ICSE Class 9 Physics Chapter 06 Heat and Energy

Heat And Energy

Syllabus

i) Concepts of heat and temperature.

Scope - Heat as energy; SI unit, joule. 1 cal = 4.186 J exactly.

ii) Anomalous expansion of water

Scope - Anomalous expansion of water, graphs showing variation of volume and density of water with temperature in the 0 to 10°C range. Hope's experiment and consequences of anomalous expansion.

iii) Energy flow and its importance.

Scope - Understanding the flow of energy as linear and linking it with the laws of thermodynamics - energy is neither created nor destroyed' and 'no energy transfer is 100% efficient'.

iv) Energy sources. Renewable versus non-renewable sources (elementary ideas with example), energy degradation.

Scope - Solar, wind, water and nuclear energy (only qualitative discussion of steps to produce electricity). Renewable energy : Bio gas, solar energy, wind energy, energy from falling of water, run-of-the river schemes, energy from waste, tidal energy, etc. Issues of economic viability and ability to meet demands. Non-renewable energy : Coal, oil, natural gas, inequitable use of energy in urban and rural areas, use of hydroelectrical power for light and tube-wells. Energy degradation : Meaning and examples..

v) Green house effect and global warming.

Scope - Meaning and impact on the life on earth; projections for the future; what needs to be done.

A - Heat And Temperature; Anomalous Expansion

6.1 Concept Of Heat (Heat As Energy)

It is our common experience that on rubbing our palms, they get heated ; on passing electric current in a metallic wire, the wire gets heated; on burning coal, we get heat; on pumping air in a bicycle tube, the barrel of pump gets heated. In all these cases, heat is produced either by doing work or by providing energy in some form other than heat. On rubbing palms and on pumping air in a bicycle tube, heat is produced by doing work i.e., from mechanical energy, while on passing current in a metallic wire, heat is obtained from electrical energy and on burning coal, heat is obtained from chemical energy. Thus heat is also a form of energy.

Each body is made up of molecules. The molecules are in random motion and each molecule exerts a force of attraction on other molecules. Thus molecules possess energy and the heat energy of a body is the internal energy* of its molecules. A hot body has more internal energy than an identical cold body. When a hot body is kept in contact with a cold body, the cold body warms up and the hot body cools down i.e., the internal energy of cold body increases and that of the hot body decreases. Thus energy is transferred from the hot body to the cold body when they are placed in contact. The energy which flows from hot body to the cold body is called the heat energy or simply the heat.

On touching, a body appears hot to us when heat energy flows from that body to our hand, while it appears cool to us when heat energy flows from our hand to the body.

Example : If we touch warm water, we feel hot because heat energy from warm water passes to our hand. Similarly, if we touch a cube of ice, we feel cool because heat energy from our hand passes to the cube of ice.

Thus we can define heat as follows :

Heat is the internal energy of molecules constituting the body. It flows from a hot body to a cold body.

Unit of heat

The unit of heat is same as that of energy. The S.I. unit of heat is joule (abbreviated as J) and its C.G.S. unit is erg, where

\[1 \text{ J} = 10^7 \text{ erg} \quad \text{...(6.1)}\]

Other units of heat are calorie (cal) and kilocalorie (kcal), where

\[1 \text{ kilocalorie} = 1000 \text{ calories} \quad \text{...(6.2)}\]

The units calorie and joule are related as :

\[1 \text{ cal} = 4.186 \text{ J (or nearly 4.2 J)} \quad \text{...(6.3)}\]

6.2 Concept Of Temperature

When a hot body is kept in contact with a cold body, it is noticed that after some time, the hot body becomes less hot and the cold body becomes less cold. Obviously, this is because of the flow of heat from hot body to the cold body. The body which imparts heat is said to be at a higher temperature than the body which receives heat. Thus, temperature determines the direction of flow of heat.

When a body receives heat energy, the particles constituting the body start vibrating more vigorously and its temperature rises provided its physical state or dimensions remain unchanged. Thus temperature is defined as below.

Temperature is a quantity which tells the thermal state of a body (i.e., the degree of hotness or coldness of the body). It determines the direction of flow of heat when two bodies at different temperatures are placed in contact.

If there is no transfer of heat between the two bodies placed in contact, they are said to be at the same temperature, but it does not mean that they have equal amount of heat in them. In fact, temperature alone does not tell us the quantity of heat energy contained in a body. Experimentally, we find that by imparting same quantity of heat energy to different bodies, they get heated to different temperatures. The amount of heat energy contained by a body depends on its mass, temperature and the material of the body.

Unit of temperature

The S.I. unit of temperature is kelvin (symbol K). The other most common unit of temperature is degree celsius (symbol °C) and degree fahrenheit (symbol °F).

The temperature on Celsius scale and Kelvin scale are related as :

\[T_K = 273 + t \text{ °C} \quad \text{...(6.4)}*\]

Thus, by adding 273 to the temperature in degree celsius, we get the temperature in Kelvin. Actually a degree on both the Kelvin and Celsius scales is equal.

The ice point is 0°C on Celsius scale, 32°F on Fahrenheit scale and 273 K on the Kelvin scale. The steam point is 100°C on Celsius scale, 212°F on Fahrenheit scale and 373 K on the Kelvin scale. Thus there are 100 equal degrees between the ice point and steam point on both the Celsius and Kelvin scales, but 180 equal divisions on the Fahrenheit scale. Thus 1 degree on Celsius scale is \[\frac{9}{5}\] degree on Fahrenheit scale.

* More precisely T K = 273.15 + t °C

Difference between heat and temperature

The zero of the Kelvin scale is called absolute zero and it is at a temperature when molecular motion ceases. It is at -273°C i.e. 0 K = -273°C.

The temperature on Celsius and Fahrenheit scales are related as :

\[\frac{C}{5} = \frac{F - 32}{9} \quad \text{...(6.5)}\]

6.3 Thermal Expansion

Almost all substances (solids, liquids and gases) expand on heating and contract on cooling.

The expansion of a substance on heating is called the thermal expansion of that substance.

A solid has a definite shape, so when a solid is heated, it expands in all directions i.e., the length, area and volume, all increase on heating. The increase in length is called the linear expansion, the increase in area is called the superficial expansion and the increase in volume is called the cubical expansion. The liquids and gases do not have a definite shape, so they have only the cubical (or volume) expansion. On heating, liquids expand more than the solids, and gases expand much more than the liquids.

Some substances such as water from 0°C to 4°C, silver iodide from 80°C to 141°C and silica below -80°C contract on heating and expand on cooling. The expansion of a substance on cooling in a certain range of temperature is called the anomalous expansion of that substance. Here we shall study the anomalous expansion of water.

6.4 Anomalous Expansion Of Water

If we take some water at 0°C and start heating it, we find that it contracts (instead of expanding) in the temperature range from 0°C to 4°C. On heating it further above 4°C, it expands. Similarly, if water initially at a temperature above 4°C is cooled, it contracts till the temperature of water reaches 4°C. On further cooling it below 4°C to 0°C, it expands. This unusual expansion of water on cooling it in the temperature range 4°C to 0°C, is called anomalous expansion of water. Thus,

The expansion of water when it is cooled from 4°C to 0°C, is known as anomalous expansion of water.

Fig. 6.1 shows the variation in volume of 1 g of water with temperature in the range from 0°C to 10°C. The volume of water first decreases on heating it from 0°C to 4°C and then increases on further heating it from 4°C to 10°C. The volume of water is thus minimum at 4°C. For 1 g of water, the volume at 4°C is 1.0000 cm³.

Fig. 6.2 shows the variation in density of water with temperature in the range from 0°C to 10°C. When water is heated from 0°C, the density of water first increases from 0°C to 4°C and then decreases above 4°C to 10°C. On the other hand, on cooling water from 10°C, the density of water first increases up to 4°C and then decreases when it is cooled further below 4°C to 0°C. Thus the density of water is maximum at 4°C which is equal to 1 g cm-3 (or 1000 kg m-3).

6.5 Hope's Experiment To Demonstrate The Anomalous Expansion Of Water

In 1805, the scientist T.C. Hope devised a simple arrangement, known as Hope's apparatus for demonstrating the anomalous expansion of water. Fig 6.3 shows the Hope's apparatus.

The apparatus consists of a tall metallic cylinder provided with two side openings P near the top and Q near the bottom, fitted with thermometers T1 and T2 respectively. The central part of the cylinder is surrounded by a cylindrical trough containing a freezing mixture of ice and salt. The cylinder is filled with pure water at room temperature. The temperature recorded by both the thermometers is observed at a regular interval of time.

Observations : (i) Initially both the thermometers T1 and T2 show same temperature, (i.e., room temperature).

(ii) First the temperature recorded by lower thermometer T2 starts decreasing and finally it becomes steady at 4°C, while the temperature recorded in upper thermometer T1 remains almost unchanged during this time.

(iii) While the temperature recorded by lower thermometer T2 remains constant at 4°C, the upper thermometer T1 shows a continuous fall in temperature up to 0°C and then it also becomes steady.

Thus finally the temperature recorded by upper thermometer T1 is 0°C and that by lower thermometer T2 is 4°C.

Fig. 6.4 shows the variation in temperature recorded by thermometers T1 and T2 with time.

Explanation : Initially water in cylinder is at room temperature (say 12°C) which is indicated by the point A in the graphs. As the freezing mixture cools water in the central portion of the cylinder, water contracts and its density increases. Consequently the cooled water sinks to the bottom and warm water from the bottom rises up to take its place. Thus by convection, water of the lower part cools, so the reading of the lower thermometer T2 falls rapidly. The reading of upper thermometer T1 does not change because the upper part does not change. This continues till temperature of entire water below the central portion reaches at 4°C. This is shown by the part AB in graphs (a) and (b). Now the reading of lower thermometer T2 becomes steady. On further cooling below 4°C, due to anomalous expansion, water of the central portion now expands, so its density decreases and hence it rises up. To take its place, water from top descends down and by convection, water above the central portion cools. So the reading of upper thermometer T1 now falls rapidly till 0°C and water near the top freezes to form ice at 0°C. Now the thermometer T1 shows the steady temperature 0°C. This is shown by the part BC in graphs (a) and (b). At this stage, the lower thermometer T2 shows the temperature 4°C at which water has the maximum density while the upper thermometer T1 shows the temperature of water and ice at 0°C.

6.6 Consequences Of Anomalous Expansion Of Water

(i) The anomalous expansion of water helps in preserving the aquatic life during the very cold weather

In cold weather (or winter), when the atmospheric temperature starts falling well below 0°C, water at the surface of a pond (or lake) initially at temperature above 4°C, begins to radiate heat to the atmosphere, so the temperature of water starts falling upto 4°C. When temperature of water at the surface falls, water contracts, so its density increases and therefore, it sinks to the bottom. This continues till temperature of entire water reaches to 4°C. Now, further cooling of top layers below 4°C results in expansion of water and so its density decreases. As a result, water does not sink further, but it remains on the surface. When the temperature of atmosphere falls below 0°C, water on the surface rejects heat to the atmosphere and gradually freezes into ice, but water well below the ice layer remains at 4°C. The water layer just below the ice in contact with it will be at 0°C. Fig. 6.5 shows the formation of ice at the surface of pond (or lake). Since ice is a poor conductor of heat, so ice now prevents the flow of heat from water of pond (or lake) to the atmosphere. Thus temperature of water in contact with ice is at 0°C. While the temperature of water layers below ice gradually increases to 4°C. As a result, fish and other aquatic creatures remain alive in water of the pond (or lake), though water on the surface has frozen into ice. Nature thus protects the aquatic life during the winter season.

(ii) The anomalous expansion of water is responsible for the burst of water pipe lines, and destruction of crop during the very cold nights

In winter nights, as the atmospheric temperature starts falling below 4°C, water in pipe lines expands and it exerts large pressure on the pipes, causing them to burst. Plants also die for the same reason as their capillaries burst when water expands below 4°C.

To protect the plants, the field is filled with water.

Exercise 6(A)

1. What is heat ? Write its S.I. unit.

2. Two bodies at different temperatures are placed in contact. State the direction in which heat will flow. Ans. From the body at high temperature to the body at low temperature.

3. Name the S.I. unit of heat and how is it related to the unit calorie ? Ans. joule (J), 1 J = 0.24 cal (nearly)

4. Define temperature and write its S.I. unit.

5. Why does a piece of ice when touched with hand, appear cool ? Explain. Ans. On touching ice, heat passes from our hand to the ice.

6. Distinguish between heat and temperature.

7. What do you understand by thermal expansion of a substance ?

8. Name two substances which expand on heating. Ans. Brass, Iron

9. Name two substances which contract on heating. Ans. Water from 0°C to 4°C, silver iodide from 80°C to 141°C.

10. What do you mean by anomalous expansion of water ? Ans. Expansion of water on cooling it from 4°C to 0°C.

11. At what temperature the density of water is maximum ? State its value. Ans. At 4°C, 1000 kg m-3

12. State the volume changes observed when a given mass of water is heated from 0°C to 10°C. Sketch a temperature-volume graph to show the behaviour.

13. Draw a graph to show the variation in density of water with temperature in the temperature range from 0°C to 10°C.

14. A given mass of water is cooled from 10°C to 0°C. State the volume changes observed. Represent these changes on a temperature-v graph.

15. Describe an experiment to show that water has maximum density at 4°C. What important consequences follow from this peculiar property of water ? Discuss the importance of this phenomenon in nature.

16. Deep pond of water has its top layer frozen during winter. State the expected temperature of water layer (i) just in contact with ice, (ii) at the bottom of pond. Ans. (i) 0°C (ii) 4°C

17. Draw a diagram showing the temperature of various layers of water in an ice covered pond.

18. Explain following :

(a) Water pipes in colder countries often burst in winter.

(b) In winter, water tank (or ocean) starts freezing from the surface and not from the bottom.

(c) Fishes survive in ponds even when the atmospheric temperature is well below 0°C.

(d) A hollow glass sphere which floats with its entire volume submerged in water at 4°C, sinks when water is heated above 4°C.

(e) A glass bottle completely filled with water and tightly closed at room temperature, is likely to burst when kept in the freezer of a refrigerator.

Multiple choice type :

1. Calorie is the unit of :

(a) heat (b) work (c) temperature (d) food Ans. (a) heat

2. 1 J equals to :

(a) 0.24 cal (b) 4.18 cal (c) 1 cal (d) 1 kcal Ans. (a) 0.24 cal

3. S.I. unit of temperature is :

(a) cal (b) joule (c) celsius (d) kelvin Ans. (d) kelvin

4. Water is cooled from 4°C to 0°C. It will :

(a) contract (b) expand (c) first contract, then expand (d) first expand, then contract. Ans. (b) expand

5. Density of water is maximum at :

(a) 0°C (b) 100°C (c) 4°C (d) 15°C Ans. (c) 4°C

Teacher's Note

When we boil water for tea or coffee, the heat energy transforms into steam. Understanding how heat differs from temperature helps us appreciate why a cup of boiling water feels hotter than a warm bath, even though the bath contains more total heat energy.

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