1967-05-03 — Page 15

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