# Frank Solutions for Chapter 5A Heat: Thermal Expansion Class 9 Physics ICSE

### Exercises

1. Define the following terms:

(i) Coefficient of linear expansion

(ii) Coefficient of superficial expansion

(iii) Coefficient of cubical expansion

Answe

(i) Coefficient of Linear expansion is equal to the change in length of a rod of length 1 m when its temperature rises by 1°C.

(ii) Coefficient of superficial expansion is equal to the change in area of a rod of area 1 m2 when its temperature rises by 1°C.

(iii) Coefficient of volume expansion is equal to the change in volume of a rod of volume 1 m2 when its temperature rises by 1°C.

2. State the relation between the three types of expansion.

If the Coefficient of Linear expansion is denoted by α

Coefficient of superficial expansion is denoted by β

And Coefficient of volume expansion is denoted by γ

Then the relation between α, β and γ is stated as

β = 2α and γ = 3 α

α : β : γ : : 1 : 2 : 3

3. What is a bimetallic strip? What is the criterion of choosing the two metals for a bimetal strip? Give two uses of a bimetal strip.

A bimetallic strip consists of two metal strips - one with high coefficient of expansion and the other with low coefficient of expansion. Two different metals are used for regulating temperature in an electrical device as the strip bends due to different coefficient of expansion on excessive heating due to current, thus breaking electrical circuit until the strip cools down to a preset point. Two applications of bimetallic strip are thermostat in electric iron and in balance wheels.

(a) When boiling water is poured into a glass bottle, it generally cracks because on pouring hot water in the bottle the inner surface heats up and expands more as compared to its outer surface. This unequal expansion between the two surfaces causes a strain and the bottle cracks.

(b) Telephone wires sag in summer because due to heat of the sun, the wire expands and increases in length, thus they sag in summer.

(c) In cold countries water pipes burst in winter because the water has maximum density at 4°C and, due to anomalous expansion of water, it expands when the water is cooled to a temperature below 4°c

(d). Even when the water in the lakes is frozen, fishes can survive due to anomalous expansion of water, water has maximum density at 4°C and this dense water remains at the bottom of the lake and the upper layer of water is less dense and freezes but the temperature of each layer of water below increases by 1°C. Thus it is warmer in the lakes below the ice layer. This helps the fishes to survive.

4. A bimetal strip is shown in the diagram (Fig. 15) before heating. Draw a diagram to show how it bends on heating and on cooling.

Diagram is to show how the given strip bends and cools. Cue to heating the copper bends more than invar due to high coefficient of linear expansion of copper.

5. What is a thermostat? Give two uses of thermostat.

Thermostat is a device for regulating temperature in electric circuits and it is made up of bimetallic strip. Two applications of thermostat are in electric iron and in refrigerators.

6. Explain the ‘anomalous expansion’ of water.

When heated, water expands normally but behavior of water is unusual over a small range of temperature between 0 and 4°C. This is known as anomalous expansion of water.

7. What properties of water slow down the complete freezing of the water in the lake?

Anomalous expansion of water slows down the complete freezing of water in the lake.

8. Around bottom flask containing coloured water, filtered with a stopper and a narrow glass tube is shown in the diagram (Fig. 16). What will happen to the water level X marked in the tube when it is placed in a hot water bath? Give reason for your answer.

The level will rise above X when the flask is kept in hot water bath because the water in the flask will heat up and expand in volume thus raising the level of water in the tube above X

9. Give reasons for the following:

(i) When boiling water is poured into a glass bottle, it generally cracks.

(ii) Telephone wires sag in summer.

(iii) In cold countries water pipes burst in winter.

(iv) Even when the water in the lakes is frozen, fish can survive.

(i) When boiling water is poured into a glass bottle, it generally cracks because on pouring hot water in the bottle the inner surface heats up and expands more as compared to its outer surface. This unequal expansion between the two surfaces causes a strain and the bottle cracks.

(ii) Telephone wires sag in summer because due to heat of the sun, the wire expands and increases in length, thus they sag in summer.

(iii) In cold countries water pipes burst in winter because the water has maximum density at 4°¢ and, due to anomalous expansion of water, it expands when the water is cooled to a temperature below 4°C

(iv) Even when the water in the lakes is frozen, fishes can survive due to anomalous expansion of water, water has maximum density at 4°C. and this dense water remains at the bottom of the lake and the upper layer of water is less dense and freezes but the temperature of each layer of water below increases by 1°C. Thus it is warmer in the lakes below the ice layer. This helps the fishes to survive C and this dense water remains at the bottom of the lake and the upper layer of water is less dense and freezes but the temperature of each layer of water below increases by 1°C. Thus it is warmer in the lakes below the ice layer. This helps the fishes to survive.

10. What will be the approximate temperature of water in the lake shown in the diagram (Fig. 17) ?

(i) At X, and

(ii) At Y?

The layer at the top level is ice so its temperature is therefore 0°C and temperature at every layer of water below ice increases by 1°C

(i) At X = 1°C since it is the first layer below ice

(ii) At Y = 4°C since it is the fourth layer below ice

11. The diagram shows a sketch graph showing change in volume of a given mass of ice at -5°C to water at 15°C (Fig. 21).

(i) Why does the temperature along BC remain constant?

(ii) When is the volume of water minimum?

(iii) At what temperature does water have maximum density ? Explain.

(i) At BC temperature is constant because at 0° C all of the heat given is used in process of formation of water from ice and no part of heat is used in changing the temperature of ice.

(ii) At point D i.e. at 4°C the volume of water is minimum

(iii) At point D i.e. at 4°C, the density of water is maximum due to anomalous expansion of water in the temperature range of 0°C and 4°C as volume decreases, density of water increases

12. Complete the following sentences:

(i) Density of water is -—---—- At 4°C.

(ii) When ice melts its volume -------— and density -----——.

(iii) In a bimetal two metals of different ----—--- of ---—---—- are used.

(iv) Water pipes burst in winter in very cold countries is due to ----—-—-

(v) CGS units of heat is —-—--—-.

(vi) SI unit of temperature is —-----—-.

(i) Maximum

(ii) Increases; decreases

(iii) Coefficient; expansion

(iv) Anomalous expansion of water

(v) Calorie

(iv) Kelvin(K)

13. A glass stopper has got stuck in the neck of a bottle. How can you remove the stopper without breaking the bottle?

We should heat the neck of the bottle because due to heating the neck will expand and loosen the stopper stuck in the neck. In this way, we can easily remove the stopper.

14. What is meant by apparent expansion and real expansion of a liquid?

When a liquid is heated in a flask. Due to heat, the flask expands along with the liquid, thus providing more space for the liquid. Hence the level of the liquid falls. Thus this dropped level shows the apparent expansion of the liquid. The actual expansion of the liquid, when heated, is the real expansion.

15. Equal volumes of alcohol and mercury are heated uniformly in identical containers separately from 10°C to 50°C. Will both the liquids occupy the same volume? Give reason.

No, the both liquids i.e. mercury and alcohol will not have same volume on heating as they have different coefficient of expansions, thus they expand differently on heating at same temperature range. Alcohol expands more than mercury.

16. How will you demonstrate thermal expansion in liquids? Describe an experiment.

Fill a round bottom flask with a coloured liquid up to the brim. Fit it with a one holed cork and pass a narrow glass tube through it. Mark the level of the liquid in the glass tube. Place the flask in a water bath and heat the bath.

You will see that the level of liquid of water rises in the glass tube. This is due to the expansion of the liquid on heating.

17. Describe an experiment to demonstrate thermal expansion in gases.

Take a round bottom flask which is filled with air and closed with a one holed rubber cork. A narrow capillary tube is passed through the cork and the flask is supported on a tripod in an inverted position as shown in the figure, so that the end of the capillary tube is under water. Now, heat the flask gently with a spirit lamp. You will observe bubbles coming out from the capillary tube. This shows that gases expand on heating.

18. Why does a hot glass chimney often crack when a drop of water falls on it?

A hot glass chimney often crack when a drop of water falls on it because the glass of the chimney expands due to constant heating by the flame of the chimney but as a cooler water drop falls on the glass, the outer surface contracts more than the inner side of the glass. This unequal contraction between the two surfaces causes a strain and the glass cracks.

19. Can we fuse an iron wire in a glass rod? Give reason to your answer.

No, we cannot fuse iron wire in glass rod because glass is bad conductor of heat and it does not allow heat to pass through it and iron wire would not be able to get sufficient heat to fuse.

20. Suggest some suitable way to remove a tight glass stopper from a bottle.

We should heat the neck of the bottle because due to heating the neck will expand and loosen the stopper stuck in the neck. In this way, we can easily remove the stopper from the bottle.

21. Name two substances which expand on heating.

Two substances that expand on heating are water and iron.

22. Name two substances which contract on heating.

Two substances that contract on heating are plastic and poly-ethene.

23. Describe an experiment to demonstrate the thermal expansion of solids.

Take a ball and ring apparatus as shown in figure. At room temperature, the ball can just pass through the ring. Now heat the ball over a flame and try to pass it through the ring. You will find that the ball when heated is not able to pass through the ring. The diameter of the ball increases when the ball is heated. This shows the expansion of solid on heating.

24. The joins in metal pipes become loose in summer. Why?

The joints in metal pipes loosen in summers because the joints get more space to expand due to the heat in summers as compared to the pipes therefore the joints become loose in summers

25. State three factors on which the increase in length of a rod depends, on heating.

The increase in length of a rod depends on the following factors

(a) material of the rod

(b) Original length of the rod

(c) Rise in temperature

26. ‘The coefficient of linear expansion for a metal is 0.000016 per °C’. Explain the meaning of the statement.

The given statement states that the change in length of the metal will be 0.000016 m when its temperature changes by 1°C.

27. fig. 19 shows a copper washer. Explain, giving reasons, whether the following characteristics of the washer increase, decrease or remain the same, when it is heated.

(a) Internal diameter,

(b) Volume,

(c) Mass

(d) Density

(e) External diameter,

On heating the copper washer

(a) Its internal diameter will increase as the copper washer expands outwards

(b) Volume will increase as the size of the metal expands on heating

(c) Mass will remain the same as the mass does not change with change in temperature

(d) Density will decrease as the volume of the washer increases on heating and density is inversely proportional to volume

(e) External diameter will increase because the copper washer expands outwards

28. Show by an experiment that the real expansion of a liquid is equal to the sum of its apparent expansion of a liquid is equal to the sum of its apparent expansion and the volume expansion of the vessel containing the liquid.

Fill a round bottom flask with a coloured liquid up to the brim. Fit it with a one holed cork and pass a narrow glass tube through it. Mark the level of the liquid in the glass tube as A. Place the flask in a water bath and heat the bath. You will see that the level of liquid of water starts falling to level B and then it rises to level C. In this, AC is the apparent expansion and is due to expansion of the flask due to heating which provides more space for the liquid. Hence the level of liquid drops. As soon as the liquid also starts getting heated up, it expands and the level of liquid rises. BC is the real expansion. So the actual expansion of the water is sum of AC and AB.

29. At what temperature the density of water is maximum?