1973-02-10 — Page 23

報日德茶

質教華僑 頁三第張六第 日八初月正年丑癸

WAH. KIU YAT PO

1973英文中學會考試題預習專欄

物理科

(十六

Physics (16)

Cont'a to measurement of S,H.

tom

(3) Electrical method.

The electrical circuit employed is shown in the fig. S is the stirrer; T is the thermometer; 6 is the resistance coil and Ris a rheostat. The liquid whose S.H. is required is weighed with the copper calorimeter whose weight is known. After switching on the current, the temperature register -ed on the thermometer is noted at regular time intervals. The current in the circuit is maint ained at a constant value by means of the rheostat. The electrical: energy that is converted into heat energy is given by IVt in Joules, where I is the current Vas the voltage across the coil land t is the time interval for the

temperature rise of the liquid (from its initial temperature.

Raggun

The heat energy absorbed by liquid and the calorimeter 18 m3,0 + m2s2e where n is the that mass of the liquid and my of the calommeter, s, and s their respective specific heats. Hence VIt = {0 9, + 02S2

By plotting a graph or t(in

ute) against (in °C),

line graph should be obtai The gradient of the straight line is equal to

VI

Hence s the S.H. of the liqui can be determined accurately, To measure the number of joul

equivalent to a calore.

Though in ST-units, there i

す

no such term as the mechanical

equivalent of heat, it is stil of interest to find the relation between the joules and the calorie The classical method of determin ing the relation involves the rotation of a number of vanes in a-copper calorimeter containing known mass of water. The vanes are set rotating by means of falling masses. The height through which the masses have been traversed is measured and the temperature rise of the water in the calorimeter is measured. Now the only cause of the temperature rise of water 18 the rotation of the vanes which arises from the falling of the masses. Hence the potential energy of the masses 19 converted into rotational kinetic energy of the vanes which in turn is transform- ed into the internal molecular

energy or the calorimeter and its content and this accompanied by a temperature. By computing the ratio.

Heat absorbed by water and calor. meter in cal.

potential energy of the falling: masses in J,

The result was

about 4.18.

Hence 1 cal.

Example 1.

Joules.

. Long

A cardboard tube or contains m kg of lead shot (S.H. 30 cal/kg°C). The ena of the tube are closed works one of which has a small hole plugged with a wooden peg. This allows the temperature of the lead shot to be taken. The initial temperat- ure of the lead shot is taken sav 6,°C-

The tube is then inverted

quickly so that the lead shot

falls through a distance hm. AIT of the P.E. of the lead short are then converted into the interna) energy when the shot is brought to rest. The tube is inverted a large number of times, e.g. 100 times, to obtain a measurable

temperature rise of the lead shot. Let be the final temperature

of the lead shot.

Then the PE of Ine snov

mgh x 100 J

The heat gained by une snov = m x 30 x (0,

The rati0.

m x 30 x (02.

mgh x 100 3(02 - 0; } cal/

Togh

the number of joules cyuz 1. cal

10 g

302

h 80 cm., and g = 9-8 m/s

the the

六期星

by the heat produced. If the

initial temperature of the bulle

is 17°C, what is the speed of the

bullet entering the target?

(S.H. of lead = 130 J/kgm°C.

Melting point of lead= 32790 Latent heat of fusion of lead

22680 J/kg)'

Let m be the mass of the bullet in kg. & V be the speed of the bullet in m/s.

The KE of the bullet

my

The heat required to melt the bullet is derived from the K.E. of the bullet

m x 130 x (327-17)

62900 d

125800

22680J

354 m/s (approximate

-1y).

(This is an example of the Joule's] mechanical equivalent of heat.)

J/cal

6.000

Solution to ex. 6. 1. Suppose that m kgm water is evaporated Then the final temperature be 10000.

of the

80

Ox9-8x

The above example provides a method of finding the so-called nechanical equivalent of heat. But the result is not accurate because there is a considerable heat loss from the tube to the surrounding

Effect of pressure on melting point.

It was found that if a substan De expands on solidifying, its melting point will be lowered un- der the application of pressure. On the other hand, substances. which contracts on solidifying have their melting point raised by pressure.

A well known example of illus trating the result said above is shown in the fig.

A than copper wire with heavy Loads at each end is hung over a black of ice resting on supports. After a certain time interval, depending on the size of the block, the wire cuts through it and falls to the floor, leaving the ice still in a solid block. This phenomenon is called regetar

meaning refreezing.

This is explaas folLUWEA

The melting point of the ice 19

of the lowered by the pressure wire in contact with it. The ide therefore melts and flows above the wire. The latent heat of fusion comes from the copper wire. As soon as the water passes over the wire, it is no longer under pressure and therefore refreezes. This time it gives out latent heat which is conducted down through the wire to provide hear for further melting of the ice beneath. The high conductivity of copper is an important factor in this process. If an iron wire which is lower thermal conductivi ty than oopper is used, it will cut through the ice more slowly.

Examples

(2) A copper calorimeter with stifier has a total heat capacıvý 60 J/00. 0.2 kg of water at 1000 is poured into the calorimeter and then dry steam at 10000 is passed in. If the final temperau- ure of the mixture is 30°C. What is the mass of the steam condens- eq?

Let m be vie mass in g or the steam condensed.

Then heat gained by the calor- meter and its content

= 60 x 20 + 0.2 x 4200 x 20

18000 J

The heat given out by steam = m x 2260 + m x 4.2 x 70 =2554 mJ.

= B

18000

15:255.4-

(3) A lead bullet moving horizon- tally enters a target. and is N brought to rest, being just meltso

Heat given by the molter lead = heat, given out in the change from 400°C to 328°C +. latent heat given out in changing from molten lead to solid lead heat given out in the change from 32800 to 10000. i.e.

+ my + m Se

st:

1 x 2260Ü 1 x 170 x 72 + +1 x 130 x 228 where 170 J/kg°C is the ɔn vi molten lead

130 J/kg°C is the S.H. O solid lead

22680 J/kg is the latent heat of fusion of lead.

64.68 kJ

heat absorbed by ice

heat requa ice from -20°C to 0°C ice + heat. required to change 0°C ice to 00( water heat required to change. 0°C water to 100 C water heat. required to change m kem of wate

100°C steam.

red to change the

0.04 2100 × 20 + 0.04

336000+ 0.04 x 4200 x 100 mx 2260000 J

·31.92 +2260 m kJ

Assume no heat loss

= 31.92 +2260 0.0145 kg 14.5 gm.

The mixture contains . gu of water and 1 kg of lead. S.H. of oil 2100 J/kg°C Heat absorbed by ice.

8 x 336 J

Heat given out by mixture

- m x 2.1 x 20 + (52−n)x4. 'where

the mass of 011 Assume no heat loss,

8 x 336 42 m + 84(5<-{ /

m = 40 gm.

∙x20

The original mixture contains 40 gm. of oil and 12 gm. water.

3. Temperature of boiling water 10000. The temperature of the mixture must be at 0°C otherwis all of the ice will be melted. Let m be the mass in gm. of the

ice melted Then 2-0.

LUC

water due Lo condensation of the steam. Heat loss by steam

(2-)x 2260 (2-m14.2 × 100

Heat gained

m x 336

لی

ice

Heat gained Heat loss.

36 m

2680(2-m 1,78, gm.

78 gm. of the ice have been melted:

Exercise 7.

1: A piece of metal or mass of gu Immersed in liquid air (-180°C

19

and is then transferred to 100 gm of water at 10°C in a vessel of mass 50 gm and S.H. 840 J/kg°C. If 5 gm. of ice are formed, what 18 the S.H. of the metal? (latent heat of fusion of ice

2260 J/gm)

2. The initial temperature of a quantity of water in a vessel is 18°C. The vessel is then heated for 4 minutes, and the water in it begins boiling. After a furthe 30 minutes, all the water has boiled away, from these observat-

one find the latent heat of vapourization of water

日十月二年三七九一骞公年二十六國民中

(接第六張第二頁)

ろくく

1973英文中學會考試題預習專欄:

堅英文海院主編

化學科(十六)

3. When quicklime is dissolved an water and carbon dioxide is passed

into the solution for a long time. the product is

A. Calcium carbonate

B. Calcium bicarbonate

C. Calcium hydroxide

D. Calcium carbonate and water

E. Calcium bicarbonate and water

Cement is a mixture ur

A clay and sand

clay, sand and water. 0. slaked lime, sand and

lim

D. limestone amm viMY E. chalk and sand

5. Motor is made by mixing

Bay and sand

quickline, water and cla

C. quicklime, water and sand D. limestone, sand and water E. cement, sand and water

6. Ordinary glass is made by heating together a mixture of

sodium hydroxide, limestone and sand

sodium carbonate and salu R. limestone and sand. S. sodium bicarbonate, line-

stone and sand

sodium carbonate, limeston

In the solvay process for the manufacture of sodium carbonate the most important reaction is

· DaCO → Ca0 + 002 CaO + 2NH 01

CaC1 HO 2NH

R. NH2 + H20 + CO

NH HCO3

+NaCl

NH CI + NaHCO 2NaHCO3 -> Na2CO3

NH HOO

8. When washing soda crystals are left in the atmosphere for a long, time, they will

Plose most of their water

crystallization.

lose all of their water or crystallization to forms soda ash.⠀⠀

absorb water and change into solution.

absorb carbon aloxide and

form bicarbonate.

>there 18 no visible reaction.

9. In the solvay rur une manu-

facture of sodium carbonate, the by-product 1s

P. Calcium chloride

Q. Ammonium chloride

R. sodium bicarbonate

S. Ammonia

Water

未完轉入第四張第二頁

3. A copper calorimeter of mas 150 gm. contains 250 gm. of

alcohol at 2500. Steam at 1009. 13 passed into the calorimeter and it is found that 20 gm. of steam are condensed. Assuming that heat is lost to the surrounding what is the amount of alcohol evaporated?

no

(S.H. of copper = 420 J/kg°C]

alcoholo

2:310 J/kg°C.

Latent heat of vaporization ory water = 2260 J/gm,

Latent heat of vaporization or alcohol 1100 J/gm.

B01

point of alcohol - 78

4. What is the temperature differ ence between the water above and below a fall of 100 m high if the whole of the gravitational energy lost was retained as heat in the water?

5. A copper calorimeter of mass 170.5 gm. contains 100 gm of water at 19.5°C. Naphthalene (Melting point 79.9°0) is melted in a test tube, and 18 then cooled to 80°C and poured into the calorimeter. If the highest temperature reache by the water is 28.7°C and the final. mass of the calorimeter anu its content 19 188,3 gm. What is the latent heat of fusion. (naphthalene ?

(S.H. of copper - 420 J/kg UT

Naphthalene 1350 J/kg°C)