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Q1. Which of the following is not a reason for using mercury as a liquid for thermometer?

• 1) Mercury does not stick to glass
• 2) Mercury is a metal
• 3) Mercury is very sensitive to expansion
• 4) Mercury is a good conductor of heat

Solution

'Mercury is a metal' is not an enough reason for it to be used as a liquid for thermometer.

Q2. (i) How does the convection current form?(ii) A hot utensil kept away from the flame cools down. How?

Solution

(i) When hot particles in a medium rise up and cold particles move down to take their place, convection currents are formed.(ii) A hot utensil kept away from the flame cools down as it transfers heat to the surroundings by radiation.

Q3. Which material is the best absorber of infra-red radiation?

• 1) Shiny metal
• 2) White paper
• 3) Dark animal fur
• 4) Window glass

Solution

Dark animal fur is the best absorber of infra-red radiation because black surfaces are both good absorbers and good emitters of infra-red radiation.

Q4. Why the feathers of birds keep them warm in cold weather? Look at the figure given alongside. Mark where the heat is being transferred by conduction, by convection, and by radiation.

Solution

The feathers of birds keep them warm in cold weather due to air trapped in them. Air being a very poor conductor of heat, prevents the body heat of birds from escaping to the cold surroundings and hence keeps the bird warm.

Q5. Convection _______.    

• 1) takes place best in solids
• 2) takes place best in vacuum
• 3) takes place best in liquids
• 4) takes place best in gases

Solution

Convection takes place best in gases.

Q6. When an unlit matchstick is held near to an extremely hot Bunsen burner, the matchstick does not get hot enough to light up. However, if the air gap were to be replaced by a piece of copper rod, the matchstick lights up easily. Which of the following could explain the above observation?  

• 1) Air is a bad conductor of heat while copper is a good conductor of heat.
• 2) The flame does not radiate any heat sideways when air is between the flame and the match
• 3) The match head reflects heat by radiation.
• 4) Air is a bad radiator of heat while copper is a good radiator of heat.

Solution

Air is a bad conductor of heat and copper being a metal is a good conductor of heat. So, when the air gap is replaced by a piece of copper rod, the match lights up easily.

Q7. (i) One end of a wooden spoon is dipped in a cup of ice cream. What change will be observed at its other end?(ii) Why do we use handle of wood in frying pan?

Solution

(i) When one end of a wooden spoon is dipped in a cup of ice cream, there will be no change in the temperature of its other end because wood is a bad conductor of heat so there will be no flow of heat from one end to other.(ii) Material of pan is a good conductor of heat. So, a handle of wood (which is a bad conductor of heat) is used in a frying pan so that it does not get heated by conduction.

Q8. Which of the following is not a possible reason for not using water as a thermometric liquid?

• 1) Water is transparent.
• 2) Water is non-volatile.
• 3) Water sticks to glass.
• 4) Water has a high specific heat capacity.

Solution

Non-volatile liquids are preferred as thermometric liquids, so this cannot be a possible reason.

Q9. (i) What is difference between conduction and convection?(ii) It is hotter at the same distance over the top of a fire than it is on the side of it. Why?

Solution

(i) In conduction, matter is not transported with heat; while in convection, matter is transported with the heat.(ii) Convection takes more heat upwards. Towards the top, the air gets heated by convection. Therefore, the hand above the flame feels hot. On the sides, however, there is no convection and air does not feel as hot as at the top.

Q10. Which of the following is not associated with the phenomenon of convection?

• 1) Heating a glass rod
• 2) Air conditioner
• 3) Candle flame
• 4) Sea breeze

Solution

Heating a glass rod involves the phenomenon of conduction.

Q11. Often before sunrise on a clear, calm, cold morning, ice (frost) can be seen on the top of parked cars, even when the air temperature is above freezing. Why does this happen?

Solution

Often before sunrise on a clear, calm, cold morning, ice (frost) can be seen on the top of parked cars, even when the air temperature is above freezing. This happens because the tops the tops of the cars are cooled by radiation.

Q12. The objects made of steel, mica, wood and brass are lying in the same room during winter season. The objects which will feel warmer to touch are:

• 1) steel and brass objects
• 2) mica and brass objects
• 3) mica and wooden objects
• 4) wooden and steel objects

Solution

Wood and mica will be warmer to the touch as these materials are bad conductors of heat. 

Q13. Convection takes place in solids, liquids and gases.

• 1) False
• 2) True

Solution

Convection takes place only in liquids and gases.

Q14. When a pan is removed from the fire, it slowly cools down. Why does it cool down?

Solution

When the pan is removed from the fire, it slowly cools down. The heat is transferred from the pan to the surroundings since heat flows from a hotter object to a colder object.

Q15. How does heat transfer occur in solids? Explain.

Solution

In solids, the heat is transferred by the process of conduction. Heat is transferred from hotter end to the colder end of the object. In this process, molecules of the substance do not move but only heat energy is transferred.

Q16. What is a clinical thermometer? What is the range of a clinical thermometer? Explain why, a clinical thermometer cannot be used to measure high temperatures.

Solution

The thermometer used for measuring the temperature of human body is called clinical thermometer. The range of a clinical thermometer is from 35°C to 42°C. A clinical thermometer cannot be used to measure high temperatures because it has been designed to measure only human body temperature which varies over a short range. If it is used to measure high temperatures, it will get damaged.

Q17. A process of heat transfer which does not need a medium is called:

• 1) Conduction
• 2) Induction
• 3) Convection
• 4) Radiation

Solution

A process of heat transfer which does not need a medium is called radiation.

Q18. In boiling of water in a steel utensil by using a stove, describe the process of heat transfer.

Solution

At first, heat is transferred from the stove to utensil by conduction .Then heat is given to water by conduction through utensil and then in water, heat flows by convection.

Q19. Explain how heat from the sun reaches the earth.

Solution

The sun is an extremely hot object. It emits heat radiations in all directions. These radiations travel through vacuum between the sun and the earth at a very high speed and reach the earth. When the sun's radiations fall on the earth and its objects, they receive heat energy and hence get heated. Thus, the sun's heat reaches the earth by the process of radiation.

Q20. There is a lot of concern over the use of mercury in thermometers. Elaborate.

Solution

There is a lot of concern over the use of mercury in thermometers since mercury is a toxic substance and is very difficult to dispose off if a thermometer breaks. Hence, these days, digital thermometers are available which do not use mercury.

Q21. One litre of water at 30^oC is mixed with water at 50^oC, then the mixture will have temperature (a) 80^oC (b) 20^oC (c) More than 80^oC (d) In between 30^oC and 50^oC

Solution

The mixture will have temperature in between 30^oC and 50^oC since the hot water (at 50^oC) will try to lose its heat to the cold water (at 30^oC).

Q22. If atmosphere of earth is not present then what will happen to the temperature of earth?

Solution

If atmosphere of earth is not present, then the earth will become extremely cold since there will be no absorption of heat radiations to keep the temperature of earth balanced.

Q23. Why do we use mercury in the clinical thermometer?

Solution

Mercury is the only metal that exists in liquid state at room temperature and it does not stick to the walls of the container and also it has high coefficient of expansion, making it ideal for use in clinical thermometer.

Q24. How does heat and temperature differ from each other?

Solution

Heat is the form of energy which can cause hotness to a body while temperature is the degree of hotness or coldness of a body.

Q25. What happens when heat falls on an object?

Solution

When heat falls on some object, a part of it is reflected, a part is absorbed and a part may be transmitted. The temperature of the object increases due to the absorbed part of the heat.

Q26. (i) Why do freezers in freeze are always at top?(ii) Why does hot air rise and cold air sink?

Solution

(i) Freezers are always located at the top of the refrigerators so as to easily circulate the cold air downwards and keep the refrigerator cold.(ii) Cool air is more dense than warm air, so the cool air falls through the warm air. Warm air being light rises up.

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