1973-03-01 — Page 31

育教儒華

頁三第張八第日七廿月正年丑癸展夏

WAH KIU YAT PO

報日僑茶

四期星

日一月三年三七九一屣公年二十六國民華中

橋

1913英文中學會考試題預習專欄

畿道游女洲院主編

物理科

(+π)

Physics

Electrostatics.

If a rod of ebonite is rubbed: with für it gains power to attract light bodies such as oleces of. paper. The rod is said to be electrified. This phenomenon opened another branch in physics

electrostatics.

Electric charges

An ebonite rod rubbed with fur is suspended by a piece of thread. A second ebonate rod similarly rubbed with fur is held near to the first. It will be observed that the first rod 18 repelled On the other hand if the rod rubbed with fur is held near to a glass rod rubbed with silk, it will be observed that the latter is attracted towards the ebonite rod. Thus the charge on the ebonite rod is different from that on the glass rod. The charge on the ebonite rod rubbed with fur is called the negative charge and the charge on the glass rod rubbed with silk is called the positive charge.

a fundamental rule is that;

Like charges repel unlike charges attract

Remark:

Attraction is not a sure test a charge. Repulsion is the only sure. test.

2. Conductors and insulators

Substances can be classified. has conductors and insulators.

Conductors include metals, earth, human body etc. It is known that a good conductor of electricity is also a good conductor of heat and vice verse. A metal rod held by hand cannot be charged perman- ently since the electric charge will leak away along the rod through the human body to earth. Insulators include non-metals, plastic, ebonite, paper etc. A poor conductor of electricity is also a poor conductor of heat., 3. Induction.

Electric charges can be obtain- ed on an object without touching it. This method is called electro- static induction. Two metal cans A & B placed on insulating tiles are allowed to touch each other..

1.11.1

charges.

The gold leaf electroscope

consists of a metal rod A. to which. gold leaf L 18 attached. The rod is fitted with a circular disc B and is insulated with a plug P from a metal case C. The metal can screens the gold leaf L from outside influences.

ly by induction, the procedures

described above are repeated wa un

a glass rod rubbed with silk This times f

negative charges flow. from the earth to the conductor along the wire to neutralize the positive charges in the conductor.

glass rod

rubbed with Bilk

When the volume is reduced to chard;

블

52.8V

or Pr

A

Fig 3)

When B 18 touched by a negatively charged rod (eg, an ebonite rod rubbed with fur), some of the negative charges on the rod pass to the cap and the leaf L. Accord- ing to the fundamental rule, like charges repel. The leaf diverges. Now, if an unknown rod X is held near B and the leaf diverges fur- ther, then the rod X is negatively charged. A positive charge is similarly tested. The electroscope is firstly charged by using a positively charged rod. The un- known rod is then held near to the cap. An increase in divergence shows that the rod is positively

harged.

Temark

If the divergence decreases when the rod under tested is hrought near to the

say that the rod can we canno

opposite

charge to that of the gold leaf. or example in the first case în which the leaf is negatively charged, the divergence will also decrease if the rod is neutral. Hence it is important to note

that repulsion is the sure test of a charge.

5. Charge by induction

To obtain a permanent( positive charge on a conductor by the method of induction, an ebonite rod rubbed with fur, for instance, is held near to a conductor on an insulating stand, Then by induct- ion, the side of the conductor close to the charged rod becomes positively charged and the side further from the charged rod receives a nett negative charge

ि

Hence we can conclude that the induced charge (charge on the conductor) 19 always opposite to the inducing charge (charge on the ebonite rod or the glass rod),

an electroscope_ 6. Charging an.

near

When charged body is brought

the leaf divergea. the cap, This can be understood in terms. of induction. If a negatively charged rod is brought near the cap, it induces a positive chan on the cap, and a negative one

on the leaf. The leaf repel each. other. Further the negative charr on the leaf induces positive. charge on the inside of the case, the corresponding negative charge running to the earth via the earthing wire. The positive chɛrg on the case attracts the negative charge on the leaf and increase the divergence.

on:

To give a permanent positive.

to the electroscope, the charge cap is earthed momentarily while keeping the inducing charge in position. The negative charge. the leaf flows along the wire to the earth, leaving the positive charge which then distribute over the rod and the Leaf

pressure. when the volume becomes

158.4 cm, and 18 greater

than the observed pressure by 1.3 cm.

From this we can see that the water vapour mist have become saturated when the volume is reduced. The initial pressure of the water vapour is 0.9 cm. = 9 mm.

4. Let the the temperature of the mixture containing ice and the liquid.

Then heat absorbed by ice

25 x 336 25. 4.2t

8400, 105t J.

The heat lost by the calorimeter

and the liquid is

= (120 x 0.42 + 150

883(26-t) 22950883

Assume no heat loss..

22950 883t 8400

988t = 14550

14.700.

52)(26-t

Let m be the mass of the liquid vapour condensed.

Heat gained by the calorimeter

+ liquid + water

(120×0.42+150x2.52+25x4.2).

.: (30-14.7)

- 988 x 15.3.

15100 J.

Heat loss by m gme. of the vapour.

1471 m x 120 m x 2.52(64.7 1120m - 26m = 1346m J gain sssume no heat loss,

15100

The 91 pressure 293°К 15

760

mm.

Applying

17.8 x 743

29.4 x

Where P = air pressure at 70°C.,

3430K)

17.8 x 343 *

29.4 x 293.

52.0.cm

atmospheric pressure

the S.V. P. at 70°C

76.0-52.0

24.0 cm. of mercury

-76.00m

unsaturated vapour pressure at 20°C

A rod positively charged is held: close to one can. The two cans are then separated by touching. only the tiles while keeping the charged rod in position.

The rod is finally removed. It can be proved by a gold leaf electroscope that the two cans are oppositely charged. The charge obtained on the cans are called induced charges. Finally the cans are allowed to touch with each. other. It will be found that the metal cans become neutral. This shows that the induced charges are equal and opposite.

4. Gold leaf electroscope.

A gold leaf electroscope is a device for testing electric,

A wire is then used to connect

the conductor to earth. In this way, the negative charges on the conductor flow along the wire to the earth.

T

The charged rod is still held in position. The wire and the charge rod are removed. The positive charges remaining on the conduct- or will distribute themselves. over the conductor so that the conductor 19 charged pe rmanently with positive charges.

This can be proved by means of gold leaf electroscope al ready charged positively.

To charge a conductor negative

Solution to revision exercise.

Mass of Liquid at 2590)

99.72 - 36.7 = 63.02 m Mass of 11iquia expelled at 65°C

99.72 -98.46 = 1.26 gm.

The coefficient of apparent expansion of the liquid

1.26

02 x 40 = 4.99 × 10-4/01

76.5

Where:

0.000019 x 10

length at 1000

76.49 cm.

3. Let V be the volume of unsatur ated

Then

52.8 x V = x P

Boyle's law holda).

- pressure when the volume is

halved

105.6 cm. which is greater than the measured value by,

105.6 - 105.2 = 0.4 cm.

= 17.5 x 0,6 10.5 B

unsaturated vapour pressure obeys Boyle's law.

285

unsaturated at 1290.

apour pressure

10.2 mm.

97.4%

100%

7. A graph of S.V.P. against t°C is plotted in fig. (4 From the graph, the S.V.P. at 16,390 is about 14 mm. of mercury, The S.V.P. at dew point (12:5°C) 25, about 11.1 mm. of mercury,

R.H.

74.0 x 100%

79.4%)

1:10:

1-44

dew pl.