1972-12-18 — Page 25

育教僑華頁一第張七第4日三十月一十年子壬展夏

1913英文中學會考試題預習專欄

英文:實院主編

(^)

物理科

PHYSICS (8)

Determination of the Specific Gravity or Density of a • Liquid:

The following methods are in. generaluuse for determining specific gravities or densities of liquids,

(a) The specific Gravity Bottle.

When the bottle is filled with liquid and the stopper is put in, the volume of liquid now in the bottle is a definite

long as the temperat-

one,

But of equal volumes A

the same. Thus,

the

of any given liquid and of w water can be found by direct weighing and their ratio give the specific gravity of the liquid.

(b) The Hydrostatic Balance.

This method is an application of Archimedes' Principle. A heavy body is weighed (1) in air, (2) suspended in water, (3) suspended in the given liquid.

(1) - (3),gives the weight of liquid displaced.

(1) - (2) gives the weight of water displaced.

The ratio of these difference

(2)

is the specific gravity of the liquid.

(c) Method of Balancing Columns

(Hare Apparatus)

B

The apparatus is an inverted U-tube. The vesseal A contains. water, and the vessel 8' con- tauns the liquid whose speci- fic gravity is required. Some air is sucked out through the side tube C, which is then closed by a clip. In this way the column of water is: obtained in one limb of the U-tube and a column of liquid in the other. They are Hi and H2 respectivley.

The pressure due to the water column is H. and due to the liquid column is gHa Where ff are the densities These pressures are equal, for each represents the differen- ce of pressure between the air. in the upper part of the U-tub and the atmosphere outside. Hence,

£f¡H1 = 852H2

Therefore, the specific gravity of the liquid is the ratio of heights of these columns, Hy H2- (d) Method of Archimedes Princi-

ple and the Erinsiple of Mo- ments:

A heavy solid is balanced by a weight vin air. The distance d cm of the solid is kept constant from the pivot and the distance dom of the weight lo noted. The solid is then immersed in water and the balance restored by moving W closer (d") cm.

The solid is then immersed in a sample of liquid of unknown S.G. The distance of W which reads d' is recorded.

d"

W

W

I

両

W

REFERENCE LIBRARY.

18 DEC1972

CITY HALL

HO報日僑筆

TAH KIU YAT PO"

weight of solid in air weight of solid in water. 1- weight of solid in liquid By taking moments about the pivo

W

S.G. of liquid full liquia loss in weight of solid in lòss in wt. of solid in wate

W

Wa

(Wd" - Wd") / ("d" - Wd")

(e) The Common Hyarometer.

This is usually made of glass The bulb E is weighted with a littel mercury or lead shot,

The bulb D contains

air, and the construc- tion is such that wher the instrument is. placed in a liquid of suitable density it will float upringht, with part of the stem above thesurface. The weight of the liquid. displaced by the hy drometer is equal to the weight of the hy- drometer itself, and is therefore always the same, whatever the density of liquid. Hence the the liquid displaced ses as the density quid iminishes, End

when the instrument is float- ing in a dense liquid than when it is floating an a light

lone. It is thus possible to graduate the stem so that the specific gravity of a liquid can be read off directly. Determination of the S.G. or Density of a Solid.

a) By direct measurement. (b) By Hydrostatic Balance. (o) By Specific, Gravity Bottle.

This method is suitable for a solid in the form of powder Three weighings are necessary to find (1) the mass my of the solid, (2) the mass IL, of the bottle full of water (3) the mass m when the so- lid is inside the bottle and all the remaining air has e been displaced by filling up with water. Then (n,+ m

3 is the mass of water which has the same vo- lume as the solid, so that the specific gravity of the solid is

(d) By the balance beam method.

SURFACE TENSION

Inte molecular Forces These forces arise from two main causes:

(1) The potential energy of the

molecules, which is due to interactions with surrounding molecules,

(2) The thermal energy of the m0-

lecules -- this is the kine- tin energy of the molecules. and depends on the temperat- ture of the substance con- cerned.

States of Matter

The molecules of soild are said to be in a condensed state.

Their thermal energy 10 then re- tatively low compared with their ingrExse potential energy and the molecules are bound to each other. They may vibrate in their mean position, Bolide have a de- fiate shape and size.

-期星

日八十月二十年二七九一屡公年一十六國民華中

As the thermal energy increase," at some particular ktemperature the molecules are able to move comparatively freely relative to neighbouring molecules. The sold then loses xits rigid form and becomes a liquid. The molecules in the liquid constantly exchange places with other molecules, wher eas in the mach solid the neigh bours of a particular molecule 'remain unchanged. Further, the

molecules of a liquid have trans- lational as well as vibrational energy, that is, they move about constatatly through the liquid, whereas molecules of a solid have vitrational energy only.

As the stemperature of the liquid rises, the thermal energy is fur- ther increased. The average dis“ tance between the molecules in- creases and so their mean poten tial energy approaches zero. At this stage a further increase il thermal energy enables the mole- cules to completely break the bonds of attraction which keep them în liquid state. The moleca- les then have 14++1A.

nn inter-

normal pressures the forces of attraction between the zes mole- cles are comparatively very small and the molecules move about Tree. iy inside the volume the occupy: Gas molecules have rotational and vibrational energies in addition. to translational energy

Surface tension: Molecular theory Surface of a liquid acts like an elastic skin covering the Liquid or is in a state to tension. This can be explained by the intermo- lecular forces exist between them. The average distance apart of the molecules is such that the attrac-

forces. Thus the average inter- molecular force between one mole-

ules inside a

19 zero: For moleculas on the surface of a Liquid, the molecules above the surface are removed to infinity. A resultant attractive force exist due to a large number of molecules below it.Consequently moleculcs in the surface have potential en- ergy. They form a bond with morg reighbouring molecules. The sur-

surrounding one quid and the

face is in a tension state.

Exercise for this week

1. A hydrometer floats in water with 6.0 cm of its graduated stem unimmersed, and in o11 of relative density 0.4 with 4.0 cm of the stem unimmersed What is the length of stem un ammersed when the hydrometer 15 placed in a liquid of rel- ative density 0.97.

A balloon has a volume of 10 000 c.ft and the envelope. weighs 100 lb. If filled with nydrometer of s.g. 0.07 (air 1.00) what force will just prevent the balloon from ris- ing? (1 c.ft, of air weighs 0.084 lb) E

The wooden rod woghs 1.0 N

19 suspended as shown.

U

31:

0.5m

It 18 first suspended from two spring balances, then one balance is removed and the rod 18 lowered into water until it rests as follows:

الله

0.4 m

.8m

(a) Calculate the readings

in the spring balances A and B. (b) Draw a daagram to show the

forces acting on the rod wher it is partially immersed in water and show the distances between these forces,

(c) Calculate the spring balan-

ce reading C and the kpthrust

on the rod,

(d) Calcualte the density of the

rodi

1978

中文中學會考試題預習專欄

英文科 (八)。徐有錄

English (8).

Essay-writing is no magic.' Like building a house, it needs materials and the skill to put the materials together in proper form. The writer has made a number of suggestions in this, column, suggesting different ways to the candidates to commence an

to select the theme and to essay, end an assay. This is only the aspect touching on skill. What is just as important is the aspect on materials. The following is what the writer thinks the candi- dates should do to gather the materials and to make use of them

efficiently.

It must be made clear at this. juncture, however, that all the suggestions made here in this

are for the purpose of

Colum the candidates to write

essays for the examination under discussion and not for the pur- pose of writing a treatise, as more sophisticated ways will have to be used for the latter purpose.

Now let us get to the aspect of materials. In essay-writing, materials are the words, the phrases and the sentences. Assemb le these materials skillfully and you have the essay.

Candidates, generally at this stage, should have a sufficiently large enough vocabalary, and have at their command, wide varieties of phrases and sentences. Having possession of these is one thing. Being able to use them skillfully As another. Just knowing the mean- ing of the words is not good en- ough for the candidates. They must know, when, where and how to use them, because several words may have the same meaning, and yet they have different emotive values good, bad or neutral. Using a word at the wrong place may throw an essay out of perspective. It is

therefore imperative that the candidates should learn more abuu the usage of the words, phrases and sentences, that are used in everyday life. They should read more newspapers, particularly those in tabloid form. The tabloid": are recommended, because the art- acles in them are usually concise. One may say that they are tailored for the consumption of the candi- dates.

Another way to gather materi- is to read short story books and essays published by polished authors. Candidates should memori- ze the stories by heart. When writ -ing their own essays, candidates should, whenever possible, copy the style, and indeed, whole sen- tences used by these authors.. Mistakes in Grammar are thus eliminated. As a matter of fact, If the candidates will follow the method suggested here to prepare for their examination, it is al- most certain that they will find the time spent in the preparation. well spent.

If candidates should want to go a step farther --- to get very they high marks for their essays, should learn how to use "Figures. bf Speech" particularly, the simile, the metaphor, and the hyperbole. A figure of speech enables a person to express him- self well in writing. It makes his writing more interesting and impressive. It makes a description more vivid and realistic by pro viding concrete pictures in place of abstract ideas. The following are examples of the simile, the metaphor and the hyperboles

The simile

Something is said to be like an- other thing, but usually in one way only. Most similes are intro- duced by the words "as" or "like": (a) He is as hungry as a starving

tiger.

(b) He drove Like a Dus-driver on

a strange route,

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