1978-04-17 — Page 34

頁二第張九第 日一十月三年午戊

WAH KIU YAT PO

育教僑華

the gloss priam is 1.5, find (i) the angle of incidence.

of the ray

B

1978

中學會考試題預習專欄

Win & Cole Prais

物理

(廿八) 魯榮家

PHYSICS (28)

Revision Exercise (cont)

5. (8) A tuning fork of un-

known frequency vibrates in

unison with a stretched metal

vire vibrating with ita fund-

amental frequency. If now the

length of the wire is increas- ed by 1/50 of its origin) length length, then it is found that 5 heats per sec-

ond can be heard. Find the

frequency of the tuning

(b) In an experiment to find the

velocity velocity of

sound in air, forks of diff... erent frequencies are used. and the shortest length of

resonance air columns are

measured (as shown in figure 7)

Fregnency

Shortest leng-

th-L. (in cm)

(in Hs)

250

29.0

320

21.6

420

15.2

500

12.0

fork:

Figure 7.

(1) Plot Lagainst 1/f. (ii) Estimate, from your graph,

the velocity of sound in air and the end correction of the tube.

6. (a) Figure 8 shows an

object AB-19cm tall placed in

front of a concave mirror of

focal length 15cm, so that it stands on the principal axis

concure

(ii) the critical angle of

of the glass prism.

SECTION 6

ELECTRICITY;

MAGNETISH & ATOMIC PHYSICS

7. (a) Figure. 10 shows the potentiometer used to measure the e..f. of an unknown cell.

E. AB is a wire of length Im,

G is a sensitive centre zero

galvanometer and the e.m.f.

of the driver cell is 2V

Figure 10.

driver celt

(1) The galvanometer G reads

zero when contact X is 0.75m

from A, what is the e.m.f.

of. E.

(ii) If the 2V driver cell.

has an internal resistance

of 1 ohm and wire AB has a

resistance of 6 ohms, where

is the balance point when a 2-ohm resistor is placed in series with the ZV battery (h) Practically, it is easy to damage the galvanometer hy large current if contact X is made on the wire AB far from the balance point. With the aid of diagrams, suggest a method to prevent from

damaging the galvanometer, (c) With the aid of diagrams, explain how a potentiometer can be used to compare the

ep,f,a of two celis,

8. (a) A galvanometer has a resistance 10 and a full scale deflection of 10mA.

(i) Describe in detail, họw

may it converted into an amme ter reading up to IA,

*

(ii) If it is to be converted. into

voltmeter with separate ranges of 0-1V, 0-10V and 0-100 full scale deflection

報日僑華

一期星

motion of the rod AB in each

cade

(i) What is the correct

operating voltage of the lamp (i) Calculate the power dig- sipated in the Jamp at its. correct operating voltage.

(iii) If the only voltage

supply in a house available to operate the lamp is 240V, hence, a resistor should be inserted in series with the lamp, find the value of the resistor.

(a) A transformer steps down from 250V to 12V with 1200

turns on the primary

(1) find the number of turne on the secondary,

(ii) If the efficiency of the

transformer is 80% and the current taken from the second- ary is 1A, find the current. taken from the 240V supply.

capper rés

Figure

Figure 14

Figure 13 and 14 show two copper rods lying in a hori- zontal plane. Electric curr- ent is flowing in each of the cases. What will happen in

both casES.

(e) Figure 15 and 16 show a espper rad AB is rolled in

the

dicated direction, what

is the direction of the in- duced current in each case

日七十月四年八七九一篇公年七十六國民華中

ooolco

8

Figrue 17

Figure 18

(e) A radioactive element is examined by means of a G-M tube connected to a scalar.

The count rate drops from 8000 per minute to 1000 per

minute în 6 minutes. Find the half-life time.

育教僑華

化學

1978

「中學會考試題預習專欄

明德社主鏞...

(廿八)朱宏林

Chemistry (28)

Solution to 2.40

(a)

(25

It is known as the hydrolysis of eater.

(11) There will be an increase in

the yield of the products. This is because the sodium hydroxide added neutralizes the ethanoic acid formed, thus shifting the position of the equilibrium towards the right side.

(iii) n is equal to 1 as it is the

methanol, CH,OH, which does. not give an alkene on dehyd- ration by cone H250

Solution to 0.41

20250 OH+ 2Na2C2H ON+H

ount of hydrogen gas

2240

22400

moles

≈ 0.1 moles

According to the equation

in (i),

no. of moles of Na dissolved.

= 2 x no. of moles of H2

= 2 x 0.1 moles

= 0.2 moles

mass of Na dissolved

0.2 x 23 g 4.6

ii) folecular mass of ethanol-46 According to the equation

in (1),

no. of moles of ethanol used.

2 x no. of moles of H2

0.2 moles

mass of ethanol used

= 0,2 x 46 g

= 9.2 g.

mass of ethanol distilled off 50 g -9.2 g. 40.8€

(iv) Use a water bath and do not place a flame near the dis- tillate because the vapour of ethanol is highly inflam mable.

Ethanol is covalent in nature so it has a low boiling point and a low melting point.

The solid is ionic in nature so it has a high melting point and a high boiling point.

(vi) The white powder is sodium

carbonate.:

Sodium, when exposed to air, is oxidizes to sodium oxide by oxygen in air. Sodium oxide reacts with thé mois- ture in air to form söddum. hydroxide.

4Na + 2

2N320

Na2+H20 2NaOH

The carbon dioxide in air then reacts with sodium hy- droxide formed to give water and sodium carbonate.

2NaOH + CO2

(b) The formulae of the oxide and the chloride of the metal are respectively MO and MOL,

Let y be the relative atomić mass of the metal. Then, y + 16 1.5 y + 71 = 3:0 1.e. 3,8(y+16)=1,6(y+71)

113.660,8

1.02.20

=52.8

y = 24

Figure 8

(i) copy figure 8 on to a graph paper, locate the image of AB by a graphical method,

Show your construction lines.

clearly.

(ii) Using the information

from your completed diagram,

find as accurately as possible

the linear magnification of the object AB,

(b) Where must an object be

placed in front of a mirror

of focal length 15cm to form images timea enlarged ? (c) A ray of light enters a

face of a triangular prism of refractive angle

60"

and can

just emerge from the other face as shown in figure 9.

BA

shown in figure 11.

Figure 11

What are the values of the

resiataza

B12 and

(h) A lamp of filament rexist- ance 40 is designed to

carry a current of 0.5A as shown in figure 12:

The ester should be methyl ethanoate.

(iv) n is equal to 3 as it is the

propanol. The ester could

be propyl ethanoate or 180- propyl ethanoate..

8

Solution to 2.42

A is copper(II)chloride;

B is iron(II) sulphate; C in magnesium chloride; Dia ammonium chloride.

*+20H ̈→→Cu(Òñ) 1⁄2

+20H-Fe(CH)2. L2++20H ̃→→Fig(OH)2

Figure 15

(v)

CHCHCH OH CH ̧CH=CH¿+H20

(11) (1)

CHCHACHCH

CH=CH2+520

Он

angle of

(glas) Triangular Priam

Figure 9

(air).

If the refractive index of

Lamp

0.3A

(mm) 600

Figure 12

Figure 16,

= (d) Figure 17 and 18 show & copper rod AD is placed on two metal rods and current is flowing on each of the cases

what is the direction of

8

Xis nitrogen, gas. NH NO2

+2H20

(i) Y is magnesium nitride.

31g +

(iii) 2 is ammonia gas.

W is magnesium hydroxide. NgzNo2 + 6H20-3g(OH)2+2MHz

(iv) The electronic structure of

W (magnesium hydroxide) 18:-

(2) CI + Ag". (3) SO+Ba

2

2+

88

A; e.g. sodium;

AgC1(e)

BBS04(s)

(ii) A; e.g. potassium hydroxide;

(iii) Ej e.g. neon gas;

(Lv) Bja

e.g. aluminium, Al3+;

(v) C; e.g. phosphorus; (vi) 0; e.g. silicon; (vii) D; e.g. chlorine, C1"; (viii) or 0; e.g. aluminium.