HEAT Class- VII Chapter 4 C.B.S.E.

LEARNING OBJECTIVES- Knowledge of heat, temperature, thermometer and reading of temperature on thermometer, Differences between clinical and laboratory thermometers, Methods of transfer of heat – conduction convection and radiation, Understanding why woolen clothes are preferred in winter?, Understanding why white or light coloured cotton clothes are weared in summer?, why the base of utensils are made up of copper?

INTRODUCTION- Heat is a form of energy. Heat is accumulated inside any object when it is heated in fire. The heat is transferred from a hot object to cold object. The S.I. Unit of heat is Joule (J). Calorie and British thermal unit (BTU) are also used as units of heat.

Note-Watt (W) is a unit of rate of transfer of heat. The rate of transfer of one joule heat per second is called a watt.

HOT AND COLD OBJECTS- If we eat or touch ice cream we feel cold. But the tea and handle of a girdle appear warm. If one liter warm water at 80˚C (degree Celsius) is mixed with one liter warm water at 50 ˚C then the temperature of mixed water becomes 65˚C . By touching we can say which is comparatively hot or cold. But it is not always trustworthy. Temperature is used to measure the hotness or coldness of any object. The degree of hotness or coldness is called as ‘temperature’. Temperature is measured by a device called ‘thermometer’.

MEASURING TEMPERATURE- The temperature of any object is measured by thermometers. The thermometers are of different types-

1. Clinical thermometers –It is used to take temperature of humans. e.g. Glass and mercury thermometer, Digital thermometer, Infrared forehead thermometers etc.

2. Laboratory thermometers– It is used to take temperature of soil and chemicals in laboratories.

3. Maximum- minimum thermometers– They are used to measure temperatures of the day.

The range of temperature of glass and mercury clinical thermometer is 35˚C to 42˚C .It is because this thermometer is used for measuring human body temperature only. The human body temperature does not go below 35˚C and above 42˚C. The average or normal body temperature is 37 ˚C. On this thermometer range remains given in degree Celsius as well as in Fahrenheit Scales.

There is a ‘kink’ near the bulb of clinical thermometer. This is a kind of bending of mercury line. It prevents the rising and falling of mercury level when taken out of mouth.

Fig.Kink in a clinical thermometer near bulb

ACTIVITY- READING OF CLINICAL THERMOMETER– Following are the steps and precautions in taking the reading of temperature through a clinical thermometer-

Fig. Clinical Thermometer

1. The Thermometer should be washed before and after use. Use of an antiseptic will be better.

2. It should be ensured before use that mercury level is below 35˚C.

3. Put the bulb of thermometer inside the mouth and gently close for 2-3 minutes.

4. Take out the thermometer and read the mercury level along the line of sight.

5. Do not hold the bulb of thermometer while taking the reading.

6. Glass and mercury thermometer should be handled with care otherwise it may break. The poisonous mercury may come out.

7. Clinical thermometers should not be kept near flame or in sunlight otherwise it may break.

LABORATORY THERMOMETER-This thermometer is longer than clinical thermometer. It range of temperature is -10 ˚C to 110 ˚C . It is used to measure the temperature of chemical reactions, soil, water etc. It is used to take temperature in the chemical reaction, soil, water etc. in real time. It is because there is no ‘kink’. When it is taken out of the chemicals the mercury level falls so at the same time temperature is noted down.

Fig. Laboratory Thermometer

DIFFERENCES BETWEEN CLINICAL AND LABORATORY THERMOMETERS-

S.N.	Characteristic	Clinical Thermometer	Laboratory Thermometer
1.	Range	35 ˚C to 42˚C	-10 ˚C to 110 ˚C
2.	Kink	Present	Absent
3.	Length	Small	Longer than
4.	Used for	Humans , animals	Chemicals , Soil ,water etc

MODES OF TRANSFER OF HEAT- Heat is transferred from source to sink, surrounding or any other object. Heat is transferred from higher temperature object to lower temperature object. There are three modes of transfer of heat-

1. CONDUCTION– The process by which heat is transferred from hotter end to the colder end of an object is known as conduction. Conduction of heat occurs in solids. But, some solids are conductors of heat while others are insulators of heat.

Fig. Melting of wax due to conduction of heat

The materials which allow heat to pass through them easily are called conductors of heat. e. g. metals like iron, copper, aluminium etc. The materials which do not allow heat to pass through them easily are called insulators or poor conductors of heat. e. g- Plastic, wood, clothes, dry leaves, dry bricks etc. The pure water called as distilled water and dry air are insulators of heat (and current).

Example 1 when one end of a metal strip is heated with the flame of candle, the other end also becomes warm after some time. The wax pieces kept over it starts melting and falling. This activity confirms that heat is conducted through metals.

Example 2 One end of a steel spoon is inserted in a hot cup of tea. The other end becomes hot after sometime. It confirms that heat of hot tea has transferred from one end to other end. So, steel is a conductor of heat.

DIFFERENCES BETWEEN CONDUCTORS AND INSULATORS-

S.N.	CONDUCTORS	INSULATORS
1.	Allow heat to pass through them easily	Do not allow heat to pass through them easily
2.	Conductors are metals, mineral water, salty water, moist air etc.	Insulators are non-metals, dry wood, plastic, paper, clothes etc.

2. CONVECTION– The transfer of heat by movement of particles of medium being heated is called convection. Convection occurs in liquids, gases and fluids.

Example 1 On boiling the milk, the milk near the base of container gets hot due to flame. Hot milk rises up. The cold milk from the sides moves down towards the source of heat. This milk also gets hot and rises and comparatively cold milk from the sides move down. This process continues till the whole milk gets heated.

Fig. Boiling of milk by Convection

Example 2 A candle was lighted. One hand was kept above the flame and other on the side of flame. The air above top becomes warmer than the sides due to convection. So, the air above top will feel hotter by top hand than the air on side by side hand.

Fig.Transfer of heat by convection of air

Example 3 SEA BREEZE AND LAND BREEZE– The people living in coastal areas experience dual nature of blowing of air due to convection of heat. The sea breeze and land breeze blows in coastal areas. The sea breeze and land breeze are due to difference in heating of the land and ocean water.

During day, the land becomes warmer than the ocean. So, air near the land becomes warm and rises above. As a result a partial lackness of air or low density air pressure is created on the land during day due to sunlight. To full fill this lackness, air blows from sea toward the land. It is called as sea breeze.

Fig. Sea Breeze and Land Breeze

During night, the sea becomes warmer than land by releasing the heat absorbed during the day. So, the air above sea becomes warm and rises above. As a result a partial lackness of air or low density air pressure is created above the sea during night. To full fill this lackness, air blows from land toward the sea. It is called as land breeze.

3. RADIATION– The process of transfer of heat from source to surrounding or other object with or without any medium is called radiation. In radiation the presence of any solid or liquid medium is not essential for transfer of heat. All hot bodies radiate heat.

Example 1 The solar radiations (sunlight) reach to the earth through radiation method. The sunlight passes through vacuum (total absence of air) and then through air of atmosphere to reach on the earth.

Example 2 The heat reaches up to us when we are sitting in front of a room heater is an example of radiation of heat.

When heat is transferred to anybody, a part of heat is absorbed, a part of heat is reflected and a part of heat is transmitted. This is the reason during the hot sunny day the use of umbrella protects us because some part of sunlight is reflected, some part is absorbed and only some part of sunlight is transmitted. So, we feel comfort.

TYPES OF CLOTHES WE WEAR IN SUMMER AND WINTER– In summer season, white and light coloured clothes are weared to reflect the sunlight. So, the person wearing it can feel comfort in summer.

In winter season, black, dark and colorful clothes are weared so that sunlight can be absorbed and transmitted inside so that the person wearing it can feel comfort in winter season.

WHY WOOLEN CLOTHES KEEP US WARM– When we wear woolen clothes in winter we feel comfort due to checking of incoming of cold air inside and out going of our body heat. The woolen clothes entrap air inside it. Air is an insulator of heat, so neither the body heat is not released outside nor the cold waves come inside.

This is the reason that instead of one thick blanket, the two thin blankets joined together will give more comfort.

SOLVED EXERCISE QUESTIONS-

Q.1 State similarities and differences between the laboratory thermometer and the clinical thermometer.

Ans-

Criteria	Clinical thermometers	Laboratory Thermometers
Similarities	It has mercury inside.	It also has mercury inside.
Similarities	Temperature Scale is degree Celsius (˚C).	Temperature scale is also degree Celsius.
Differences	It is used to measure temperature of the body.	It is used to measure temperature of water, chemicals
Differences	Its range is 35 ˚C to 42 ˚C	Its range is -10 ˚C to 110 ˚C

Q.2 Give two examples each of conductors and insulators of heat.

Ans- Conductors- 1. Copper 2. Aluminium

Insulators- 1. Wood 2. Plastic

Q.3 Fill in the blanks:

(a) The hotness of an object is determined by its ………………

(b) Temperature of boiling water cannot be measured by a …………….thermometer.

(d) No medium is required for transfer of heat by the process of ………………

(e) A cold steel spoon is dipped in a cup of hot milk. It transfers heat to its other end by the process of …………………..

(f) Clothes of ………………colours absorb heat better than clothes of lighter colours.

Ans- (a) temperature (b) clinical (c) Celsius

(d) radiation (e) conduction (f) dark

Q.4 Match the following:

(i) Land breeze blows during (a) summer

(ii) Sea breeze blows during (b) winter

(iii) Dark coloured clothes are preferred during (c) day

(iv) Light coloured clothes are preferred during (d) night

Ans-(i) –night (ii)- day (iii)- winter (iv)- summer

Q.5 Discuss why wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing.

Ans- The air is an insulator of heat. So, on wearing more layers of clothing during winter an insulation layer is created which prevents our body heat to go in surrounding and we feel comfort. This type of insulation layer is not formed if wear just one thick piece of clothing.

Q.6 Look at Figure. Mark where the heat is being transferred by conduction, by convection and by radiation.

Ans-

Fig. Showing- 1. Conduction 2. Convection 3. Radiation

Q.7 In places of hot climate it is advised that the outer walls of houses be painted white. Explain.

Ans- White wall will reflect the sunlight and only little transmission occurs. So, such a wall will become comparatively less hot than the other coloured walls. The people living behind these walls in their room will feel more comfort.

Q.8 One litre of water at 30˚C is mixed with one litre of water at 50˚C. The temperature of the mixture will be

(a) 80˚C

(b) more than 50 ˚C but less than 80˚C

(d) between 30˚C and 50 ˚C

Ans- (d) between 30˚C and 50 ˚C

Q.9 An iron ball at 40˚C is dropped in a mug containing water at 40˚C .The heat will

(a) Flow from iron ball to water.

(b) not flow from iron ball to water or from water to iron ball.

(d) increase the temperature of both.

Ans-(b) not flow from iron ball to water or from water to iron ball.

Q.10 A wooden spoon is dipped in a cup of ice cream. Its other end

(a) becomes cold by the process of conduction.

(b) becomes cold by the process of convection.

(d) does not become cold.

Ans-(d) does not become cold

Q.11 Stainless steel pans are usually provided with copper bottoms. The reason for this could be that

(a) copper bottom makes the pan more durable.

(b) such pans appear colourful.

(d)copper is easier to clean than stainless steel.

Ans- (c) copper is a better conductor of heat than stainless steel.

availetek

December 24, 2022 at 8:55 am

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