1973-02-21 — Page 20

育教備藥頁四第張五第日九十月正年丑癸夏

1973英文中學會考試題預習專欄

齪道英文將院主編

物理科

(十八)

Physics (18))

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Heat can be transmitted from one place to another by one of, three ways, namely conduction convection and radiation. Thermal conduction.

The process of conduction of heat involves the flow of heat from places of high temperature. to those of low temperature. The actual amount of flow depends jointly on the material of the conductor and on the temperature! distribution along the conductor. The property of the body which determines the flow of heat under certain standard conditions is the thermal conductivity. A good con ductor of heat has a very high thermal conductivity while a poor conductor of heat has a very low. thermal conductivity at ordinary temperatures. Silver is known as the best heat conductor among « metals while bismith is one of the poorest. Most non-metals have appreciably lower conductivities, but an exception is carbon which has e conductivity higher than most metals. According to the kinetic theory, the atoms or molecules inside a solid are hela together by strong molecular forces. High temperature in one part of a solid will imply violent motions of the particles in that part which in turn will produce. the same type of motion on its neighbours. Thus, a state of local agitation conesponding to a local high temperature diffuses out into the rest of the soląd. This] is conduction of heat in a solid. In a fluid where the molecules. are farther apart, collision of the thermally agitated molecules with their neighbours transfer the excess energy among molecules? But this process is not efficient and this explains why fluids have low thermal conductivity and, there -fore poor heat conductor. Convection

In addition to the random motions of the molecules of a fluid there may be a motion of the fluid as a whole. The motions of the molecules are then super- imposed on a drift of the whole collection of molecules. The trans -ference of heat from one place to another which is due to the Imotion OT the fluid is called

convection

When a vessel containing a 1iquid is heated at the bottom, the liquid at that part expands Since its mass remaine unaltered. it becomes less dense and there- fare rises. Thus a current of hot Liquid flows upward and its place is taken by a cold current moving. downward. This process continues until the whole volume of the liquid is at the same temperature convection currents occur widely."} throughout the atmosphere and the joceans. The formation of breeze

is one example. A breeze generally blows in from the sea during the day while at night its direction is reversed. (see fig. 1)

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вееза

NIGHT-TIME

LAGO AND BREEZE

Radiavi 011

The flow or near energy I Eval the sun across empty space to the Earth is referred to as radiation, This is the only one of the modes of transmission of heat which cami proceed without the presence of any material medium. Substances. are in general continuously emitt- ing and absorbing energy in the form of radiation: Radiant

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magnetic waves. The radiation emit Eted by hot bodies depends on the

WAH KIU YAT PO

temperature of the body and the nature of its surface and is distributed over a range of wave- Lengths. At, a temperature of 300°0 the most intense radiation emitted by a hot body has a wavelength in the infra-red region. As the. temperature is raised to 86000. the radiation is emitted in th visible region and the body be comes red hot. At about 3000°0, the body appears white hot.

The radiant heat emitted by hot body can be detected most,* convenicently by a thermopile which consists of large number of pairs of antimony and bismuth bars joined inseries, providing a large number of junctions on which the radiant heat is allowed to fall.

三期星

日一廿月二年三七九一暨公年二十六國民樂

b) If the volume is doubled, 1.e. = 21,

748 V = p' (2V) temperature

stant

or pl = 374 mm of mercury

The air pressure at 10090

764

P = 273 × 373 = _103.8

1973 中學入學試

試題預習專欄、

The total pressure - 374 + 12)

386 mm of mercury,

智慧出版社主版

英文科(十八)

NSWERS TO PREVIOUS EXERCISES

SECTION AT. I

5.B

2. B

10. P

7.2

8. S

SECTION BII

mercury.

By definition of boiling point of liquid, the 5.V.P. of water inside the bottle external atmospheric

pressure, which is 76 cm. of) mercury.

Hence the pressure inside the. bottle at 10000 103.876

= 179.8 cm of mercury.

The pars are placed side by side and are insulated from one another by papers. The whole is mounted in a short brass cylinder provided 4. From the definition of R.H. with two terminals connected to

the free ends of the two end bars.

BISMUTH

Wantimony

THERMOALE

Upon the incidence of radiant heat a current called the thermoelectri current is produced in the circuit and the galranometer gives a de- flection. The magnitude of deflect- ion gives the amount of radiant heat incident.

Applications.

(A) The vacuum flask or the ther mos flask.

silvorad

wallsy

shock ped

vacimo real

THERMOS FLASIC?

The vacuum flask consists of al double-wall glass vessel with a vacuum between the walls No heat can enter or leave the inner flask by conduction or convection across the vacuum. A small amount of heat is lost to or gained from the sur roundings by radiation, but this is reduced to a minimum by silver- ing the walls on the vacuum side. There is a small amount of heat loss by conduction at the neck and through the cork. The sun total of heat los9 19 very small so that a cold or hot liquid inside remains at constant temperature for a long time.

(B) Ventilation by convection.

Extraction of state air may achieved by using natural or forced convertion. For the firsu, ventilating apertures should be placed high, because exhaled air 18 warm and tends to rise. For the latter, a fan may be used to pro-, 'vide for the air motion.

Mason to ex. 8 Solution

1. The air pressure at 6000!

1168 -350 818 mm of mercury Let p be the total pressure at 120°0

APPLY

818 313

p=764 mm of mercury.]

2. Let V be the initial volume.

-

At this volume, the, air pressure 760 12 748 mm of mercury (a) If the volume is halved,

1.6.

748 x V = p x 1⁄2 |

at the same temperature or p = 1496 mm of mercury. Whe total pressure 1496 1508 mm of mercury

unsaturated vapour pressure

at 20°C(P)

saturated vapour pressure, at 20°C. (5.U.P.)

P5.25 mm of mercury, The unsaturated vapour obeys Boyle's law.

F' = unsaturat

-ed vapour pressure at 1090

5,09 mm of mercury

5.07 at 1000 = 3 V.P. at 10°C 5:07

is not any coup in

the bowl.

You ought not to wake

him up.

She doesn't need to t

take a rest.

4. Won't they come again? 5. Don't tell her to walt

heri

SECTION"

(multiple choice)

Choose the best answer by drawing a

firm line across the right box.. James said, an listening to the radio

now.

James said that'

A. he was listening to the radio now, B. he is listening to the radio then. C. he was listening to the radio then D. he had listened to the radio then, ha listened to the radio then.

ABCD F

DOOOO

v, have you seen it befür

X 100% J

2. He said, " iary!

He asked Hary if

55,1%

Revision exercise on neat

16. An empty S.G. bottle has a mass 36.7 gm. When filled with a liqu The mass become 99.72 gm., the room temperature being 25°C. Afte placing the bottle in a water bat at 6500, removing, and wipe it, th mass is 98.46 gm. What is the coefficient of apparent expansion of the liquid?

2. A brass scale measures 76.5 cm/ at 2000. What will it read at 109. coefficient of linear expansion of brass = 0.000019/°C.

3. A closed vessel contains air mixed with water vapour. Initiall the air is unsaturated and the pressure is 52.8 cm., when the volume is halved the pressure becomes 105.2 cm and when the volume to reduced to one third the pressure is 157.1 cm. Deter mine (a) whether the water vapour becomes saturated when the volume is reduced, (b) the initial pressure of the water vapour.“ 4. A copper calorimeter of mass 120 gm. contains 150 gm. of a liquid at 26°0. 25 gm. of ice at 000 are added and the minimum temperature of the mixture is recorded. At this temperature, vapour from the game boiling "liquid is introduced into the

mixture until the final temperat- ure is 3090. Find the mass of vapour condensed. The S.H. of copper = 420 J/kg°C, liquid = 2520 J/kg°C. Boiling point of (liquid = 64.7°C.

The latent heat of evapouration of the liquid = 1120 J/gm. Latent heat of fusion of ice

336 J/em.

5. A uniform capitlary tube sealec at one end, contains air enclosed by a water index, which keeps the air saturated with water vapour. The length of the air column is 17.8 cm at 2000 and 29.4 cm. at 70°C. The S.V.P. at 20°0 is 17 mm and the atmospheric pressure is 176 cm of mercury, The S.V.P. at (70°0.

6. The R.H. in a closed room (AT) 2000 is 60%. What is the R.H. at 1290?

6.V.P. at 20°C 17.5 mm of mercury 5.V.P. at 12°C 10.5 mm of mercury 7. What is the R.H. of an atmos phere at 16.300 if its dew point ¿a 12.5°C?

The following table gives the 6.V.P. of water vapour in mm of mercury at temperature t. B.V.P.|9.2|10.5 12.0 13.6 15.5 17.5

10 12:

16 10 20

A. he had seen it before. B. you had seen it before.

C. I had seen it before before

D. they had seen it before, E. she had seen it before.

un, Jack

00000

asid the teacher,

A told Jack to stand up.

requested Jack to stand C. asked Jack to stand up.

said Jack to stand up. command Jack to stand up.

AN B

OOODU

say, We want some more food. They say that

we want some more food.. they wanted some more food. C. we wanted some more foodi D. they want some more food. E. they will want some more food,

said to him,

A B C D

□□□□□

"Are you late today?",

asked him if-

TAY I was late that day. B. he was late that day

C. we were late that cay D. they were late that E

he was late today.

4. J

Change the following sentences anto

Indirect Speech.

Tom said, "I am very tirea".

She said, "I lost my bag yesterday.

He said to me, "Where are you going?

4. David said, "Nother, may I go out?”

5. They said to me, "What time is it

now?

II. Change the following sentences into

Direct Speech.

1. Robert asked me if I had spoken to

- The man asked the beggar what hey

wanted.

Susan said that she was ill,

They asked John how he would go there.

They said that they would leave for

ndon the next day.

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