Class 7 Science Heat Notes

1.3 Reading a Thermometer

 

Let us learn how to read a thermometer. Wash the thermometer, preferably with an antiseptic solution. Hold it firmly and give it a few jerks. Jerks will bring the level of mercury down. Ensure that it falls below 35°C. Now place the bulb of the thermometer under your tongue. After one minute, take the thermometer out and note the reading.

This is your body termperature.. The temperature should always be stated with its unit,°C.

 

The normal temperature of human body is 37°C or 98.4° F. The temperature of every person may not be 37°C. It could be slightly higher or slightly lower. Actually, what we call normal temperature is the average body temperature of a large number of healthy persons.

1.4 Transfer of Heat

There are three modes of transfer of heat. They are conduction, convection and radiation.

 

Conduction : The process by which heat is transferred from the hotter end to the colder end of an object is known as conduction.

 

In solids, generally, the heat is transfererred by the process of conduction.

Depending on type heat conduction, the materials are of 2 types :

 

(i) Good conductors : Those materials which allow the flow of heat through them. e.g. Metals, Iron, Copper, etc.

 

(ii) Bad conductors : Those materials which don’t allow the flow of heat through them. e.g., Wood, Plastic, etc.

Convection is the mode of heat transfer by actual motion of matter.

 

When water is heated, the water near the flame gets hot. Hot water rises up. The cold water from the sides moves down towards the source of heat. This water also get hot and rises and water from the sides moves down. This process continues till the whole water gets heated.

 

Radiation is the mode of heat transfer without any medium. From the sun, the heat comes to us by the process of radiation.

 

All hot bodies radiate heat. When this heat falls on some object, a part of it is reflected, a part is absorbed and a part may be transmitted. The temperature of the object increases due to the absorbed part of the heat.

1.5 Land and Sea Breeze

During the day, the land gets heated faster than the water. The air over the land becomes hotter and rises up. The cooler air from the sea rushes in towards the land to take its place. The warm air from the land moves towards the sea to complete the cycle. The air from the sea is called the sea breeze. The water cools down more slowly than the land. So, the cool air from the land moves towards the sea. This is called the land breeze.

1.6 Conductors and Insulators

Conductors: The materials which allow heat to pass through them easily are called conductors of heat. Eg. Aluminium, Iron and Copper.

Insulators: The materials which do not allow heat to pass through them are called insulators of heat. Eg. Plastic and Wood.

1.7 Thermodynamic Processes

i) Isothermal Process: The process in which temperature of the system remains constant is called isothermal proce

ii) Adiabatic Process: The process in which no heat enters or leaves the system is called adiabatic process i.e. the heat of the system remains constant.

iii) Isobaric process: The process in which pressure of the system remains constant is called isobaric process.

iv) Isochoric process: The process in which volume remains constant is called isochoric proce

1.8 Effects of heat :

i) Heat causes the change in the temperature of a material.

ii) Heat can lead to the expansion of the material This expansion can be in the length, area or volume of the materials

⇒ Linear expansion : Expansion in the length of the material by heating.

⇒ Superficial expansion : Expansion in the area of the material by heating.

⇒ Cubical expansion : Expansion in the volume of the material by heating. iii) Heat can change the state of a material.

Liquid -----beat-----> gas (vaporization)

Solid -----beat-----> liquid (fusion or melting)

Solid -----beat-----> gas (sublimation)

Note: The process of converting solid directly into gas by heating without going through the intermediate liquid state is called sublimation.

⇒ The process of conversion of gases directly into solid without going through the intermediate liquid state in called frosting.

⇒ Heat always flow from higher temperature to lower temperature, under normal circumstance.

⇒ Inside a refrigerator, heat is made to flow from low temperature to higher temperature, by doing an external work.

Q1. Define the following terms:

a.) Temperature b.) Thermometer

c.) Conductor of heat d.) Insulator of heat.

Ans: a.) Temperature is a measure of the degree of hotness & coldness of an object. {Note: S.I unit of Temperature is Kelvin. Other Units are degree Celsius & degree Fahrenheit}

b.) Thermometer is a device used for measuring temperatures.

c.) Materials which allow heat to pass through them easily are called conductors of heat. E.g. iron, aluminum, copper etc.

d.) Materials which do not allow heat to pass through them easily are called insulators of heat.

E.g. plastic, wood etc..

{Note: Water & air are poor conductors of heat.}

Q2. I.) What is Clinical thermometer?

ii.) Name the liquid used in this thermometer.

iii.)What is the range of this thermometer?

Ans.: I.) Thermometer that measures our body temperature is called a Clinical thermometer.

ii.) Mercury.

iii.) 35oC to 42oC.

{Note: Mercury is a poisonous metal. If a thermometer breaks, when in mouth, one is exposed to

mercury. To overcome this, digital thermometers are used these days.}

Q3. What is the normal temperature of healthy human body?

Ans: 37oC.

Q4. What is the range of Laboratory thermometer?

Ans: - 10oC to 110oC.

Q5. We cannot use Laboratory thermometer to measure our body temperature. Why?

Ans: There is no ‘kink’ in laboratory thermometer. Therefore the temperature will fall as soon as we remove the thermometer from the mouth & we will not be able to measure the correct temperature of the body.

Q6. What is the use of ‘kink’ in clinical thermometer?

Ans: ‘Kink’ is provided in the clinical thermometer which does not allow the mercury to come down on its own & we can note the measured temperature even after some time.

Q7. Why is mercury used in thermometer?

Ans: Mercury is used in thermometer because of following reasons:

i) It does not stick to the walls of capillary tube.

ii) Its silvery shine helps us to see temperature accurately.

iii) Its expansion is uniform.

iv) It does not vaporize easily.

Q8. Give reason for each of the following:

a.) Cooking utensils are made up of metals.

b.) Cooking utensils & tea kettles are provided with handles made up of wood & ebonite.

c.) Light coloured clothes are more suitable in summer & dark clothes in winter.

d.) Woollen Clothes keep us warm in winter.

e) The base of cooking utensil is painted black.

Ans: a.) Cooking utensils are made up of metals. Being very good conductors, they heat up very quickly& therefore cooks food in shorter time.

b)Wood & ebonite are poor conductors of heat, therefore the handle does not get heated & we can hold the hot utensil quite comfortably.

c) White or light coloured clothes are more suitable in summer because they absorb very little of the sun’s heat & keep our bodies cool.While black or dark coloured clothes are suitable in winter because they absorb most of the radiant heat of the sun & keep us warm.

d) Wool is a poor conductor of heat.Moreover, there is air trapped in between the wool fibres. This prevents the flow of heat from our body to the surroundings. So we feel warm.

e) So that it absorbs more heat & cooks food in shorter time.

Q9. Suppose you are given the choice in winter of using either one thick blanket or two thin blankets joined together. What would you choose & why?

Ans: We prefer two thin blankets joined together because the air trapped between two blankets acts as an insulator & does not allow the body heat to escape.

Q10. Name the three processes of transfer of heat & explain them?

Ans: Conduction, Convection & Radiation are three processes of transfer of heat. Conduction: It is the process of transfer of heat in solids. In this process the molecules of the solid pass the heat from one to another, without themselves moving from their positions.

Convection: It is the process of transfer of heat in liquids & gases. In convection, the molecules themselves move from one place to another, carrying heat with them.

{Note: Solids are not heated by convection because the molecules of a solid are not free to move from one place to another; they can only vibrate about fixed position.}

Radiation: It is the process of heat transfer from a hot body to a colder body without heating the space between the two. The transfer of heat by radiation does not require any medium.

Q11. What are Land & Sea Breezes?

Ans: Sea Breezes: During the day, the land is heated by sun to a higher temperature than the water in the
sea. Air over the land is heated, becomes lighter & rises while the cooler air fromthe sea blows towards land to take its place. This is called the sea breeze.

Land Breeze: At night, the land is no longer heated by the sun. It therefore cools. But the land cools much faster than the sea. Therefore, the sea is warmer than the land at night. So at night a current of air blows from the colder land to the warmer sea. This is called the land breeze.

Q12. Name the physical quantity that determines the direction of heat flow?

Ans: Temperature.

{Note: Heat flows from high temperature to low temperature till the temperature of both the bodies in contact is same.}

Q1 Define Radiation.

A1 The process of transfer of heat that does not require any material medium.

Q2 Distinguish between Heat and Temperature.

A2 Heat: It is a form of energy. Heat is the cause. Temperature: Degree of hotness or coldness of a body. Temperature is the effect.

Q3Whymercury is preferred as thermometric liquid in making thermometers?

A3 It does not stick to the walls of thermometer.

It has uniform contraction and expansion.

It is only metal which is found in liquid state.

Q4Why is alcohol preferred as thermometric liquid in making thermometers?

A4 For measuring very low temperatures, alcohol thermometers are used, because alcohol freezes at –117degree Celsius.

Q5What do you understand by lower fixed point?

A5 The temperature at which pure ice melts at normal atmospheric pressure is taken as a standard called lower fixed point.

Q6 Define upper fixed point.

A6 Temperature at which pure water boils at normal atmospheric pressure is called upper fixed point.

Q7What are the different modes of transfer of heat?

A7 The different modes of transfer of heat are Conduction, Convection & Radiation.

Q8 What are the scales commonly used to measure temperature?

A8 The scales commonly used to measure temperature are Celsius ,Kelvin & Fahrenheit scale.

Q9 What happens to radiant heat when it falls on body?

A9 The radiant heat when falls on body get absorbed

Q10 Give two difference between clinical thermometer and laboratory thermometer.

A10 Clinical thermometer:

(a) The temperature range of this thermometer is from 35o C to 43o C.

(b) It is used to measure body temperature.

Laboratory thermometer:

(a) The temperature range of this thermometer is from -10o Celsius to 110o Celsius.

(b) It cannot be used to measure body temperature because it does not have a kink near the bulb to prevent mercury level falling on its own.

Q11 Name a liquid which is a good conductor of heat.

A11 Mercury is a good conductor of heat.

Q12 In what way is conduction different from convection?

A12 In conduction heat is transferred from one particle to another in the direction of lower temperature without the actual movement of the particles of the medium is called conduction where as in convection heat is transferred from one place to the other by the actual movement of the particles of the medium.

Q13 What is convection? Does it need any medium for transfer of heat?

A13 In convection is the process of heat transfer in which heat is transferred from one place to the other by the actual movement of the particles of the
medium. Yes it needs medium for transfer of heat.

Q14 Why do heating coils of a room heater have a shining covered metal surface?

A14 Heating coils of a room heater have a shining covered metal surface so that they absorb very little heat and reflect most of it.

Q15 State two applications of convection currents.

A15 (1) In winter many hotels and houses are heated centrally on the principle of convection currents.

(2) Ventilators in houses are provided at the top keeping in view that the air we breathe out is hotter and lighter, Ventilators provide outlet for the stale and warm air, while fresh air enters the room through doors and windows.

Q16 Why do woolen clothes keep us warmer than cotton clothes?

A16 Wool and air are poor conductors of heat. The air trapped in between the wool fibres prevents the flow of heat from our body to the surroundings. This keeps us warm.

Q17 Give reasons:

1 Handles of cooking utensils are made up of wood or ebonite Handles of cooking utensils are made up of wood or ebonite as these materials are bad conductors of heat which do not get heated up and can be held comfortably.

Q18 What is the temperature of a healthy human body?

A18 The temperature of a healthy human body is 98.6degreeFahrenheit or 37degree Celsius.

Q19 Give two application of Radiation.

A19 (a) Radiators of cars and air conditioners are painted black so as to radiate maximum amount of heat to produce cooling effect.

(b)Dark colored clothes are more suitable in winter because they absorb most of the radiant heat of the sun and keep us warm.

Q20 A clinical thermometer cannot be used to measure temperature of boiling water. why?

A20 A clinical thermometer cannot be used to measure temperature of boiling water because if it is placed in very hot water , the expansion of mercury will break the thermometer

Q21 List two important conditions required for conduction of heat.

A21 (a) Two objects need to be in contact with each other.

(b) The temperature of two objects should be different.

Q22 What are Good and Bad Conductors of heat?

A22 Materials which allow heat to pass through them easily are called good Conductors of heat. For example: silver, copper, iron, aluminiumand mercury. On the other hand materials which do not easily allow heat to pass through them are called bad conductors of heatorinsulators.wood,plastic,mud,cork,cotton,wool,ebonite are bad conductors of heat

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