Physics Class 9th Notes Chapter 9 Transfer of Heat Mardan board

Physics Class 9th Notes Chapter 9 Transfer of Heat according to KPK Mardan board syllabus. Contains solved exercises, review questions, MCQs, important board questions, and chapter overview.

conceptual questions of Physics Class 9th Notes Chapter 9

Q.1) Why white clothes are preferred to wear in summer? Explain briefly.

Answer :
White clothes are more suitable for summer because they reflect maximum sun radiation and absorb minimum sun radiation and thus keep our body cool. Thus, we feel comfort by wearing white clothes in summer.

Q.2) Why is the freezer compartment kept at the top of a refrigerator? Explain briefly.

Answer :
In the refrigerator, the cooled air in the freezer compartment is kept at the top because it cools the surrounding air and makes it denser as compared to the air at the bottom. The cool denser air moves down while the warmer air at the bottom moves upward where it is cooled. That’s why the freezer compartment is kept at the top of a refrigerator.

Q.3) A black car standing in the sun warms up more quickly than any other. why?

Answer:
The black color is a good absorber of heat radiation. It can absorb all the sunlight falling on it. Therefore a black car standing in the sun warms up quicker than other cars.

Q.4) Why a tile floor feels colder to bare feet than a carpeted floor?

Answer:
A tile floor is a good conductor of heat as compared to the carpeted floor. When a bare foot is put on the floor, more heat is lost by the foot which is absorbed by the tiles. As a result, we feel cool. If we put the feet on the carpeted floor, we feel less cool because in this case our feet losses no heat or very less amount of heat to the carpet. That’s why, a tile floor feels colder to bare feet than a carpeted floor.

Q.5) How woolen sweaters keep us warmer in winter?

Answer :
Wool is a bad conductor of heat. It allows very small amount of heat to pass through. It has air trapped in between the fibers. That’s why, woolen sweaters keep us warm in winter

Q.6) In certain places, birds can fly for hours without
flapping their wings.   Explain.

Answer:
In certain places, there are streams of the hot air which arise from the sun, such streams of hot air are known as thermals. Basically, thermals are convection currents. The birds can fly for hours on thermals without flapping their wings.

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Q.7) Good-quality thermos bottle is double-walled and evacuated between these walls, and the internal surfaces are like mirrors with a silver coating. How does this configuration combat heat loss from all three transfer methods and keep the bottle’s contents coffee hot?

Answer:
The purpose of the silvering is to reflect all radiant heat trying to leave the vessels. The space between the walls is highly evacuated (vacuum is created) to prevent convection. The glass, being a poor conductor minimizes conduction of heat as well. The heat loss through the flask is so small that a hot liquid placed in the flask will remain hot for a very long time.

Q.8) A piece of wood lying in the sun absorbs more heat than a piece of shiny metal. Yet the metal feels hotter than the wood when you pick it up. Explain.

Answer:
A piece of wood lying in the Sun absorbs more heat than a piece of shining metal, yet the wood feels less hot than the metal when you pick it up because the metal being a good conductor of heat conducts all the heat present in it to our hands whereas wood being a bad conductor conducts very little heat from its surface to your hands and hence feels less hot than the metal.

Q.9) Some pot handles remain cool during cooking while others become unpleasantly hot. What determined which handles remain cool and which become hot?

Answer:
With the help of thermal conductivity of substance, we can determine which handles remain cool and which become hot.
The wood has 0.08-0.16 Jm-1k-1s-1 thermal conductivity, which is a poor conductor of heat.  If the pot handle made up with wood will remain cool.
The steel iron has 80 Jm-1k-1s-1 thermal conductivity, which is a good conductor of heat. If pot handle is made of steel iron will become hot.

Q.10) When sunlight warms the land beside a cool body of water, breeze begins to blow from the water toward the land. Explain. 

Answer:
Convection causes coastal breeze. During the day the land heats up more quickly than the sea. The hot air over the land rises and the cold air from the sea blows to replace it. Thus there is a sea breeze during the day.

Comprehension Questions Physics 9th chapter 9

Q.1) Explain conduction of heat and its mechanism. Describe any three of its practical applications.

Answer:
Conduction
 
    “Conduction is the flow of thermal energy (heat) through matter from places of higher temperature to places of lower temperature without movement of the matter as a whole”.

Mechanism of Heat Conduction:
        The mechanism of heat conduction can be explained by the behavior of atoms within the material. The solid iron rod is made of closely packed iron atoms. According to the kinetic-molecular description of matter the greater the temperature, the more is the kinetic energy of atoms or molecules. The atoms in the hotter part of the rod vibrate more violently (thus possess more kinetic energy) than those in the colder part.

Initially, before the rod is inserted into the flame, the iron atoms are vibrating about their equilibrium positions. As the flame heats the rod, the iron atoms near the flame begin to vibrate at greater speeds and wider distances. These wildly vibrating atoms collide with their neighboring atoms and transfer some of their energy to these atoms. Which in turn pass thermal energy to their own neighbors and so on. Iron is metal and contains a large number of electrons that are free to move through the metal called free electrons.

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These free electrons also play a big part in the conduction of heat. For example, when one end of the iron rod is heated, the atoms in the heated part vibrate more with greater speeds. The free electrons that collide with these atoms gain kinetic energy and move faster. They diffuse into the colder part of the metal where collisions with other free electrons and atoms occur which results in the transfer of energy. In light of this discussion, we can define the conduction of heat as that process by which heat energy is transferred from particle to particle.

Q.4) Explain radiation of heat and its mechanism. Describe any three of its applications.

Answer :
Radiation:

When we sit next to an open campfire, the heat from the fire does not reach us by conduction because air is a bad conductor of heat. It does not reach us by convection because hot air rises up. In this case, the heat reaches us by radiation of heat. The hot fire radiates heat rays just as light rays. When these rays fall on anything they make that object hotter. Sun heat reaches us after passing through millions of kilometers in empty space. Sun heat also warms up anything on which it falls. Neither conduction nor convection can take place in a vacuum as no material is present for the transfer of heat through conduction or convection. Sun heat reaches us by radiation of heat. Based on the above observations, we define radiation of heat as
        “The process by which heat energy is transferred from a hotter place to a colder place with or without a material medium is known as radiation”.
Unlike conduction and convection transfer of heat by radiation, does not necessarily require a material medium.
Mechanism of heat radiation:
The mechanism of radiation is energy transfer by electromagnetic waves. Electromagnetic radiation comes from accelerating electric charges. On a molecular level, that’s what happens as objects warm up – their molecules vibrate harder and harder, causing acceleration of electric charges which emits those radiations. Heat energy transferred through radiation is as familiar as the light; in fact, it is the light but not visible or barely visible. Electromagnetic waves (radiation) can transfer energy via vacuum or empty space as well as via a material medium like glass.
            Every object around us is continually radiating, unless its temperature is at absolute zero 0 K (which is a little unlikely because you can’t physically get to a temperature of absolute zero, with no molecular movement). For example, a scoop of ice cream has a temperature of about 237 K, therefore it radiates.
                Even we radiate all the time, but that radiation isn’t visible as light because it’s in the infrared part of the spectrum. However, that light is visible to infrared scopes, as you’ve probably seen in the movies or on television.
Practical applications of radiation:
A) Colouring Material
                        The cooling fins on the heat exchanger at the back of a refrigerator are painted black so that they lose heat more quickly. By contrast, saucepans that are polished are poor emitters and keep their heat longer. In general, surfaces that are good absorbers of radiation are good emitters when hot.
B) The texture of the surface: 
                        One type of radiant barrier material, ARMA foil, produced by Energy Efficient Solutions.
C) Satellite Protective Coating: 
                            The highly reflective metal foil covering this satellite (the Hubble Space Telescope) minimizes temperature changes.

Q.5) Discuss the greenhouse effect. Explain its importance and global warming concern.

Answer :
Greenhouse effect
‘The greenhouse effect is the process by which radiation from a planet’s atmosphere warms the planet’s surface to a temperature above what it would be without its atmosphere’. The strength of the greenhouse effect – will depend on the atmosphere’s temperature and on the amount of greenhouse gases that the atmosphere contains.
Mechanism of the Greenhouse effect
Earth receives energy from the Sun in the form of ultraviolet, visible, and near-infrared radiation. Of the total amount of solar energy available at the top of the atmosphere, about 26% is reflected to space by the atmosphere and clouds and 19% is absorbed by the atmosphere and clouds. Most of the remaining energy is absorbed in the surface of Earth. Because the Earth’s surface is colder than the photosphere of the Sun, it radiates at wavelengths that are much longer than the wavelengths that were absorbed. Most of this thermal radiation is absorbed by the atmosphere, thereby warming it. In addition to the absorption of solar and thermal radiation, the atmosphere gains heat by latent heat fluxes from the surface.

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Importance and global warming concern
The greenhouse effect is important for the survival of life on earth. On Earth, an atmosphere containing naturally occurring amounts of greenhouse gases (water vapor, carbon dioxide CO2, methane CH4, and ozone O3) causes air temperature near the surface to be about 33 °C (59 °F) warmer than it would be in their absence. Without the Earth’s atmosphere, the Earth’s average temperature would be well below the freezing temperature of water.
             Human activity has increased the amount of greenhouse gases in the atmosphere leading to global warming (increase in the temperature of the earth). Due to human activities in the period from 1880 to 2012, the global average temperature has increased by 0.85 °C.


         The largest human influence has been the emission of carbon dioxide from factories and motor vehicles. Currently, about half of the carbon dioxide released from the burning of fossil fuels is not absorbed by vegetation and the oceans and remains in the atmosphere. Now in order to decrease global warming we have to reduce the emission of greenhouse gases and to plant more vegetation to absorb the produced carbon dioxide.

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