Heat
Science Class Seven
ncert exercise solution: heat
Question (1) State similarities and differences between the laboratory thermometer and the clinical thermometer.
Answer
Similarities between laboratory and clinical thermometer
(1) Laboratory and clinical both thermometers are made of glass tube.
(2) In both of the thermometers generally mercury is used to measure the temperature. (Although in modern thermometer mercury is not used)
(3) Both of the thermometers are calibrated with degree Celsius or degree Fahrenheit.
(4) Both of the thermometers need to put in contact with body of which temperature is to be measured.
Differences between laboratory and clinical thermometer
(1) Laboratory thermometer is used in laboratories to measure the temperature of liquids, solutions, etc.
While clinical thermometer is used to take body temperature.
(2) In laboratory thermometer no kink is present which can prevent the level of mercury to come down on its own.
In clinical thermometer a kink is present which prevents the level of mercury to come down on its own.
(3) In laboratory thermometer mercury level comes down automatically after removing from hot solution or liquid.
In clinical thermometer some jerks are needed to be given to bring down the level of mercury.
(4) The laboratory thermometer is generally marked from –10oC to 110oC
While a clinical thermometer generally marked from 35oC to 42oC or 94oF to 108oF.
Question (2) Give two examples each of conductors and insulators of heat.
Answer
Two examples of conductors: iron and copper.
Two examples of insulators: plastic and wood
Explanation
All metals are conductor or good conductor of heat and electricity. And all non-metals are bad conductors i.e. insulator of heat and electricity.
Question (3) Fill in the blanks:
(a) The hotness of an object is determined by its _____.
Answer
The hotness of an object is determined by its temperature.
(b) Temperature of boiling water cannot be measured by a _______ thermometer.
Answer
Temperature of boiling water cannot be measured by a clinical thermometer.
Explanation
The temperature of boiling water cannot be measured by a clinical thermometer because of the following:
(i) Water boils at 100oC. And clinical thermometer is graduated up to 42oC only. This means using a clinical thermometer a temperature maximum up to 42oC can be measured only.
(ii) If a clinical thermometer would be put in the boiling water, it may be broken.
(c) Temperature is measured in degree _____.
Answer
Temperature is measured in degree Celsius.
Or, Temperature is measured in degree Fahrenheit.
Or, Temperature is measured in degree Kelvin.
(d) No medium is required for transfer of heat by the process of _____.
Answer
No medium is required for transfer of heat by the process of radiation.
Explanation
Radiation is a process by which heat is transferred without any medium. The heat from sun reaches to us without having any medium present in the space. The heat from sun reaches to us by the process of radiation.
A hot body transmits heat by the process of 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 _____.
Answer
A cold steel spoon is dipped in a cup of hot milk. It transfers heat to its other end by the process of conduction.
(f) Clothes of ______ colours absorb heat better than clothes of light colours.
Answer
Clothes of dark colours absorb heat better than clothes of light colours.
Question (4) Match the following:
| Question Table | ||
|---|---|---|
| Column A | Column B | |
| (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 |
Answer
| Answer Table | ||
|---|---|---|
| Column A | Column B | |
| (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 winher keeps us warmer than wearing just one thick piece of clothing.
Answer
More layers of clothes have more space to trap air within them. The air trapped in the space between layers of clothes gives extra insulation. And keep a person more warm who wears more layers of cloths. While by wearing only one thick piece of clothes do not provide that very extra layer of insulation.
So, wearing more layer of clothing during winter keeps us more warm compare to wearing just one thick piece of clothing.
Question (6) Look at figure. Mark where the heat is being transferred by conduction, by convection and by radiation.
Answer
Question (7) In places of hot climate it is advised that the outer walls of houses be painted white. Explain.
Answer
Objects having light colour reflect most of the heat while dark coloured objects absorbs most of the heat falls on them.
Thus, white coloured wall will reflect most of the heat of sunlight from it. Because of absorption of less heat, it will keep inner temperature lower compare to walls painted with dark colour.
Because of this it is advised that outer walls of houses be painted white in places of hot climate.
Question (8) One litre of water at 30oC is mixed with one litre of water at 50o. The temperature of the mixture will be
(a) 80oC
(b) more than 50oC but less than 80oC
(c) 20oC
(d) between 30oC and 50oC
Answer : (d) between 30oC and 50oC
Explanation
By mixing of same volume of 30oC and 50oC of water together, some of the temperature of 50oC water will be used to make the same volume of water having 30oC equal. Because heat always transfer from hotter object to colder object. This will decrease the temperature of whole water from 50oC but will keep the temperature above the 30oC.
So, the temperature of mixture of water will be between 50oC and 30oC. This means temperature of mixture will be more than 30oC but lower than 50oC.
Question (9) An iron ball at 40oC is dropped in a mug containing water at 40oC. 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 : (b) not flow from iron ball to water or from water to iron ball.
Explanation
Heat is transferred from hotter body to colder body. Since the temperature of iron ball and water is equal. So, neither heat will flow from iron ball to water nor from water to iron ball.
Question (10) A wooden spoon is dipped in a cup of ice cream. Is 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.
Answer : (d) does not become cold.
Explanation
Since wood is the bad conductor of heat i.e. wood is an insulator. And an insulator does not allow heat to transfer through it. So the wooden spoon is when dipped in a cup of ice cream. It does not become cold.
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 appeared colourful.
(c) Copper is better conductor of heat than the stainless steel.
(d) Copper is easier to clean than the stainless steel.
Answer : (c) Copper is better conductor of heat than the stainless steel.
Explanation
Gold, Copper, and silver are better conductor of heat than that of steel or stainless steel. Because of this stainless steel pans are usually provided with copper bottoms. So, that it can observe more heat.
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