Physics Chapter no 9|9thClass||tutoronlines

Transfer of heat

Table of contents:

Number of topics Name the topics
1 The three process of heat transfer
2 Conduction
3 Convection
4 Radiation
5 Consequences and everyday application of heat transfer

The three process of heat transfer:

  • Conduction
  • Convection
  • Radiation

Conduction:

The mode of transfer of heat by vibrating atoms and free electrons in solids from hot to cold parts of a body as known as conduction. Insulators are the substances which allow current to pass through them.

Example:

Conduction Example are wood, cork, cotton, wool, glass, rubber, etc. are the bad conductor or insulators.

Convection:

process by which heat is transferred by movement of a heated fluid such as air or water.
Natural convection results from the tendency of most fluids to expand when heated i.e., to become less dense and to rise as a result of the increased buoyancy. Circulation caused by this effect accounts for the uniform heating of water in a kettle or air in a heated room. the heated molecules expand the space they move in through increased speed against one another, rise, and then cool and come closer together again, with increase in density and a resultant sinking.

Radiation:

Radiation is energy that comes from a source and travels through space at the speed of light. This energy has an electric field and a magnetic field associated with it, and has wave-like properties. You could also call radiation “electromagnetic waves”.

 

Exercise short Questions

Q.1. Why metals are good conductors of electricity? Heat.

Ans. 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 one part of the object to the other as they move. Thus, changes reach from one part of the metal object to the other more quickly than non-metal.

Q.2. Explain why?
(a) A metal feels colder to touch than wood kept in a cold place?

Ans. Conductor have good conduction property. So by touching cold conductors, there is a rapid transfer of heat from our hand to cold conductor and it feels colder. As wood is a bad conductor, so transfer of heat from our hand to wood is very low. Due to this reason, we feel less cold.

(b) Land breeze blows from land toward sea?

Ans. At night, the land cools faster than the sea. Therefore, air above the sea is warmer, rise up and the cold air from the land begins to move towards the sea.

(c) Double walled glass vessel is used in thermos flask?

Ans. Air is one of the bad conductor or good insulator. That is why double walled glass vessel i.e. two walls separate by an air space and double glazed flask keeps the tea hot.

(d) Desserts soon get hot during the day and soon get cold after sunset?

Ans. As the specific heat of sand is low, so it absorb the heat more quickly and gets hot in day. So dessert day is very hot. In night, it releases heat more quickly and become cold quickly after sunset.

Q.3. Why conduction of heat does not take place in gases?

Ans. Gases are poor conductors of heat. The gas molecule are far apart. So conduction of heat does not take place in gases. However, heat is transferred through gases by another method called convection.

Q.4. What measures do you suggest to conserve energy in house?

Ans. We take the following measures to conserve energy in house:

Plastic or foam laggings are used to insulate Hot Water Tank.
Wall cavities are filled with plastic foam or wool.
Ceiling of room is converted by insulating material ( false ceiling)
Double glazed windows panes are used, these windows panes have air between glass sheets that provide good insulation.

Q.5. Why transfer of heat in fluids takes place by convection?

Ans. Liquid and gases are poor conductors of heat. However, heat is transferred through fluids ( liquid or gases) ” an other method called convection. Transfer of heat in fluids takes place by convection because movement of molecules is easy fluids.

Q.6. What is meant by convection current?

Ans. Gases also expand on heating, thus convection current are easily set up due to the differences in the densities of air at various parts in the atmosphere.

Example:

Convection current set up by electric, gas or coal heaters help to warm our homes and offices.

Q.7. Suggest a simple activity to show convection of heat in gases not given in the book?

Ans. In summer, the intense radiation of sun warm the surface of earth. The air near the surface heat up and expand. Its density decrease due to increase of volume and it rises up. A colder air comes to fill this gap, due to which convectional current of air are produced.

Q.8. How does heat reach us from the sun?

Ans. Our sun is the major source of heat energy. Heat reaches us neither by conduction nor by convection, because the space between the sun and earth‘s atmosphere is empty. Beat reaches us through radiation (light waves) from the sun.

Q.9. How various surfaces can be compared by Leslie cube?

Ans. The nature of different sides of Leslie cube are different. So transfer of heat from different side of the cube is different. So, various surfaces can be compared by Leslie cube.

Q.10. What is greenhouse effect?

Ans. Glass and transparent polythene sheets allow radiation of short wavelength to pass through easily but not long wavelengths (infrared) of thermal radiation. Thus, green house become a heat trap. Radiations from the sun pass easily through glass and warms up the object in a greenhouse.

Q.11. Explain the impact of greenhouse effect in global warming?

Ans. Earth’s atmosphere contains carbon dioxide and water vapor. It causes greenhouse effect and thus maintains the temperature 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.

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