eot, long on the 2011
rson speaks
an alternating
snetio
het iron ande md in the Law The hàg- VIRVILE depend.
rns in the two
17 18
step dow
a U-shaped horf pe] anent:
XO
into th
B\pda Pree
electric resistance between carbon" blods
varies in a corresponding manner. A steady current is passed through the transmitter by connecting it in series with a battery and the primary of a step-up transformer. Thus, when sound waves enter the transmitters the changes in resistance will cause the current in the nrimary circuit to vary. A high voltage: A/C will be set up in the se- condary of the transformer with the same frequency as that of the original sound waves entering the transmitter. This is transmitrd / to the receiver at the other end and is then converted back into sound energy
Tonics for revision this week:
Network solutions by Ohm's Laws,
2. Internal resistance of cells; more calculatione
on notentiometer.
Questiona.
53.(a) Resistances of 20 ohms and 5 ohme are con-..
nected in parallel and the joint resistance. connected in series with another resistance, of 24 ohms. A voltage of 14 is applied to the ends of this cirouit. Find (1) the ef- fective: resistance of the circuit,(ii) the) ourrent passing through each of the reela-l tances.
FRIES
) A cell of e.d.f, 2 volts, a 5 ohms 001)
and a 2-ohm coil, are connected as a shunt across the terminale of the 5 ohma ooil / reade 1-3 volts, What is the internal re→ sistence of the cell? If a second ohm cox is now connected in parallel with the first how will the reading of the voltmeter be affected71:
A Dentell cell has an
and internal resistance
of 1.1 volta
as measuredy
by a potentiometer. A poor vo tmeter of resistance 32 ohmsg is connected to the velLT What is the reading of the voltmeter? Repeat for a better voltmeter of resistance 252 ohms and draw your conclusion.
The wire
and has a re
Dandell cel
poter
with a galvanometer
enia 200 am,lor.
When a
@tested on the
potentiometer wire, no current flows ougho the galvan eter when the points tapped are 125.om apart. Find the current flowing in the potentiometer wire,
END
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仍日
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日四十月三年米丁麼宴
'WAH KIU YAT PO