翼四·第·張五第日二十月正年丑癸居
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1913英文中學會考試題預習專欄
物理科
|Physics (17)}
(FE)
Evaporation explained by kinetic) theory.
The Molecules of a liquid have an average kinetic energy. When the surface molecules have kinetic energies greater than average, they can escape from the attract- ion due to their neighbours and jump out of the liquid. Some of these molecules may collide with other molecules above the liquid and bounce back into it. But many. others may escape into the atmos- phere
Thus the liquid surface is always covered with a quantity of the liquid vapour even at ordinary temperature.
Hygrometry.
The water vapour in the atmos- phere is important because it affects our comfort. The rate at which water evaporates from the skin or anywhere else, depends on the pressure of the water vapour surrounding it. Since the atmosph ere contains other gases besides. water vapour, such as oxygen and nitrogen, we have to refer to the partial pressure of the water Wapour.
Vapour pressure *
| Consider a closed container containing some quantity of a liquid, some of the surface mole cules will leave the liquid and go into the space above.
liquid
At the same time, a number of the liquid molecules in the space will return to the liquid because of collision with other liquid mole- oules. If the escapes in a given time are more numerous than the retums, evaporation occurs, But if returns are in excess of the escapes, we have condensation. A dynamic equilibrium is said to be established when the processes of evaporation and condensation are in balance. At this point, the space above the liquid is saturated with vapour and the pressure exerted is called the. saturation vapour pressure (S.V.R At a given temperature, the S.V.P. of a liquid is always the same whether there is air or other gases in the space or not. When the state of dynamic equilibrium ▼¶ is not reached, the vapour press- ure is said to be unsaturated. The magnitude of the S.V.P. ((a) depends on the liquid.
(b) increases with increase of temperature and at the b.p. of the liquid is equal to the atmospheric pressure.
(c) does not obey Boyle's law. On the other hand, when the space above the liquid is not saturated With the vapour, the pressure of the unsaturated vapour is appro-
ximately jnversely proportion=1, to the volume of the space.
measurement of Liquid.
To measure the S.V.P. of a Liquid, a simple barometer may be used. As shown in the diagram, the barometer A 19 used for compari- Sion with the barometer B. A bent pipette C containing the liquid, 19 used to introduce the liquid into the space above the mercury" The liquid is introduced into the pace above mercury drop by drop/ until a small quantity of the 11quid appears above the mercury surface. At this point, the vapour Pressure of the liquid is the S.V.P. The depression of the mercury column in B 19 due to the
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日四十月二年三七九一腐公年二十六国民奉中育敦備堂
Hence R.H, at 20°0
WAH KIU YAT PO
S.V.P. Hence by measureing this
depression, hom., the S.V.P.
the liquid can be determined
under reduced
prese
Since the S.V.P. of a liquid 13 equal to the external atmospher -10 pressure at the hip. of the liquid, a liquid can be made to boil without heating it simply by reducing the atmospheric pressure above it to a value less than the 5.V.P. of the liquid.
to futer
As shown in the figure, the press- ure of the air above water can be. reduced by switching on the filei pump. At the point when the press- ure of air is less than the SVP of water at the temperature of the experiment, the liquid boils. But the temperature is no longer 100%. On the contrary, the water temper ature Falls-rapidly because the necessary latent heat of vapouri zation 19 due to water itself.
Relative humidity
The state or the atmosphere, with regard to the presence of water vapour in it is expressed, by the quantity moan as the relative humidity. (R.H. which! is defined as the ratio of the mass of water vapour actually present in a given volume of air to the mass of water vapour re- quired to saturate the same volume of air at the same temperature.
R.H.
mass of water vapour in a given volume of air mass of equal volume of saturated water vapour the same temperature.. density of water vapour in air.
density of saturated water vapour at the same temper- ature:
since an unsaturated water vapour. obeys Boyle'q law, the density of the unsaturated water vapour 18 inversely proportional to its pressure. Hence
partial pressure of water vapour presenta
S.V.P. at the temperature of atmosphere
It is abvious that the R.H. will never be greater than unity. It jis usually expressed in percentage
Dew point
In the evening, the earth cools more rapidly than the air above it Then a thin film of moisture can often be found on smooth surfaces such as metals, The surface has cooled to a temperature such that the water vapour in contact with it has become saturated and has
begun to condense. The temperature of a cold surface on which dew just appears is called the dew point. It is the temperature at: which the S.V.P. of water is equal to the partial pressure of water vapour present in the atmosphere.
In terms of dew point, the R.H. can be rewritten as
R.H
S.V.P. at dew point 5. V.P. at temperature of atmosphere
Dew point hygrometer
Hygrometry is the measurement of R.H. and a hygrometer is an instrument for measuring it.
The Raynault's hygrometr consists of 2 glasa tubes C, D with silverplated thimbles A, Di cemented on to their lower ends. G contains wther into which dip a thermometer and a glass tube R. Air is gently blown through the ether carries away its vapour ana".. makes it evaporate containuously. In doing so it cools. A film of 7 mist appears on the thimble A. The thimble B is for comparision puIH pose. At the moment when the dew appears. The thermometer I is read The glow of aar through the ether is stopped, and the ether is allow
ed to warm up. The temperatur at which the dew disappeared is noted. The average of these two temperature readings is taken as – the dew point. Lastly, the temper- ature of the room is taken.
From these readings, the Rh can be determined the S.V.P. at dew point and at room temperature are found from tables.
Wet and dry bulb hygrometer
The bulb of a thermometer is wrapped with a piece of muslin and dipped into a small jar of water. It is mounted, with a second dry bulb thermometer in a louvred draught-shield. The rate at which water evaporates from the muslin increases as the R., of the at- mosphere falls, the cooling of the wet bulb therefore increases. The greater the difference in reading of the two thermometers, the less is the R.H. the actual R.ll. can be obtained from the thermometer re- ings with aid of a table.
Hair hygrometer. 7
A hair hygrometer consists of a bundle of hairs fized to a spring at one end, and wrapped round a spindle at the other. The expansi or of the hair turns the spindle and moves a pointer over a scale dir— ectly calibrated in R.H.
Examples
1. A closed vessel contains air, saturated water vapour and al excess of water. The total press- ure in the vessel is 760 mm. of mercury at 2500. Find the total pressure when the temperature is 100°C (S.V.F. of water at 25°C is 24 mm ot ↑`morcury),
The partial pressure of saturated water vapour is 24 mm. of mercury, the air pressure
760 - 24 = 736 mm of mercury.
Let p be the air pressure at 100°C For a fixed mass of air, we have.
736, 373
p921 mm.
At 10000, which is the b.p. ater, the S.V.P. is 760 mm. mercury
of
of
The total pressure in vessel 760921 1,681 mm.
2. The R.H. in a closed room at 15°C is 60%. If the temperature rises to 20°C, what will the R.. become?
(S.V.P. Or water at 15°C = 12.67mm
01 Ury, at 2000, - 17.3.6 mm.D
Let p the actual water vapour pressure at 15°0,
·60
60%
P = 700 x 12.67
7.6 mm.
The unsaturated water vapour obeys Boyle's law,
273+201
7.73 mat.
7.73
3. The H.H. of air at 19.500 is 75%. Calculate its dew point,
given that S.V.P. of water at 19.500 15 17 mm of mercury, the b.p. of water under pressure of 12 mm, 14mm of mercury is 1400, and 16.45°C. respectively."
From definition of R.Hi
75%
P. at dew point.
S.V.P. 7
of mercury.
ASUS VIP at dew point = 12.75m
dew point
(16.45-14175
1. The final temperature of mixture is 0°C. *)
Heat given out by water
= 100×4.2x1.0+5x336+50x0.84x10
vessel
= 4200 + 1680 + 420
Heat absorbed by metal, =83 x 190 where s 1b the în J/gm°C.
AMANS.H. of the metal
Assume no
no heat loss
83 x 130s
6300
0.0422 J/m°C
422 J/kg°C
The heating curve of sketched as shown.
(more)
By Newton's law of cooling, the rate of heat loss in the time in- serval 45 min. is the same as that in the interval 30.25 min.
if m. grams of water are used in the experiment,
4.2 x (100–18
latent heat of vaporization.
4.2 x 82 x 30.25
2320 J/gm
3. Let m be the gm. of alcohol evaporated.
Heat given out by steam
1- 20x2260+20x4.2x22
- 47048 J.
Heat absorbed by alcohol and the
calorimeter
150 x 0.42
2310
2 x (78-25) + 250
x 1000 x (78-25) + m x 1100
1100 m.
no heat loss to the
roundings,
33940 1100 m. 47046
12.1 gin.
4. Water above the fall is at a cemperature t°C. All of the P.B. of water is converted into heat after falling a height of 100 m.
The heat developed.
m x 9.8 x 100 – 930 m J.
heat mat
= 4200 T/k660. The temperature difference
980
42000.23300.
5. Let be the latent heat o fusion of napthalene.
Heat absorbed by water and the calorimeter.
= 70.5 x 0.42 x (28.7-19.5.
x 4.2 x (28.7-19.5)
= 4132 J
Mase of napthalene.
1.00
188.3 – 100 – 70.5 = 17.8 gm. Heat given out by napthalene
17.8 1.35 x 0.1 +17.8 x I +17.8 1.35 x (79.9-2387) 1232.4+ 17.86
Assume no heat loss,
4132 1232:4
L = 163 J/gm
. = 16,000 J/Kg,
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