Matric Part 1/9th Class Physics Chapter 9 Long Question With Answer for Unit 9 Transfer of Heat

Question # 1

Describe use of Leslie cube

Ans 1: Leslie's cube: a Leslie cube is a meta box having forces of different nature and it used to identify the emission of radiation.
Ans 2: Faces of Leslie's cube: the four faces of Leslie's cube may be as follow:
1. A shining silvered surface
2. A dull black surface
3. A white surface
4. a colored surface
Ans 3: Explanation: hot water is filled in the Leslie cube is placed with one of its face toward a radiation detectors. The rate a t which various surface absorb heat also depends upon the nature of those surface.
Ans 4: Example: take two surfaces, one is dull black and the other is a silvered polished surface, with a candle at the middle of the surface. It is found that
- A dull black surface is a good absorber f eat.
- A polished surface is a poor absorber of heat
Ans 5: Conclusion: the conclusion draw from the above is shown below
Surface: emitter, absorber, reflector
Dull black surface: best, best, worst
Colored surface: good, good, bad
White surface: bad, bad, good
Shining silver surface : worst, worst, best

Question # 2

What causes a glider to remain in air? describe

Ans 1: Gliding: a glider looks like a a small aeroplane without engine. Glider pilots use upward movement of hot air currents due to convection of heat. these rising currents of hot air are called thermals.
Gliders ride over these thermals. The upward movement of air currents in thermals hep them to stay in air for a long period.
Ans 2: Uses; the birds stretch out their wings and circle in these thermals. The upward movement of air helps birds to climb up with it. Eagles hawks and vultures are expert thermal climbers. after getting a free lift, birds are able to fly for hours without flapping their wings . They glide from one thermal to another and thus travel through large distances and hardly need to flap their wings.

Question # 3

what measures may b taken to save energy?

Ans 1: In houses good thermal insulator means lower consumption of fuel.
Ans 2: Measure to save energy: for this , following measures may be taken to save energy
- Hot water tanks are insulated by plastic or foam lagging
- Wall cavities are filled with plastic foam or wool
- Ceiling of rooms is covered by in insulating material( false ceiling)
- Double glazed window panes are used. These windows panes have air between sheets that provides good insulation.

Question # 4

Write the uses of conductors and non-conductors.

Ans 1: Uses of conductor: good conductors are used when quick transfer of heat is required through a body. Thus, cooker , cooking plates, boiler , radiators and condensers of refrigerators etc. are made 0f metals such or aluminum as copper . Similarly, metal box are used for making ice creams, etc..
Ans 2: Uses of insulators: insulators or bad conductor are used in home utensils such as handles of sauce-pans, hot plate, spoons etc. They are made of wood or plastic. Air is one of the bad conductor or best insulator.
- Cavity walls separated by an air space and double glazed windows keep the houses warm in winter and cool in summer
- Materials which trap air i.e. wool, felt, fur, feathers, polystyrenes, and fiber glass are also bad conductors. Some of these materials are used for laggings to insulate water pipes, hot water cylinders, ovens, refrigerators,, walls and roofs of houses.
- Woolen cloth is used to make warm winter clothes.

Question # 5

What are the factors due to which radiation occurs

Ans 1: Various factors such as:
- Color and texture of the surface
- Surface temperature
- Surface area
Ans 2:
1. Decrease temperature by radiation: all the objects lying inside a room including the walls , roofs and floor of the room are radiating heat. however, there are also absorbing heat at the same time . When temperature of an object is higher than its surroundings then it is radiating more heat than it is absorbing. As a result, its temperature goes on decreasing till it becomes equal to its surroundings. At this stage, the body is giving out the amount of heat equal to the amount of heat it is absorbing.
2. Increase temperature by radiation: when temperature of an object is lower than its surroundings, then it is radiating les heat than it is absorbing. As a result, its temperature goes on increasing til it becomes equal to its surroundings.
3. Color and texture of the surface; the rate at which various surfaces emit heat depends upon the nature of the surface. Various surfaces can be compared using Leslie's cube.
4. Surface area: The transfer of heat by radiation is affected by the surface area of the body emitting or absorbing heat large is area, greater will be the transfer of heat . It is due to reason that large number of slots are made in radiators to increase their surface area.

Question # 6

Write the uses of conductors and non-conductors

Ans 1: Uses of conductor: good conductors are used when quick transfer of heat is required through a body. Thus, cooker , cooking plates, boiler , radiators and condensers of refrigerators etc. are made 0f metals such or aluminum as copper . Similarly, metal box are used for making ice creams, etc..
Ans 2: Uses of insulators: insulators or bad conductor are used in home utensils such as handles of sauce-pans, hot plate, spoons etc. They are made of wood or plastic. Air is one of the bad conductor or best insulator.
- Cavity walls separated by an air space and double glazed windows keep the houses warm in winter and cool in summer
- Materials which trap air i.e. wool, felt, fur, feathers, polystyrenes, and fiber glass are also bad conductors. Some of these materials are used for laggings to insulate water pipes, hot water cylinders, ovens, refrigerators,, walls and roofs of houses.
- Woolen cloth is used to make warm winter clothes.

Question # 7

What is conduction. Explain it briefly

Ans 1: Conduction: the mode of transfer of heat by vibrating atoms and free electrons in solids from hot to cold parts of a body is called conduction of heat.Example: the handle of metal spoon held in hot water soon gets warm. But in case of a wooden spoon, the handle does not get warm. Bot the materials behave differently regarding the transfer of heat. Metals are better conductors of heat than non-metals.
Ans 2: Conduction in solids: in solids, atoms and molecules are packed close together. They continue to vibrate about their mean position. The atoms or molecules present at the end begin to vibrate more rapidly. They also collide with their neighboring atoms or molecules. In doing so, they pass some of their energy to neighboring atoms or molecules during collisions with them with the increase in their vibrations. These atoms or molecules in turn pass on a part of the energy to their neighboring particles. In this way some heat reaches the other parts of the solids. This is a sow process and very small transfer of heat takes palace from hot to cold parts in solids.
Ans 3: Conduction in metals: metals have free electrons. These free electrons move with very high velocities within the metal objects. They carry energy at a very fast rate from hot to cold parts of the object as they move.Conclusion: thus heat reaches from hot parts of metals to cold parts much more quickly
Ans 4: Conduction in non-metals: non-metals are bad conductor of heat and the substances through which heat does not conduct are called bad conductors or insulators.Example: wood, cork, cotton, wool, glass, rubber, etc. are bad conductors or insulators.Conclusion: non-metals are insulators thus heat does not conduct through it.

Question # 8

What is sea an land breezes? also explain the gliding and their uses

Ans 1: Sea breeze: on a hot day, the temperature of the land increases more quickly than the sea. It is because the specific heat of land is much smaller as compared to water. the air above land gets hot and rise up. Cold air from the sea begins to move towards the land as illustrated. It is called sea breeze.
Ans 2: Land breeze: at night, the land cools faster than the sea. Therefore, air above the sea is warmer, rises up and the cold air from the land begin to move toward s the sea as illustrated. It is called land breeze.

Question # 9

What is greenhouse ? also explain
t he greenhouse effect in global warming.

Ans 1: Greenhouse: a greenhouse is a heat trip. It is made of glass or transparent polythene sets that allow only radiations of short wavelength to pas through them. Radiation of long wavelength cannot enter or escape from green house.
Uses; greenhouse is usually used for plant growth
Ans 2: Phenomenon of maintenance of temperature in greenhouse : light from the sun contains thermal radiations ( infrared) of long wavelength as well as light and ultraviolet radiations of short wavelength to pas through easily but not long wavelength of thermal radiations. Thus, a green house becomes a heat trap.
radiations from the sun pass easily through glass and warms up the objects in a green house . These objects and plants give out radiations of much longer wavelengths. Glass and transparent polythene sheets don not allow them to escape out easily and are reflected back in the green house . This maintains the inside temperature of the green house . Greenhouse effect promise better growth of some plants.
Ans 3: Greenhouse effect: green house effect is rising of temperature because of trapping of heat energy by green house gases such as carbon dioxide , etc. in the temperature.
Ans 4: mechanism of green house effect: carbon dioxide in the atmosphere and water behave in a similar way to radiations as glass or polythene. Earths atmosphere contains carbon dioxide and water vapors. It causes greenhouse effect and thus maintains the temperate of the earth. During the recent years, the percentage of carbon dioxide has been increased considerably. This has caused an increase in the average temperature of the earth by trapping more heat due to greenhouse effect
Ans 5: Global warming: the average rise in the temperature of the earth by trapping more heat due to greenhouse effect is called global warming.
Effect of greenhouse in global warming: during the recent years, the percentage of carbon dioxide has been increased considerably by rapid use of fuels. This has caused an increase in the average temperature of the earth by trapping more heat due to greenhouse effect. This phenomenon is known as global warming. This has serious implications for the global climate.

Question # 10

Define and explain the radiation
OR
How does heat reach us from sun?

Ans 1: Radiation: radiation is the mode of transfer of heat from one place to another in form of waves called electromagnetic waves. Radiation is that process of heat transfer in which heat energy reaches in the form of waves from one place to other without affecting the medium on its way.
Ans 2: Explanation: it is process by which solar energy reaches the earth passing through space. Radiation is all time continuous process around us . When a body is at a higher temperature than its surrounding objects , it emits more heat by thermal radiation and absorbs less heat by thermal radiation from nearby objects, thus temperature of the body falls. Similarly, emission of les heat by radiation cause and absorption of more heat by radiation cause the rise in the temperature of nearby objects.
Ans 3: Examples:
- Heat of the sun: it is through radiation that heat reaches us from the sun. Our sun is the major source of heat energy. It reaches us neither by conduction nor by convection, because the space between the sun and the earth's atmosphere i empty. It is radiation by which heat travels from one place to another.
Ans 4:
- Heat from fireplace; transfer of heat by radiation is the heat received from a fire place. Air is a poor conductor of heat. heat does not reach us by conduction through air from the fire place, because air poor conductor of heat. Heat from the fire places reaches us directly by the process of eradiation in the form of waves called radiation. A sheet of paper o cardboard kept in the path of radiations stop these waves to reach us.