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NCERT Solutions of Science for Class 7 Lesson – 4 HEAT

TEXTUAL QUESTIONS

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

Ans- Similarities –

(i) The principle of both ‘material expands with heat’ is the same.

(ii) Both are used for measuring the temperature of materials.

(iii) Generally mercury is used in both.

(iv) Both use Celcius scale to measure the temperature.

Differences –

Clinical ThermometerLaboratory Thermometer
1. It is used to measure the temperature of human body.1.It is used to measure the temperature of other objects in laboratory.
2. A kink is present near the bulb to prevent the fall of mercury level.2. A kink is absent in laboratory thermometer.
3. The range of the temperature is from 35°C to 42°C.3.The range of the temperature is from -10°C to 110°C.

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

Ans- Conductors – Iron, Silver, Copper.

Insulators – Glass, plastic, dry wood.

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.

(c) Temperature is measured in degree _____.

(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 _____ absorb heat better than clothes of light colours.

Ans- (a) temperature

(b) clinical

(c) celsius

(d) radiation

(e) conduction

(f) dark

Q.4- Match the following :

AB
(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-

AB
(i) Land breeze blows during(d) Night
(ii) Sea breeze blows during(c) Day
(iii) Dark coloured clothes are preferred during(b) Winter
(iv) Light coloured clothes are preferred during(a) Summer

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

Ans- This is because the air gets trapped in between the layers of clothing. Air is a bad conductor of heat. It does not allow the body heat to escape out. Hence keeps us warm.

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

images 10

Ans-

images 11

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

Ans- This is because the white colour does not absorb heat, instead it reflects back most of the heat. This helps in keeping the house cool.

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

(c) 20°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

(c) flow 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.

(c) becomes cold by the process of radiation.

(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.

(c) copper is better conductor of heat than the stainless steel

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

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

Chapter At A Glance

  • Our sense of touch is not always a reliable guide to the degree of hotness of the object.
  • Temperature is the measure of degree of hotness of an object.
  • Thermometer is a device used for measuring the temperature.
  • Clinical thermometer is used to measure our body temperature. The range of this thermometer is from 35°C to 42°C. For other purposes we use laboratory thermometer. The range of this thermometer is from -10°C to 110°C.
  • The normal temperature of the human body is 37°C.
  • The heat flows from a body of higher temperature to the body of lower temperature. There are three ways in which heat can flow from one object to another. These are conduction, convection and radiation.
  • In solids heat is transferred by conduction, in liquids heat is transferred by convection, no medium is required for the transfer of heat by radiation.
  • Materials which allow the heat to pass through them easily are the conductors of heat.
  • Materials which do not allow the heat to pass through them easily are the insulators.
  • Dark coloured objects absorb more heat than the light coloured objects. That is the reason we feel more comfortable in light coloured clothes during 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.

FAQs related to Lesson – Heat Class 7

Q. Write the precautions to be taken while reading a clinical thermometer.

Ans- The following precautions should be taken while reading a clinical thermometer –
(i) Thermometer should be washed with anticeptic solution before and after use.
(ii) Ensure that before use the mercury level is below 35°C.
(iii) Read the thermometer keeping the level of mercury along the line of sight.
(iv) Don’t hold the thermometer from the bulb.

Q. Write a note on sea breeze and land breeze.

Ans- Sea breeze – During the day, the land gets heated faster than the water in coastal areas. The air above the land gets heated 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. This is called 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 and the warm air from the sea moves towards the land. This is called land breeze.

Q. Define conduction.

Ans- The process by which the heat is transferred from the hotter end to the colder end is called conduction. In solids heat is transferred by the process of conduction.

Q. Define convection.

Ans- The process by which the heat is transferred from one place to another due to the bulk movements of molecules of fluids is called convection. In liquids and gases, the heat is transferred by the process of convection.

Q. Define radiation.

Ans- The transfer of heat from one place to another without any medium is called radiation. The heat of the sun comes to us by the process of radiation.

Q. Why does the temperature of clinical thermometer range between 35°C to 42°C?

Ans- This is because the temperature of human body doest not go below 35°C and above 42°C.