| 2 |
Thermal Physics
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Where to go! |
| 2.3 |
Transfer of Thermal Energy
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C
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Radiation
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| Core-1 |
Identify infra-red radiation as a part of the electromagnetic spectrum |
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| Sup-1 |
Describe experiments to show the properties of good and bad emitters and good and bad absorbers of infra-red radiation |
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- What is meant by heat radiation?
Heat radiation is a form of electromagnetic waves called "infra-red" waves?
- How heat reaches us from the Sun?
By the heat radiations emitted from the sun which can travel in vacume.
- What is the relation between the heat in an object and and the wavelength of emitted EM?
The hotter the object the shorted the wave length.
- What happens when heat radiations fall on an object?
Part of the radiation is absorbed and the other part is reflected
 
- How heat radiation can be detected?
They can be detected using a " thermopile" and a "blackened bulb meter"
 
- What is meant by "bad emitters", "bad absorbers", "good emitters" and "good absorbers"?
"bad emitters", "bad absorbers" are objects that reflect heat radiation
"good emitters" and "good absorbers" are objects that absorb heat radiation
 
- Why people wear dark clothes in winter and white clothes in summer?
People wear dark clothes in winter to keep warm because dark cloths are good absorbers of heat radiations. On the other hand, people wear white clothes in summer to keep cool because white cloths are good reflectors of heat radiations.
- Why roofs of petrol tanks are painted in shiny aluminum?
Because a shiny aluminum will reflect heat radiation and thus prevent overheating petrol which might generate to much gases that may lead to an explosion or a fire.
- Why vacuum flasks are made if double-walled glass vessels with vacuum between the walls and the walls are silvered?
They are made of double-walled glass with vacuum in between to prevent heat transfer from or to the liquid inside them. They are silvered because a silvered surface reflects heat radiation.
 
- Why Greenhouses are made of glass walled rooms to keep warm climate in cold nights?
| Because glass will prevent convection between the air trapped in rooms and the air outside rooms at the same time it allows heat radiation to enter from glass which supply heat to the insides of the room. During cool night, the heat energy gained from radiation is emitted back inside the house which keeps warm during the night. |
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Why cloudy nights remain warm?
When air rises up it cools down and becomes more dense. Thus it sinks down until it reaches the Earth surface which in return cools the surface. When cloud are present, they prevent cool air to sink and reach the surface. Also, the reflect heat radiations and prevent them from reaching ground.
Explain an experiment to show that black surfaces are good absorbers of heat radiation and shiny surfaces are good reflectors of heat radiations.
 
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