HUMAN DIGESTION-IMPORTANT POINS
HUMAN DIGESTION GAME 1
HUMAN DIGESTION GAME-2
TRANSFER OF HEAT
IMPORTANT POINTS
. Application of radiation in real life
Radiation can be found in...
-Teapots
Teapots have shiny surface and since shiny surfaces are bad emitters of radiation, shiny teapots are able to keep the liquid in it warm for a longer period of time as compared to black-coloured teapots.
And in addition, shiny surfaces are bad absorbers, thus shiny teapotsor container can keep cold liquid cold for a longer period of time.
-Greenhouses
Greenhouses are used in cold climates to help grow plants by trapping heat.
Daytime, infared radiation from the sun pass through glass roof into the greenhouse.
This warm up the plants and help them grow. However, the plants tooemits infared radiation but the infared radiation emitted is different and cannot pass through the glass roof thus it gets trapped inside.
Overtime, the temperature in the greenhouse will increase.
-Vacuum/Thermos Flasks
The main use of the vacuum flasks is to keep the liquid in it hot by minimising the loss of heat.
And it is done through 4 possible ways: conduction, convection, radiation and evaporation.
-The stopper of the vacuum/thermos flasks is usually made of plastic which is a poor conductor of heat.
-Conduction will not take place in the trapped air since air is a poor conductor of heat.
-Conduction and convection also cannot take place through the sides as there is a vacuum between the double-glass walls of the flask.
-And to prevent radiation, the walls of the glass are silvered to radiant heat back into the hot liquid to keep it hot.
-The only time conduction and convection can occur is when the plastic stopper is removed.
-However, heat loss through radiation is hard to stop as radiant heatcan even pass through vacuum.
-Teapots
Teapots have shiny surface and since shiny surfaces are bad emitters of radiation, shiny teapots are able to keep the liquid in it warm for a longer period of time as compared to black-coloured teapots.
And in addition, shiny surfaces are bad absorbers, thus shiny teapotsor container can keep cold liquid cold for a longer period of time.
-Greenhouses
Greenhouses are used in cold climates to help grow plants by trapping heat.
Daytime, infared radiation from the sun pass through glass roof into the greenhouse.
This warm up the plants and help them grow. However, the plants tooemits infared radiation but the infared radiation emitted is different and cannot pass through the glass roof thus it gets trapped inside.
Overtime, the temperature in the greenhouse will increase.
-Vacuum/Thermos Flasks
The main use of the vacuum flasks is to keep the liquid in it hot by minimising the loss of heat.
And it is done through 4 possible ways: conduction, convection, radiation and evaporation.
-The stopper of the vacuum/thermos flasks is usually made of plastic which is a poor conductor of heat.
-Conduction will not take place in the trapped air since air is a poor conductor of heat.
-Conduction and convection also cannot take place through the sides as there is a vacuum between the double-glass walls of the flask.
-And to prevent radiation, the walls of the glass are silvered to radiant heat back into the hot liquid to keep it hot.
-The only time conduction and convection can occur is when the plastic stopper is removed.
-However, heat loss through radiation is hard to stop as radiant heatcan even pass through vacuum.
3. Radiation
Radiation is the continual emission of infared waves from the surface of all bodies, transmitted without the aid of a medium.
Radiation..
-unlike conduction and convection, it does not require a medium for energy transfer.
-can take place in a vacuum (without any matter)
For radiation, heat is transferred by infra red waves.
eg.
-The heat from the Sun is tranferred by radiation to the Earth.
-This is something conduction and convection cannot do as there isno medium between the Sun and the Earth.
-Another example will be campfire.
-We could feel the heat from the campfire even though there is no medium in between.
Emission of infared radiation
All objects and surfaces emits infared radiation and this emission causes the temperature of the objects themselves to fall.
What are the factors affecting the rate of heat tranfer by radiation (infared radiation)? [both emission and absorbsion]
1. The colour and texture of the surface of the body
Black or dull suraface
- best radiator of heat, better absorer of infared radiation.
- better emitter of infared radiation
Shiny or white smooth surface
- worst radiator of heat, worse absorber of infared radiation.
2. The surface temperature of the body 
The higher the temperature of the surface, the higher the rate of radiation.
3. The surface area of the body
The larger the surface area, the higher the rate of radiation.
Other facts on radiation...
-If the temperature of the object is higher than the surrounding, it
is a net emitter of radiation.
-If the temperature of the object is lower than the surrounding, it
is a net absorber of radiation.
-A rough and dull black surface is both a good emitter and a good absorber.
-A smooth and polished surface is both a poor emitter and a poor absorber.
Infared radiation cannot be seen with our naked eyes but the heat sensitive camera can shot the hot spots.
eg.
The hottest spot of the cat is its nose as seen by the camera.
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