買四第張五第日三廿月三年展丙摩護

WAH KIÙ, YAT PO

報日僑華

1976 中學會考試題預習專欄

物理科 (廿四)

何辉明

PHYSICS (24)

SOLUTION FOR LAST WEEK

1.Let the resistance of.each

through Dy and Ly are equal

electric bulb be R.

(a)(i) The currents flowing

·and both equal to.

V.

·R

both would give the same brightness..

(ii) The currents flowing.

through I and by are the same and equal tó

both would give the same brightness.

(b)(i) If V.

Vand from the result in (a) the current. flowing through each electric bulb in fig. 1. is 2 times that in: fig..2..

Therefore the electric bulbs.

in fig. 1 is brighter than that in: fig.2.

(ii)Given that V2 =

=2y1

the current through each electric bulb in fig•

V

= 2R

R

the current through each electric bulb in fig. 1.

the electric bulbs in

fig.1 and that in. fig.

would give the same brightness.

(c)(1)The resistance of each

electric bulb =

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(ii)With reference to fig. ∵the\current flowing

through I in fig.1

A

(a)

2. (a)

With reference to fig. 2,

the equivalent resistance of the circuit = 6+6=12:52 the current through In

12

resistor installed

The current through In=

But the current through

the brightness of Ly is not affected but the brightness. of is is reduced.

Rag = 1002.

G

·To convert a galvanometer into an ammeter, a shunt resistance should be connected.

The current through the shunt resistance R =200.0001

= 19.9999 A

The potential different across the shunt resistance

(0.0001)(100) = 0.01 V the shunt resistance required

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日二十月四年六七九一层公年五十六國民輩中育款備造

To convert the galvanometer into a voltmeter, a multiplier.

resistance should be installed.

The potential difference across

the multiplier resistance

= 100 - 0.01 = 99.99 v

The current through the multiplier resistance".

≈ 0.1 mA = 0.0001 A

the multiplier resistance required

99.99- 0.0001

= 999900 N

(c)The current flowing through the galvanometer is 0.06 mA although the current in.the. external circuit is greater than 0.06 mA. The remaining part is conducted away through the shunt resistance. RA ahown in the fig.

G

··06 0.06m

100

008

Original

Scale

(0.08) 80

as

·Scale after

midi

the potential difference between the points to be measured

-80. v

uestion for next week

∴{a} State Lenz's law and Faraday's

law of electromagnetic. Aductione

(1) Determina the polarity of

the magnetic field produced at A when the switch is closed:

(ii) When the switch $ is juat

closed, would a current

be induced in coil Y? If it does, what is the. direction of the induced current in the resistor

Ry?

(iii) With the switch s kept on closed and Ry. being.

unchanged, what changes have been taken place in the: induced current in. COLI Y? (iv) When the resistance of Ry

is increasing, would the" direction and magnitude of the induced current vary?. (v) with the resistance n

unchanged, the coil is. brought nearer to the coil X. What is the direction

of the induced current in: the resistor Ry? (vi) When the velocity of coil

Y towards coll X is increasing, would. the magnitude and direction of the induced current in the coil Y be altered?

(In each answer above, mention:

the law involved in each casé. ) :(a)(i) State the factors that

affects the resistance of a:conducting wire at constant temperature. (ii) What is the relation.

between the resistance of the wire and these factors? State the relation mathe- "matically. What is the

name of the proportional. Pconstant?:

(b) Prove that at constant tem- perature when the length of the conducting wire is... increased by n times by

pulling the resistance of the

conducting wire is increased. by natimes.

END:

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0.01

19.9999

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(b).

(b).

1000

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(0·0001)(100)

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刊月錶與鐘頁一第張六第日三十月三年辰丙歷要

京只瑞士表壇 世界高級名表

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