1970-05-31 — Page 13

育教僑 頁第張四第 七廿月四年戌庚属 WAH KIU YAT PO

XU5%$@%%$#SAD #8%S #SIKIKI STSII%$##%# & #3 #4 #SES #ESS

報日僑華

日期

日一卅月五年〇七九一曆公年九十五國民華中

Lustance or image from mirror in GET TO

罗僑

#3%$##$%$#$%$#$%&#8%$}8%;

英中會考物理科答案

(續) 堅道英文書院撰答

Suggested Answers

for

PHYSICS (Conventional Questions)

Hong Kong Certificate of Education Exam.

"of

∙1970

LIGHT

(a) 1) Simple experienta to find the focal

(b)

30

length of a

mirror.

concave mirror is placed on the base of the stand and the pin adjusted in position until it coincide with its imge at C.. This is ful- filed when there is no-para- llax between the exploring pia and the image of it. The pin is now at the centr of curvature of the mirror. The focal length of the mir rer is half of the length of PC.

PO

Concave mirror forming a real and mammified

Image

The object should be between the principal. foous and the centre o curvature.

The image 19 inverted and mamified. It is formed outaide the centre of curvature of ̈ the minner.

A and B, total internal reflection cocur,

angle of incident » critical angle

-4-1.414

над с

Doseo C

14414-JE

cot. C

AB

the Tagine or the circular pool of light seen on the eurface of the licuid

- CH 100, om.

The diameter of the circular pool in 200 cm.

SECTION D.

5. (a) (1)

ELECTRICITY AND MAGNETISM

(1) Faraday's two Laws of Lectrolysis.

Law I. The mass of any suhetagse liberated aur-

ing electrolysis is proportional to the quantity of electricit passed through the electrolyte.

Law II When the same quantity of electricity o

passes through various electrolytes, the madees of the Prerent substances liber- ated are proport: onu is their chemical equivalente,

Mass of silver aspositi Volume of nydrogen Density of hy

mass of hyarozen liberated

Volume of oxygen liberated Density of oxygen given Mass of oxygen liberated

gm/c.c.

30

From the tale of atomic weights we find that the atomic weights of hydrogen, cxen, and silver tre respectively,

1, 15, 108

Hydrogen and silver are monvalent while oxygen is divalent..

By dividing atomic weight by valency the chemical equivalents are thus.

8,

103

The Masɛse of hydroyan, oxygen, and silver obtained by electrol is in the experiment 215, respeo

If Faraday's second law is true tha

es ought to be in the same ratio chemical equivalent weights.

it will be verified in

(a)

Maes of zinc disolves liquid gains weight

0.00034 x 2 x 7 x 60 .3

Mase of copper deposites( loses weight]

0.00033 x 2 x 7 x 60 gm.

Balancing the lose and gain in weight the solution is found in gaining

(0.00034 0.00033) 2 x 420

0.00001x 2 x 420

0.0084 N.

A galvanometer is designed to investigate the presence of current in a circuit. It is not used to measure the magnitude of current. The wire wound on the coil is too thin to carry current larger than the speci- fied ones which is normally a few milliamp eras,

To measure currents or voltage (p.d.j, modi. fication should be made.

(i) Use as an ameter

A Low resistance is connected in parallel with the meter to bys pasa a greater part of the current and only. a snall known fraction" of the total current passes through the mET- er itself

(ii) Use as a voltmeter

A high resistor, known as a multiplier, is in series with the meter. Only a very small known voltage- drop is across the meter. The larger partion in on the multiplier,

(b) Resistance of the wire is directly

Zonal to the length of the wire,

ROL

If the length is doubled, the resistance will be proportionally be doubled.

(1) Therefore, the resistance becomes o onme

Resistance of the wire ia inversely proporw tional to the cross sectional area

ROC

(ii) If the area in doubled, ita resistance will

then be halved. that 18, 2 ohmaj

(a)

3 volte

R = 6 ohms

Resistance of wire AB, 1 m. Long - 4 ohms Resistance of wire AC,0.75m Long = 3 ohms Resistance of wire CB, 0.25m Long - 1 ohm

Current in AG's current in E

10

Distance of object from mirror in cm

20

30

Inac

to 1-

(1) unoone an aroitrary 001n't on the graph AB,

Say,(20730)** This means thịt the object is placed at a point 30 cm from the mirror and the corresponding position of its image ie 20 m from the mirror;

From the equation of airror, spherical

12

The focal length of the mirror 18 12 om.

(11) The grapn AB 110s in the rarst quadrant

where both u and v are positivo. The image formed is real..

(111) The graph 00 1ies in the fourth quadrant

where the image distance is negative. Virtual images are formed.

(c) Rays incident from the electric bulb will

undergo a total internal reflection when the angle of incidence is equal to the critical angle, C.

Depth of the bulb - 100 cm Refractive index of the liquid

108

of this experiment arз dus

The clouding of hydrogen around the electrodes

The temperature rims of the acidulatec water,

The hydrostatic pressure of the liquid column inside the burette..

Water is saturated with hydrogen dur ing the preliminary adjustement,

elivers a current of Daniell cu for a period of 7 x 60 secolds

(1) Energy dissipated within the cell-

2

H(current) (internal resistance;

x (time)

I2rt joules

4 x 7 x 60 joules

* 420/4.2 400 calories;

ii) The change in weight of the copper

electrodes

Mass of copper deposited

(e.c.e) x (current) x (time) 0.00033 x 2 x 7 x 60

-0.2772 g

(iii) Inside the sell, a sino electrode immer-

ses in a solution of zinc ione (Zn++) and

a copper electrode in a solution or bi-

valent copper ions (Cu++),

As current is drawn, zinc goes into solu tion and copper is deposited on the copper electrode.

Current in CB

Total current I

(1) Current in C

Current in AC

Current in

ampere-

* 2/3 amp. 1/3 amp.

1 ampere

(11) If R is replaced by a cell of enp. 2.2 valta

Se followe

with current notations indicated. et. A

In Loop FAGY

2,25 - (1 + £!)3

In loop MABN.

3 − £ x 1 + (i + 1')3 .............(b) The difference of equatione (a) and (b)

PIVOS

0.75 -11 +0

75 am)

The portion AC is of the whole length

so of 3 volte is across AC.

of 8 VOLTE 18. exactly 2.25 volto

There will be no current flowing in branch of XY. The current flowing in the driving circuit is 0.75 amperes.