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

Question # 1

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 # 2

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 # 3

What is convection and convection currents in air? also write its uses

Ans 1: Convection; transfer of heat by actual movement of molecules from hot place to a cold place is known as convection
Ans 2: Explanation: liquids and gases are poor conductors of heat. however, heat is transferred through fluid(liquid or gases) easily by another method called convection. A liquid or a gas becomes lighter as it expands on heating. Hot liquid or gas rises up above the heated area. The cooler liquid or gas from the surroundings gills the place which in turns is heated up. In this way, all the fluid is heated up. Therefore, transfer of heat through fluids takes place by the actual movement of heated molecules from hot to cold parts of the fluid.
Ans 3: Experiment: take a beaker r and fill two third of it with water. heat the beaker by keeping a burner below it. Drop two three crystals of potassium permanganate in the water. It will seen that colored streaks of water formed by the crystals move upwards above the flame and then move downward from side ways. These colored streaks show the path of currents in the liquid. when the water at the bottom of the beaker gets hot , it expands ,become lighter and rise up. While the cold but denser water moves downward to take its place.
Ans 4: Convection of currents: gases also expand on heating, thus convection currents are easily set up due to the difference in the densities of air at various parts in the atmosphere.
Ans 5: Uses of convection currents: convection currents set up by electric, gas or coal heaters help to warm our homes and offices central heating system in building work on the same principle by convection. Convection currents occur on a large scale in nature. The day-by-day temperature changes in the atmosphere result from the circulation of warm or cold air that travels across the region. land and sea breeze rea also the example of convection current.

Question # 4

What is thermal conductivity ? explain on which factors it depends

Ans 1: Thermal conductivity: the rate of flow of heat across the opposite faces of a meter cube of a substance maintained at a temperature difference of one kelvin is called the thermal conductivity of that substance.
Unit: Wm^-1K^-1 is the unit of thermal conductivity.
Ans 2: Definition of rate of flow of heat; the amount of heat that flows in unit time is called the rate of flow of heat.
Representation: it is represented as
Rate of flow of heat= Q/t
Example: conduction of heat occurs at different rates in different materials. In metals, heat flows rapidly as compered to insulators such as wood or rubber.
Explanation; consider a solid block. One of its opposite faces each of cross-sectional area A is heated to the temperature T1. Heat Q flows along its length L to opposite ace at temperature T2 in t second
Ans 3: Factor affecting the rate of flow of heat: it is observed that the rate at which heat flows through a solid object depends upon various factors.
- Cross-sectional area of solid: large cross-sectional area A of a solid contain large number of molecules and free electrons on each layer parallel to its cross-sectional area and hence greater will be the rate of flow of heat through the solid
Ans 4:
- Length of the solid: larger is the length between the hot and cold ends of the solid thus, ore time it will take to conduct heat to the colder end and smaller will be the rate of flow of heat
Ans 5:
- Temperature difference between ends: greater is the temperature difference T1-T2 between hot and cold faces of the solid, greater will be there rate of flow of heat.

Question # 5

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 # 6

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 # 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 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 # 9

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.

Question # 10

What are applications and consequences of radiation.

Ans 1: Applications and consequences of radiation:
Absorption of heat: different objects absorb different amounts of heat radiations falling upon them reflecting the remaining part.
Dependence: the amount of heat absorb by a body depends upon the color and nature of its surface.
Example: a black and rough surface absorb more heat than a white or polished surface. Since good absorbers are also good radiators of heat. Thus, a black colored body gets hot quickly in summer by absorbing heat from its surrounding. It cools down quickly by giving out its heat to its surroundings. The bottom of cooking posts is made black to increase the absorption of heat from fire.
Ans 2: Reflection: like light rays, heat radiations also obey the laws of reflection.
Dependence: the amount of heat reflected from an object depends upon its color and nature of the surface.
Example: white surface more heat than colored or black surfaces . Similarly , polished surfaces are good reflectors than rough surfaces and reflection of heat radiations is greater from polished surfaces. We wear white or light color clothes in summer , which reflect most of the heat radiations reaching us during the hot day. We polish the interior of cooking and hot pots for reflecting backmost of the heat radiation within them.

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

9th Class English Medium Subjects

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Online Long Questions For Chapter 9 "9th Class Physics Chapter 9 Long Question Preparation "

Physics (New Book) - 9th Class Physics Chapter 9 Long Question Preparation

What are the factors due to which radiation occurs

Write the uses of conductors and non-conductors

What is convection and convection currents in air? also write its uses

What is thermal conductivity ? explain on which factors it depends

Describe use of Leslie cube

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

What is conduction. Explain it briefly

What is greenhouse ? also explaint he greenhouse effect in global warming.

Define and explain the radiationORHow does heat reach us from sun?

What are applications and consequences of radiation.

Prepare Complete Set Wise Questions For Chapter 9 "9th Class Physics Chapter 9 Long Question Preparation "

Set 1

9th Class Physics Chapter 9 Long Question Preparation - Set 1

What are the factors due to which radiation occurs

Write the uses of conductors and non-conductors

What is convection and convection currents in air? also write its uses

What is thermal conductivity ? explain on which factors it depends

Describe use of Leslie cube

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

What is conduction. Explain it briefly

What is greenhouse ? also explaint he greenhouse effect in global warming.

Define and explain the radiationORHow does heat reach us from sun?

What are applications and consequences of radiation.

9th class Physics Chapter 9 Long Q's (English Medium)

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

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

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

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