Class 7 Science Heat Modes of Heat TransferRadiation.pptx

Class 7 Science Heat

1. Radiation Radiation is the process of heat transfer in which heat directly passes from one body at a higher temperature to another body at a lower temperature without affecting the medium No medium is required for the transfer of heat by the process of radiation In vacuum, heat transfer takes place only by the process of radiation Radiation is the transfer of heat through electromagnetic waves

2. Heat radiations are also called thermal radiations , or simply, radiations . They travel in straight lines with a speed of 3 x m/s in vacuum or air. They are similar in properties to light with the only difference being heat radiation does not give the sensation of sight as light does, but heat radiations give the sensation of hotness.  

3. When radiations fall on a body, the body absorbs some of them and reflects the rest. The body becomes warm because of the absorbed radiations Different substances absorb different amounts of radiations falling on them. A body which absorbs more radiations and reflects less, becomes more warm, while a heat by body which absorbs less radiations and reflects more, becomes less warm.

4. Experimentally it has been observed that The black and dull surfaces absorb more radiations and reflect less radiations The white and shiny surfaces reflect more radiations and absorb less radiations

5. Solar Cookers Solar cookers use the heat energy of the sun to cook food. The inner surface is painted black so that it absorbs maximum solar energy falling on them

6. Applications of black and white surfaces in daily life We prefer to wear white clothes in summer. The reason is that white clothes reflect most of the sun’s heat and absorb very little of it, thus they keep our body cool.

7. We prefer to wear black and dark coloured clothes in winter. The reason is that black or dark coloured clothes absorb most of the sun's heat and keep our body warm.

8. The bottom part of a cooking utensil or pan is painted black. The reason is that a black surface absorbs more heat and so the contents of the utensil or pan get cooked rapidly if its bottom part is painted black.

9. Room heaters are provided with shiny reflectors. The reason is that the shiny surface absorbs very little heat and reflects most of it, so almost the entire heat produced by the filament of heater is reflected.