1973-01-03 — Page 19

三期星

日三月一年三七九一公年二十六國民華中

|| – 3 JAN 1973

CITY HAL 育教儒韻頁三第張五第日九十月一十年子壬展夏 WAH KIU YAT PO

鄩日橋

1973英文中學會考試題預習寫場

物理科

PHYSICAL (11)

3.5.

Expansion of liquide

Since a liquid assumes the

Vessel,

shane of its containing more or less, linear and super- ficial coefficients of expansion bave no significance

Most liquids do not expand uniformly and the coefficient cubical expansion and is not constant over a wide range of temperaturen

If we try to find the coeff- icient of expansion of a liquid by warming it in a vessel, the vess- el also expands the observed expansion is the difference betw- een the increase in volume of the liquid and the vessel.' This observed expansion is alled the apparent expansion."

The coefficient of apparent expansion of a liquid is defind aþ the apparent increase in volume. of the liquid per degree rise in temperature when the liquid is aceted in an expansible vessel. The cubical expansion coefficient in the first paragraph. is the true (or absolute)) expansion coefficient of the liquid. In words, the coefficient of true (or absolute) expansion of @ liquid is the absolute increase in volume per degree rise in temperature.

It is obvious that the true ox bansion 19 always greater than the apparent expansion. Most methods of measuring the expan-y sión of a liquid, whether true or apparent, depead on the ch change 15 density of the liqula when it expands.|

Let X be the absolute expansion of the liquid.

liquid

M be the mass of the

and are the volumes. of the fiquid at temperatures: ty and t2 respectively

[Suppose t„ is greater-than

Jacket X contains ice-water and steam is passed through Y. For the mercury to be in equilibrium the hydrostatic pressure at B and C must be equal. As shown in the figure, pressure at B helog and pressure at C =

where P is the density=hing.

t

mercury at temperature

ho Pog htPtq.

dence

Jaing the result in

Hence is the coefficient real expansion of mercury

Hence

By measuring

and

can be calculated this Upe apparatus is inaccurate te ause

(1) the expansion of the tube. CD ad different from that of AB so that the arm BC 19 notw zorizontal. This means that. the pressure a- 5 is not equal to the preaburo ar Co

(2) the wide separation of end D makes the measurement of

Inaccurate.

The value os obtained 18 the- refore only an approx118 value

An improved form of the apparatus of Dulong and Petit was due to Regnault. Thể formula for using Regnaut's apparatus is rather complicated and will not be considered. 3.6.2. Weight Dilatometer.

This 19 the method of finding by weighing. In a Laboratory, e spŕcific gravity. bottle may be used lus end of a weight dilatometer the general chape of a di la rometer 18 ns shown in fig. 3.6 2. and it requires special filling

techniques

Hence to a good)

approximation we can write:

8 = 8 79

The above. formula shows tha true expansion 18 equal to apparent expansion plus the expansion of the container?

Ir the experiment, the weight

the empty container 18 obtauet.

The container is then filled

with liquid and reweighed. The difference of these two reading

Sven M The contained filleu

with liquid is then immersed in

a water bath or beaker conteinler

water and heated to 100 0. It

then taken out after the exter 10

of the container has been drieu ( with filter paper.

This is M

From

lculated.

g 19 known, r

derermined.

can ce

can be

Solution to exercise 1. (1) Apply the formula

OF 125 9

and let F = 20, we find

32:

100+ 60

320.

formula

2) We can apply

x 100

thus the tempera

-20.x

100

Again apply the formule

when he true lesPFTA ure 19

The ratio

60

100

Now the faulty therzone! da has a temperature range of

(-1) = 102 C. The reading on the thermorecer 18therefore given by

× 102 = 61.2°C

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and

But V2

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where Pand Fare the densities of the liquid

and

Hence

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Thus we see that the densitys or the liquid 19 inversely propor t- 1onal to temperature there is one exception for the above conclus) on. This 19 the anomal→ bus expansion of water which will be discussed in detail.as

36. Measurement of coerT1 0182TA of true expansion.” 3.6.1 The first measurement

the true xpansion of W: liquid was made by Dulong and Refit in 1817 A simple form of their apparatus is shown in fig.3.6.1. che apparatus consists of a glass tube ABCD containing mercury and surrounded av class Jackets XY.

Weight dilatometer

fry_363

However, no matter whether:

ome:

the Le

a S.G bottle 18 under lying, theory

be the mass of the liquid Just fills the container (a S.G. bottle or a weigh dilatometer) volume V at 0-9.

Atxtemperaturest, the density Pu

18 related to the ice-point den

by wher

ALBO

(1+8+)

the Coefficient of on of the liquid.

temperature, some

of the liquid overflows. Thus

is the remaining mase of

the liquid, then

(1+gt)P

where g in the coefficient of]

cal expans

ον

But

Hence

Now

LOT

liquid, 8

the

at o°c

<t)

is the coefficien

expansion of the

we have X-X+

In most experimen to the mass' expelled, M

18 only. small fraction of M. ao that

then

Let

230 = 1 x [1

1.2

( 25 )

1+1.2x10x15.

00018 M.

10

temperature be to 0000125

001: 00001

-800°C/-

be the radius of the hole at 800 c.

Then use A, = 1. | 1 + 28(b_to]

THE =π (0.2)" (1+2×0,0000/2EXFTD]

OXI mi

(6) The length of the copper

tmp at 400°C 12

+ 0.000017 1.0068m.

400

The length of the iron i at 300 C 19-

1+ 0,000012 Z 300 =1.00362

The difference in lengthe the strips 18

1.0063 1.0036 = 0.00320

Exérci8a-2

gn of at 0

5.0. bottle contains 50,20 a liquid when it is full

Banand 47.65 gm of the

120uld when full at 40 c. If the linear onefficient of glase 18 0.000017 07 Find (1) the ratio of the densities of the liquid at 0 and 40°C; (11) the real ooerf101ent of expansion of the liquid. (2) In the Dulong and Petit's "belancing column" experment, the height of the cld column was 50.00cm. a 4°C and the height of the hot column was 50.24cm at 96 C. Calculate the

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coefficient of expansion of the

liquid.

(3) A loaded glass bulb weigh ta 156.25gm. in air, 57.50gm. When immersed in a liquid at 150. If the coefficient of linear expansion of glass is 0.000009/ C, find the coefficient of real expansior of the liquid./

(4) A litre flagk correctly calibrated at 4C 18 filled the mark with water at 80°c. what is t the weight of the Gater) in the lfask? (coefficient of linear expansion of glass = 0.000009/C and coefficient or cubical expansion of water = 0.0005/C)

(5) A clock and keeps

How can a vendylw

at 2010. dosa por day a at 30 07 The coefficient of linear expansion of copper 191 0.000019/ CY