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NCERT Class 7 Science Chapter 4 Heat
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Heat
Chapter: 4
NCERT TEXT BOOK EXERCISES
Q.1. State similarities and differences between the laboratory thermometer and the clinical thermometer.
Ans: Similarities:
(i) Both thermometers consist of long narrow uniform glass tubes.
(ii) Both have a bulb at one end.
(iii) Both contain mercury in bulb.
(iv) Both contain Celsius scale on the glass tube.
Differences:
(i) A clinical thermometer reads tempera- tures 35°C to 42°C while the range of laboratory thermometer is-10°C to 110°C.
(ii) Clinical thermometer has a kink near the bulb while there is no kink in the laboratory thermometer.
2. Give two examples each of conductors and insulators of heat.
Ans: Conductors: aluminum, iron.
Insulators: plastic, wood.
Q.3. Fill in the blanks:
(a) The hotness of an object is determined by its ________.
Ans: Touching.
(b) Temperatures of boiling water cannot be measured by a ________ thermometer.
Ans: Clinical.
(c) Temperature is measured in degree ___________.
Ans: Celsius.
(d) No medium is required for transfer of heat by the process of ________.
Ans: Radiation.
(e) A cold steel spoon is dipped in a cup of hot milk. It transfers heat to its other end by the process of __________.
Ans: Conduction.
(f) Clothes of _________ colors absorb heat better than clothes of light colors.
Ans: Dark.
Q. 4. Match the following:
| (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:
| (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: More layers of clothing keep us warm in winters as they have a lot of space between them. This space gets filled up with air. As air is a bad conductor, it does not allow the body heat to space out.
Q. 6. Look at Fig. 4.13. Mark where the heat is being transferred by conduction, by convection and by radiation.
Ans:
Q. 7. In places of hot climate it is advised that the outer walls of houses be painted white. Explain.
Ans: In places of hot climate it is advised that the outer wall of houses be painted white because white color absorbs very little sun’s heat rays, it reflects most of the sun’s heat rays. This keeps the house cool in the hot days of summer.
Q. 8. One liter of water at 30°C is mixed with one liter 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: (b) More than 50°C but less than 80°C.
Q. 9. An iron ball at 40°C is dropped in a mug containing water at 40°C. The heat will:
(a) Flows from iron ball to water.
(b) Not flows from iron ball to water or from water to iron ball.
(c) Flows from water to iron ball.
(d) Increases the temperature of both.
Ans: (a) Flows from iron ball to water.
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 colorful.
(c) Copper is a better conductor of heat than the stainless steel.
(d) Copper is easier to clean than the stainless steel.
Ans: (c) Copper is a better conductor of heat than the stainless steel.
ADDITIONAL IMPORTANT QUESTIONS
Very Short Answer Questions
Q. 1. Name the liquid which is commonly used in making thermometer.
Ans: Mercury.
Q. 2. What prevents the mercury level in the glass tube of a clinical thermometer from falling on its own when its bulb is removed from the mouth of a patient?
Ans: Kink (or constriction).
Q. 3. What is the name of the thermometer which has temperature range:
(a) from-10°C to 110°C
Ans: Laboratory thermometer.
(b) from 35°C to 42°C
Ans: Clinical thermometer.
Q. 4. What is the name of those thermometers which do not use mercury?
Ans: Digital thermometers.
Q. 5. When one end of an iron rod is heated, its other end also becomes hot soon. Name the process by which heat flows in the iron rod.
Ans: Conduction.
Q.6. Name the process by which a frying pan transfers heat from the gas stove through its bottom to the food kept inside it.
Ans: Conduction.
Q.7. When does the flow of heat from a hot object to a cold object stop?
Ans: When temperature of both the objects becomes the same (or equal).
Q. 8. Name the process by which heat is transferred within a liquid (or gan).
Ans: Convection.
Q.9. Name the process in which heat is transferred by the movement of a liquid (or gus) itself.
Ans: Convection.
Q.10. Name the process in which:
(a) Water in a beaker gets heated by the burner kept below it.
Ans: Convection.
(b) Air above the candle flame gets heated.
Ans: Convection.
Short Answer Questions
Q. 1. The bulb of thermometer A is coated with lamp black and the bulb of thermometer B is coated with silver. Both the thermometers are placed in sun shine for equal time:
(a) Which thermometer will show a higher rise in temperature?
Ans: Thermometer A will show higher temperature.
(b) What conclusion is obtained from the above observation?
Ans: Black objects are better absorbers of heat.
Q. 2. Which would you prefer to keep you warm on a cold winter night, one thick woolen blanket or two thin woolen blankets joined together? Why?
Ans: I would prefer to use two thin woolen blankets joined together to keep warm on a cold winter night. This is because the two thin blankets joined together will have a layer of air trapped in between them. This extra layer of trapped air (being a very poor conductor of heat) will prevent the body heat from going away more efficiently and hence keep me more warm.
Q. 3. We wear dark-coloured clothes in winters while light-coloured clothes in summers. Why?
Ans: Dark coloured clothes are warm in winters as they absorb more heat from the sunlight and keep our body warm. We wear light-coloured clothes during summer because light coloured clothes absorb lesser heat and keep us cool during summer.
Q. 4. Why is the handle of a pressure cooker is covered with thick plastic?
Ans: The handle of a pressure cooker is covered with thick plastic, because plastic is a bad conductor of heat. Hence, the heat from the hot cooker does not flow to the handle, with the result that we can hold easily.
Q.5. Why do we wear woolen clothes in winter?
Ans: Woollen clothes have fine pares filled with air. Wool and air are bad conductors of heat They do not allow heat of our bodies to escape out. Therefore woolen clothes keep the body warm in winter. And for the same reason we wear woolen clothes in winter.
Q. 6. One end of the objects such a steel spoon, a plastic scale, a pencil and a divider is put in a beaker of hot water. In which of these objects the other end will get hot? Why?
Ans: In steel spoon and divider; the other end will get hot because they are made of metals which are good conductors of heat.
Q. 7. Kites and eagles fly without flapping their wings. Why?
Ans: During the day, hot air surrounding the land gradually rises up and a convection air current develops. Kites and eagles move along this rising current without flapping the wings to fly high up in the sky.
Q. 8. Why do the birds puff up their feathers in winter?
Ans: The birds puff up their feathers in winter, because in doing so they trap large amount of air, which in turn acts as an insulator and does not allow their body heat to flow out.
Short Answer Questions
Q.1. What are the precautions needed while reading a laboratory thermometer?
Ans: (i) The thermometer should be kept upright not tilted.
(ii) Bulb should be surrounded from all the substances of which the temperature is to be measured.
(iii) The bulb should not touch the surface of the container.
Q. 2. Why do we use mercury in the thermometer in place of water?
Ans: (i) Mercury expands more than water.
(ii) Mercury do not stick on the walls of the thermometer like water
(iii) Mercury can be easily collected when thermometer breaks.
Q.3. Why are air conditioners fitted at higher level on the wall of the room?
Ans: Cold air is heavier than warm air, the cold air from the cooler/air conditioner being heavier moves downwards, whereas the hot air at the lower level moves upwards and gets cooled and comes down once again. Thus the convection currents are set in the air and room gets cooler faster.
Q.4. On a bright sunny day, a boy standing on a sea shore is flying a kite. In what direction will his kite fly ? Explain your answer.
Ans: When a boy standing on a sea shore is flying a kite on a bright sunny day, the kite will fly above the land mass.
During a sunny day, land mass becomes more heated than the sea because the land absorbs more heat than water. Therefore, the cold air blows from the sea towards the land during the day time. Hence, the kite will fly in the same direction as the air blows.
Q. 5. Why a wire gauge is generally used when a glass vessel is heated over a flame?
Ans: A wire gauge is generally used when a glass vessel is heated over a flame because the wire gauge is made from the metal and the metals are good conductor of heat. So, the heat from the flame is distributed uniformly over the wire gauge and the glass vessel is uniformly heated. This saves it from cracking.
Long Answer Questions
Q. 1. Explain conduction, as a mode of heat transfer.
Ans: When one end of a metallic strip or rod is heated, the particles at that end absorb energy and start vibrating more rapidly. These rapidly vibrating particles collide with their neighbours, and transfer a part of heat energy to them. As a result, these particles also start vibrating more rapidly and in turn cause their neighbour to vibrate more rapidly. This process continues until the last particle also starts vibrating more rapidly. Thus, heat energy is transferred from particle to particle through the whole length of the strip (or rod).
Q. 2. Explain briefly how oceanic currents take place.
Ans: The ocean water near the equator is heated by the sun to a much higher temperature than the water near the poles of the earth. This is because the sun rays fall almost straight in the equatorial region but slantingly in the polar region. The ocean water in the equatorial region expands and becomes lighter, but the water in the polar region remains cold and heavy. Therefore, convection currents of warm water flow on the surface of ocean from equator towards the poles. Below the surface of the ocean, currents of cold water flow from the poles towards the equator. These currents control the temperature of the ocean and are called the oceanic currents.
HOTS QUESTIONS
Q. 1. A person has a white shirt and a black shirt. Which shirt will make him more comfortable in
(a) winter. and
(b) summer?
Give reasons for your answer
Ans: (a) The black shirt will make the person more comfortable in cold winter season. This is because the black coloured shirt will absorb more heat rays from the sun and keep him warm and comfortable in cold winter season.
(b) The white shirt will make the person more comfortable in the hot summer season. This is because the white coloured shirt will reflect most of the sun’s heat rays and keep him cool and comfortable in hot summer season.
Q. 2. Name a liquid suitable for the use in thermometer designed to measure a temperature between-10°C and 250°C. Give a reason.
Ans: Mercury is suitable for the use in thermometer designed to measure a temperature between -10°C and 250°C because its freezing point is-39°C and boiling point is 357°C.
SKILL BASED QUESTIONS
Q. 1. Observe the following figure and tell what does this figure show?
Ans: (i) In the figure, we see that a girl is reading the temperature in thermometer.
(ii) This is a correct method of reading a clinical thermometer.
Q. 2. Observe the following figure and answer the questions:
(a) What do these two figures show?
Ans: (a) These two figures show the sea breeze and land breeze.
(b) What is the indication of these two figures?
Ans: (b) During the day wind flows from the sea, hence called sea breeze. During night time wind flows from land to sea, hence called land breeze.
FORMATIVE ASSESSMENT
1. Fill in the following blanks with suitable words:
(i) A cold steel spoon is dipped in a cup of hot milk. It transfers heat from its other end by the process of _________.
Ans: Conduction.
(ii) In solids, heat is transferred by ______.
Ans: Conduction.
(iii) The two examples of insulators of heat are ________ and _________.
Ans: Plastic, wood.
(iv) Air is ________ conductor of heat whereas copper is a ________ conductor of heat.
Ans: Poor, good.
(v) Feathers and furs keep birds and animals warm during cold weather because they have _________ trapped in them.
Ans: Air.
(vi) The hotness of an object is determined by its _________.
Ans: Temperature.
(vii) Temperature is measured in degree __________.
Ans: Celsius.
2. State whether the following statements are true or false:
(i) The device used for measuring temperature is called barometer.
Ans: False.
(ii) The transfer of heat by convection can take place in solids and liquids.
Ans: False.
(iii) A laboratory thermometer can be used to measure the body temperature of humans.
Ans: False.
(iv) A clinical thermometer can be used for measuring temperature of the boiling water.
Ans: False.
(v) Solids are heated mainly by conduction.
Ans: True.
(vi) Water is a good conductor of heat.
Ans: False.
3. Match the items of column A with those given in column B.
| Columu – A | Column – B |
| 1. Cold objects | (a) Measures body temperature. |
| 2. Hot objects | (b) Measures the temperature of inanimate objects |
| 3. Clinical thermometer | (c) Substitute for clinical thermometer. |
| 4. Laboratory thermometer | (d) High temperature. |
| 5. Digital thermometer | (e) Low temperature. |
Ans:
| Columu A | Column B |
| 1. Cold objects | (e) Low temperature. |
| 2. Hot objects | (d) High temperature. |
| 3. Clinical thermometer | (a) Measures body temperature. |
| 4. Laboratory thermometer | (b) Measures the temperature of inanimate objects |
| 5. Digital thermometer | (c) Substitute for clinical thermometer. |
4. Multiple Choice Questions:
(i) The degree of hotness or coldness of a body is called:
(a) heat
(b) light
(c) temperature
(d) energy
Ans: (c) Temperature.
(ii) Which of these is a conductor of heat?
(a) wool
(b) paper
(c) copper
(d) wood
Ans: (c) Copper.
(iii) Which of these is an insulator?
(a) copper
(b) wood
(c) iron
(d) silver
Ans: (b) Wood.
(iv) Heat from the sun reaches us due to
(a) conduction
(b) convection
(c) radiation
(d) vibration
Ans: (c) Radiation.
(v) Conduction, convection and radiation are:
(a) processes through which heat transfers
(b) scales of temperature
(c) different forms of energy
(d) types of thermometer
Ans: (a) Processes through which heat transfers.
(vi) The normal temperature of human body is:
(a) 37 K
(b) 37°C
(c) 37°F
(d) All of the above
Ans: (b) 37°C.
(vii) Solids transfer heat by:
(a) conduction
(b) convection
(c) radiation
(d) all of the above
Ans: (a) Conduction.
(viii) Ventilators in rooms is due to:
(a) conduction
(b) convection
(c) radiation
(d) all of the above
Ans: (b) Convection.
5. Write one word for the following:
(i) The form of energy that causes sensation of hotness and coldness.
Ans: Heat.
(ii) A device used for measuring temperature.
Ans: Thermometer.
(iii) Temperature scale having one hundred
divisions between lower fixed point and upper fixed point.
Ans: Celsius Beale.
(iv) The mode of heat transfer from a distance source.
Ans: Radiation.
(v) The flow of ocean water due to difference in temperature.
Ans: Ocean currents.
(vi) The degree of hotness or coldness of a body.
Ans: Temperature.
(vii) The substances which can conduct heat easily.
Ans: Good conductors.
(viii) The substances which do not conduct heat readily.
Ans: Insulators.
