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Q1. What is the other name of poor conductors of heat?

Solution

Poor conductors of heat are also called insulators.

Q2. Why room heaters should be placed at the floor of the room?

Solution

Room heaters are placed at the floor of the room so that the air heated by it rises up and sets up the convection current in the room quickly and uniformly.

Q3. (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.

Q4. If a 1cm³ iron cube is heated to 700K, what change would you expect in the size of the cube?

Solution

Objects expand on heating, so size of cube will be more than 1cm³.

Q5. Which of the following properties is not a characteristic of mercury? It sticks to the sides of a glass tube. Its freezing point is about 39^oC. Its boiling point is about 357^oC. It is a good conductor of heat.

Solution

It sticks to the sides of a glass tube.

Q6. 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).

Q7. 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.

Q8. A mercury thermometer is suitable for directly measuring the flame temperature of a candle. True or false.

Solution

False, because the temperature of flame is around 700^oC, while boiling temperature of mercury is 357°C.

Q9. 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.

Q10. What is the common unit of measurement of temperature?

Solution

The common unit of measurement of temperature is degree Celsius.

Q11. Why does the mercury not fall or rise in a clinical thermometer when taken out of the mouth?

Solution

The mercury does not fall or rise in a clinical thermometer when taken out of the mouth because of the presence of kink. It prevents mercury level from falling on its own.

Q12. An iron ball at 40°C is transferred to a mug containing water at a temperature of 40°C. In which direction will the heat flow?

Solution

Heat will not flow because both, the iron ball and the water, are at the same temperature.

Q13. 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.

Q14. (i) An iron ball at 40^oC is dropped in a mug containing water at 40^oC. What will be the passage of flow of heat in the system. Explain.(ii) How the heat flows in a body having one end cool and other at high temperature?

Solution

(i) There will be no flow of heat either from iron ball to water or from water to iron ball since both water and iron ball are at same temperatures so heat transfer will not take place.(ii) Heat flows from higher temperature to lower temperature. So heat will go from hotter end to colder end.

Q15. 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.

Q16. 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.

Q17. 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.

Q18. Stainless steel pans are usually provided with copper bottoms. Why?

Solution

Stainless steel pans are usually provided with copper bottoms because copper is a better conductor of heat than stainless steel hence cooking of food becomes faster.

Q19. Write three precautions in using a laboratory thermometer.

Solution

The precautions needed while reading a laboratory thermometer are: 1. It should be kept upright not tilted. 2. Bulb should be surrounded from all sides by the substance of which the temperature is to be measured. 3. The bulb should not touch the surface of the container.

Q20. What is radiation?

Solution

Transfer of heat from a hot body to cold body by means of heat rays, without any material medium between them, is called radiation.

Q21. (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.

Q22. How does a mud house remain cool in summers and warm in winters?

Solution

Mud is a bad conductor of heat. It does not allow heat to go out of the house in winters or enter the house in summers. Therefore, mud houses remain warm in winters and cool in summers.

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. A tile floor feels colder than a wooden floor when you walk on it, even when both are at same temperature. Why?

Solution

All materials conduct heat to different extents. Some are good conductors of heat and some are bad conductors of heat. In this case, tile floor is a good conductor of heat. Hence, it feels colder compared to wooden floor.

Q25. (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.

Q26. Why should air coolers be kept higher up in the room, and not down like the heaters?

Solution

Cool air is denser so it moves down whereas warm air is lighter so rises up hence convection currents are formed that helps in circulating the cold air and keeps the room cool. If the air cooler is not kept higher up, then the cool air will remain at the surface layer and warm air will remain at the top, hence no circulation of air will take place and room will remain warm.

Q27. How does the heat flow in a body having one end cool and other end at high temperature?

Solution

Heat flows from higher temperature to lower temperature. So heat will go from hotter end to colder end.

Q28. Can a clinical thermometer be used to measure the temperature of boiling milk?

Solution

No, because the range of clinical thermometer is 35^o C to 42^oC, while the boiling temperature is far more than 42^o C.

Q29. Heat cannot cause the following(a) increase in state of an object.(b) chemical change in an object.(c) motion in an object.(d) expansion and contraction of an object.

Solution

Heat cannot cause motion in an object.

Q30. Classify the following as bad or good conductor of heat: Iron, air, aluminium, water

Solution

Air and water are bad conductors of heat. Iron and aluminium are good conductors of heat.

Q31. (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.

Q32. A hot utensil kept away from the flame cools down. How?

Solution

A hot utensil kept away from the flame cools down as it transfers heat to the surroundings by radiation.

Q33. What is the usual temperature range of a laboratory thermometer?

Solution

The temperature range of a laboratory thermometer is generally from -10^oC to 110^oC.

Q34. 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.

Q35. How is convection involved in the boiling of water?

Solution

When water is heated, the water near the flame gets hot. Hot water rises up. The cold water from the sides moves down towards the source of heat. This water also gets hot and rises and more water from the sides moves down. This process continues till the whole water gets heated. This mode of heat transfer is known as convection.

Q36. In what ways the heat can transfer from one place to other?

Solution

Heat can transfer by means of(a) conduction(b) convection(c) radiation

Q37. When you put ice in coffee, why does the ice melt and the coffee becomes cold?

Solution

Heat flows from coffee (which is at a higher temperature) to ice (which is at a lower temperature). Hence ice melts and coffee becomes cold.

Q38. How do ocean currents form?

Solution

Ocean currents are caused due to the movement of ocean water, resulting from the temperature difference between ocean water at equatorial and polar regions.

Q39. Why does hot air rise and cold air sink?

Solution

Cold air is denser than warm air, so it falls through the warm air. Warm air being light rises up.

Q40. 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.