Chapter 4 Science CBSE 7th

Science

Class 7

Chapter 4

Heat

Important topics covered in NCERT for Class 7 Science Chapter 4 – Heat

  • Hot and cold
  • Temperature measurement
  • Thermometers
  • Heat transfer
  • Clothes in different climatic conditions

Important Terms

  • Celsius scale : The scale in which the temperature is represented in °C is called celsius scale.
  • Conduction : The process by which heat is transferred from the hotter end to the colder end of an object is known as conduction.
  • Conductor : Those materials which allow heat to pass through them easily are called the conductors of heat.
  • Convection : The process of heat-transfer in a liquid by the movement of liquid is called convection.
  • Insulator : Those substances which do not conduct heat well are called insulators.
  • Land Breeze : After sunset, the air above the sea is warmer which makes the air rise and the cooler air above the land, moves out towards the sea. This is called ‘Land Breeze’.
  • Radiation : Radiation is the process by which heat travels without the help of a material medium.
  • Sea Breeze : During the day the air in contact with the land becomes hot and rises. Then the cooler air above the sea rushes in towards the land to occupy its space. This set-up is known as sea breeze.
  • Temperature : The temperature of a substance is the measure of degree of hotness of the substance.
  • Thermometer : The instrument which is used to measure the temperature is called thermometer.
  • Degree Celsius: Unit of measurement of temperature.
  • Clinical Thermometer: A device to measure the temperature of human body.
  • Laboratory Thermometer: Used to measure the temperature in laboratory.
  • Maximum-Minimum Thermometer: Use to measure the temperature of climate.

Revision Notes

  • A reliable measure of the hotness of an object is its temperature.
  • Temperature is measured by a device called thermometer.
  • Clinical thermometer is used for measuring our body temperature.
  • A clinical thermometer reads our temperature from 35°C to 42°C.
  • The normal temperature of a human body is 37°C.
  • In order to measure the temperature of other objects, we make use of Laboratory thermometer.
  • The range of laboratory thermometer is from– 10°C to 110°C.
  • The heat flows from a body at a higher temperature to a body at a lower temperature. Heat can flow from one object to another in three different ways, namely, conduction, convection and radiation.
  • In solids, generally, the heat is transferred by conduction. In liquids and gases, the heat is transferred by convection. No medium is required for the transfer of heat by radiation.
  • Heat always flows from hotter objects to colder objects.
  • The materials which allow heat to easily pass through them are conductors of heat.
  • The materials which do not allow heat to pass through them easily are called the insulators of heat.
  • When heat falls on some object, then some part of the heat gets reflected, some portion gets absorbed and some part gets transmitted.
  • Dark coloured objects absorb radiation better than the light coloured objects. That is the reason, we feel more comfortable in light coloured clothes during the summer.
  • Woollen clothes keep us warm during winter. It is because wool is a poor conductor of heat and it has air trapped in between the fibres.

NCERT Solution

Question 1: State similarities and differences between the laboratory thermometer and the clinical thermometer.

Similarities:

  1. Both are made of glass and consist of long narrow glass tube.
  2. At one end both of them have a bulb.
  3. Bulbs of both the thermometers consist of mercury
  4. Celsius scale is present in both the thermometer

Differences:

Clinical ThermometerLaboratory thermometer
(1) Temperature range if 35 to 42 °C(1) Temperature range is -10 to 110 °C
(2) Used to measure human body temperature(2) Used to measure temperature in the laboratory
(3) It has kink which prevents immediate backflow of mercury(3) It does not have a kink

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

Conductors: Iron and Copper
Insulators: Plastic and wood

Question 3: Fill in the blanks :
(a) The hotness of an object is determined by its __________.

The hotness of an object is determined by its temperature.

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

Temperature of boiling water cannot be measured by a clinical thermometer.

(c) Temperature is measured in degree ______________.

Temperature is measured in degree Celcius.

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

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

(e) A cold steel spoon is dipped in a cup of hot milk. Heat is transferred to its other end by the process of ______________.

A cold steel spoon is dipped in a cup of hot milk. Heat is transferred to its other end by the process of conduction.

(f) Clothes of ______________ colours absorb more heat better than clothes of light colours.

Clothes of dark colours absorb more heat better than clothes of light colours.

Question 4: Match the following :

Column IColumn II
(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
Column IColumn II
(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

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

More layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing because air gets trapped in-between layer. As air is a bad conductor of heat it does not allow the escape of the heat from the body.

Question 6: Look at Fig. 4.13. Mark where the heat is being transferred by conduction, by convection and by radiation.ncert-solutions-for-class-7-science-04-04

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

In places of hot climate, it is advised that the outer walls of houses be painted white because white colour reflects back most of the heat that is incident on it as white is a poor absorber of heat. This helps in keeping the house cool.

Question.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

(d) between 30°C and 50°C
The temperature of the mixture will be between 30°C and 50°C because hot water loses the heat and simultaneously cold water gains heat this keeps the temperature in between 30°C and 50°C.

Question.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.

Answer is (b) not flow from iron ball to water or from water to iron ball because both of them have the same temperature of 40°C.

Question.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.

The answer is (d) does not become cold because wood is a bad conductor of heat.

Question.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 a better conductor of heat than stainless steel.
(d) copper is easier to clean than stainless steel.


The answer is (c) copper is a better conductor of heat than the stainless steel.

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