Heat Class 7 Notes NCERT and MCQs

24-08-2023 14:50 PM IST Priyanka Chaudhary

The chapter describes the concept of temperature and its measurement through thermometer. It also describes different ways of heat transfer and terms such as sea breeze and land breeze.

What is Temperature?

Temperature is a reliable measure of the hotness of an object. We cannot reliably measure degree of hotness of an object through our sense of touch. Temperature is a measure of the degree of hotness of an object.

Measurement of temperature

We measure temperature with a thermometer.

A clinical thermometer is a thermometer that measures our body temperature. It consists of a long, narrow, uniform glass tube with a bulb at one end. The bulb contains mercury.

The use of mercury in thermometers is a matter of concern. Mercury is a toxic substance and is very difficult to dispose of if a thermometer breaks.

These days, digital thermometers are available which do not use mercury.

The temperature range of clinical thermometer is from 35°C to 42°C because the temperature of human body normally does not go below 35^oC or above 42^oC.

The clinical thermometer is designed to measure the temperature of human body only.  The normal temperature of the human body is 37°C.

The range of a laboratory thermometer is generally from –10°C to 110°C.

Transfer of heat

The heat flows from a hotter object to a colder object. Heat can flow from one object to another through three ways. They are conduction, convection and radiation.

• Conduction
  • Conduction is the process by which heat is transferred from the hotter end to the colder end of an object.
  • It generally transfers heat in solids. 
  • The materials which allow heat to pass through them easily are conductors of heat. For example, aluminum, iron and copper.
  • The materials which do not allow heat to pass through them easily are poor conductors of heat such as plastic and wood.
  • Poor conductors are known as insulators. The water and air are poor conductors of heat.
• Convection: Convection is the second method of transfer of heat. In liquids (water) and gases the heat is transferred by convection.
• Radiation: Radiation is the process through which heat comes to us from the Sun. It does not require any medium. Our body also gives heat to the surroundings and receives heat from it by radiation. All hot bodies radiate heat.

Sea Breeze and Land Breeze

Sea breeze: During the day, the land gets heated faster than the water and 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.

Land breeze: At night, 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.

Clothes in summer and winter

Dark-coloured objects absorb more heat than the light-coloured objects. That is the reason we feel more comfortable in light-coloured clothes in the summer.

Woollen clothes keep us warm during winter. It is so because wool is a poor conductor of heat and it has air trapped in between the fibres.

MCQs based on NCERT Class 7 Science Chapter 4: Heat

1. What is the temperature range of clinical thermometer?

a. 35°C to 42°C

b. –10°C to 110°C

c. –20^oC to 75^oC

d. 30°C to 45°C

Ans. a

Explanation: Clinical thermometer is the thermometer that measures our body temperature. It consists of a long, narrow, uniform glass tube with a bulb at one end. The bulb contains mercury. The temperature range of clinical thermometer is from 35°C to 42°C because the temperature of human body normally does not go below 35^oC or above 42^oC. The normal temperature of the human body is 37°C. The range of a laboratory thermometer is generally from –10°C to 110°C.

2. Which of the following process of heat transfer do not/ does not require any medium?

a. Conduction

b. Convection

c. Radiation

d. b and c

Ans. c

Explanation: Conduction is the process by which heat is transferred from the hotter end to the colder end of an object. It generally transfers heat in solids. In liquids (water) and gases, the heat is transferred by convection. Radiation is the process through which heat comes to us from the Sun. It does not require any medium. 

3. 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

c. 20°C 

d. between 30°C and 50°C

Ans. d

Explanation: The temperature will be between 30°C and 50°C as hot water will lose heat and cold water will gain it. 

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

a. flow from the iron ball to water.

b. not flow from the iron ball to water or from water to the iron ball.

c. flow from water to the iron ball.

d. increase the temperature of both.

Ans. b

Explanation: The heat flows from a hotter object to a colder object. But in this case, the temperature of both of them is the same, therefore heat will not flow. 

5. 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.

c. becomes cold by the process of radiation.

d. does not become cold.

Ans. d

Explanation: The wood is a bad conductor of heat, therefore, the other end of a wooden spoon dipped in a cup of ice cream does not become cold. 

6. 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.

c. copper is a better conductor of heat than stainless steel.

d. copper is easier to clean than stainless steel.

Ans. c

Explanation: Stainless steel pans are usually provided with copper bottoms because copper is a better conductor of heat than stainless steel.