UNIT 9:

TRANSFER OF HEAT...

Q. Why metals are good conductor of heat?

Ans. Metals have free electrons which transfer heat rapidly. That is why metals are good conduction of heat.

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Q. Explain why?

a. A metal feels colder to touch than wood kept in a cold place?

Ans. Metals have property of conduction of heat and wood is an insulator. On touching metal, it transfers heat more rapidly. In case of wood heat conductivity is very slow. That’s why metal feels colder to touch than wood.

b. Land breeze blows from land towards sea?

Ans. At night the air near surface of water becomes hotter due to higher temperature of sea water than land nearby. So air above the sea rises up and colder air from land flows towards sea.

c. Double walled glass vessel is used in thermos flask?

Ans. Double walled glass vessel is used in thermos flask because it has vacuum between double walled glass vessel due to which it stops the conduction of heat and temperature remains constant.

d. Deserts soon get hot during the day and soon get cold after sunset?

Ans. Deserts have sand whose conductivity is high. That’s why deserts soon get hot during the day and soon get cold after sunset.

Q. Why conduction of heat does not take place in gases?

Ans. Conduction of heat does not take place in gases because molecules of gases are at greater distances. In gases, heat transfers by convection process.

Q. What measures do you suggest to conserve energy in houses?

Ans. To conserve energy in houses following measures should be suggested.

o   Doubled walls should be made with some insulator at their middle.

o   Water tanks should be covered with some insulator.

o   False ceiling should be done by some insulator material.

Q. Why transfer of heat in fluids takes place by convection?

Ans. In fluids (liquid or gas) molecules are at greater distance as compared to the solids. So heat is not transferred by conduction. Transfer of heat is done in fluids by convection process.

Q. What is meant by convection current?

Ans. Convection is the transfer of heat through a fluid by the physical movement of the fluid itself, and convection currents are those movements. They can form in air, in water, and within the earth itself.

Q. Suggest a simple activity to show convection of heat in gases not given in the book.

Ans.

Place a burning candle under one chimney of a smoke box.

Place the shouldering tip of wet string or wet paper over the top of the second chimney.

Note that the smoke goes down the chimney across to the candle and out the other chimney.

The path of the smoke indicated the direction of airflow by convection.

Q. How does heat reach us from sun?

Ans. Heat is transferred from sun to earth by radiation process. Solar Radiation is essentially photons, or packets of energy, emitted from the suns surface, which are able to travel through the vacuum of space at the speed of light. This comes to us in a range of wavelengths, including Visible light, Ultra violet and Infra red radiation.

Q. How various surfaces can be compared by a Leslie cube?

Ans. A metal box in the shape of a cube in which each of the four vertical sides have different surface finishes. When hot water is placed in the cube, the emissivity of the finishes can be compared. The device was first used by Sir John Leslie.

Q. What is greenhouse effect?

Ans. The phenomenon whereby the earth's atmosphere traps solar radiation, caused by the presence in the atmosphere of gases such as carbon dioxide, water vapor, and methane that allow incoming sunlight to pass through but absorb heat radiated back from the earth's surface.

Q. Explain the impact of greenhouse effect in global warning

Ans. The earth is surrounded by a cover of gases as atmosphere. This atmosphere allows most of the light to pass through, which reaches the surface of earth. This light from sun is absorbed by the earth surface and converts into heat energy. This heat energy is re-emitted by the surface of the earth during night. Due excessive presence of some gasses in the atmosphere, this escape of heat from earth surface is prevented, resulting in heating of earth called ‘global warming’. The gasses which are responsible for causing global warming are called ‘greenhouse gasses.

The harmful effects of presence of greenhouse gasses in atmosphere are global warming, climate change, ozone depletion, sea level rise, adverse effects on biodiversity etc. One way or another these adverse impacts are all directly or indirectly related to the presence of greenhouse gases in the atmosphere. A number of human activities, processes and consumptions produce waste gasses or greenhouse gasses that are harmful to the environment.