Class 7 Science Heat Chapter Notes

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Heat

1.1 Introduction

We wear woollen clothes during winters when it is cold outside. Woollen clothes keep us warm. We prefer to wear light coloured cotton clothes when it is hot. These give us a feeling of coolness. There are various objects in our surroundings which may be classified into hot or cold objects depending upon their temperatures. For example tea is hot whereas ice is cold.

1.2 Measuring Temperature

The degree of hotness of an object is measured by its temperature. Thermometer is a device which measures the temperature of an object. The thermometer that measures our body temperature is called a clinical thermometer. A clinical thermometer looks like the one shown in figure 1.1.

A clinical thermometer consists of a long, narrow, uniform glass tube. It has a bulb at one end. This bulb contains mercury. Outside the bulb, a small shining thread of mercury can be seen. Scale of thermometer are broadly of 3 types :

(i) Celsius Scale (C) : The lower limit for a celsius scale is 0° C while the upper limit is considered to be 100°C.
(ii) Fahrenheit Scale (F) : The lower limit for a Fahrenheit Scale is 32°F while the upper limit is 212°C.
(iii) Kelvin Scale (K) : The lower limit for a Kelvin Scale is 273.15° K while the upper limit is 373.15° K.

Relation among different scales :

Note :
(i) Lower limit and upper limit are two fixed points of any temperature measuring scale.
(ii) A clinical thermometer reads temperature from 35°C to 42°C.

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 process.

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 process.

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 materials. 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  →^heat gas (vaporization)

Solid  →^heat  liquid (fusion or melting)

Solid →^heat 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.

Objective Questions

1. The celsius scale reading is 40°C. The Fahrenheit scale reading will be

(a) 100°F

(b) 90°F

(b) 104°F

(d) 50°F

2. No medium is required for transfer of heat by the process of

(a) Conduction

(b) Convection

(c) Radiation

(d) Relegation

3. Which of the following is good conductor of heat?

(a) Wood

(b) Iron

(c) Plastic

(d) Paper

4. When a body is heated. Its

(a) length increases

(b) breadth increases

(c) volume increases

(d) All of the above

5. The range of clinical thermometer is

(a) from 30°C to 40°C

(b) From 35°C to 42°C

(c) from 20°C to 25°C

(d) from 25°C to 30°C

6. The range of Laboratory thermometer is

(a) from –10°C to 110°C

(b) from 10°C to 100°C

(c) from –10°C to 100°C

(d) from 0°C to 100°C

7. One litre of water at 25°C is mixed with one litre of water at 40°C. The temperature of the mixture will be

(a) 25°C

(b) 40°C

(c) more than 40°C but less than 60°C

(d) between 25°C and 40°C

8. If the refrigerator is kept open in a room, the temperature of the room will

(a) decrease

(b) increase

(c) remains unchange

(d) either (a) or (c)

9. A metal sheet with a circular hole is heated. The hole

(a) gets larger

(b) gets smaller

(c) remains of the same size

(d) gets deformed

10. An iron ball at 50°C is dropped in a mug containing water at 40°C. The heat will

(a) flow from iron ball to water

(b) flow from water to iron ball

(c) not flow from iron ball to water or from water to iron ball

(d) increases the temperature of both

Subjective Questions

1. Define temperature. Name a device used to measure temperature.

2. Explain the construction of a clinical thermometer with a neat diagram.

3. Convert the following temperature measurement in Fahrenheit scale?

(a) 30°C

(b) –40°

(c) 40°C

(d) 100°C

Heat Class VII-Physics

4. Convert the following temperature measurement in celsius scale?

(a) 98°C

(b) 36°F

(c) 42°F

5. Explain land and sea breeze with a neat diagram.

6. Define conduction. Explain an activity where heat transfer takes place due to conduction.

7. Define convection. Explain an activity where heat transfer takes place due to convection.

8. Define radiation. Explain an activity where heat transfer takes place due to radiation.

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

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

11. If the refrigerator is kept open in a room, what will happen ? Explai

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