Radiation
- by Soktaik
what is RADIATION? Setting the context
Why do we feel hot outside in the summer?
Well, of course it is the sun. Everyone knows the sun is hot!
So the sun, which 149600000 kilometres away, is making me hot?
Well, yes! it is because the sun is very hot and very big. It is a very big heat source
Ok, but how does sun's heat come to us?
Well, all things give out heat radiation. The sun is very hot. It gives out a lot of radiation! This radiation travels a long distance to come to us.
RADIATION - FACTS
-
Radiation is a method of heat transfer. Heat is transferred from the more hot body to the less hot (cold) body through radiation.Things heat and cool down because of radiation.
- Everything around give out heat radiation. The hotter something is, the more heat radiation it gives out
- Yes, everything around us, the book, the walls, our bodies, the bulb, a hot flame, a hot metal rod, the chair, a car - all radiate heat. Sun is the biggest, baddest radiator we know or see
- FUN FACT : Everything which has temperature above -273.15 degree celcius radiates heat
Can we see objects radiating heat ?
How can we confirm that everything around us radiates heat?
-Our eyes can't see the radiation, but special cameras can!
These cameras are called infra-red cameras.
- Can you guess one use for these cameras?
Well, because they use heat radiation to take images, we can use them to see in the dark! The army uses these in night operations.
FUN FACT : INFRA-RED CAMERAs
HEATING & Cooling due to radiation
-
All things around give out (emit) heat radiation
- At the same time , all things around us also take in (absorb) radiation
- If a body takes in ( absorbs) more radiation that it gives out, it starts getting hot. That is, its temperature increases
- If a body gives out more radiation than it takes in, it starts getting cold. That is, its temperature decreases
cooling due to radiation
RADIATION WORKS WHEN THERE IS A MEDIUM
Have you experienced heat radiation?
HAVE YOU EXPERIENCED HEAT RADIATION?
HAVE YOU EXPERIENCED HEAT RADIATION?
HAVE YOU EXPERIENCED HEAT RADIATION?
HAVE YOU EXPERIENCED HEAT RADIATION?
Radiation happens even if there is no medium
Conduction works only in solids.
Convection works only in liquids & gases.
Radiation works everywhere, even without a medium
WHAT HAPPENS WHEN RADIATION FALLS ON SOMETHING
Radiation falling on a body can be :
- Absorbed (Goes in the body)
- Reflected (Does not go in the body)
- Transmitted (Goes through the body)
How much radiation falling on a body will be reflected, absorbed or transmitted depends on the properties of the body
Absorption of radiation raises the temperature of an object. Reflection & Transmission do not raise the temperature of the object
Have you ever experienced Heat radiation?
Heater in a room
Hot utensils cool down
Hot sun
Warming Hands in fire
Ice melting in the room
HOW IS RADIATION DIFFERENT FROM CONDUCTION & CONVECTION?
Conduction - Touch, Heat transfer from one part of solid to another OR heat transfer between 2 bodies in contact
Convection - In liquids and fluids. Happens because hot things rise up.
- Other methods of heat transfer (Conduction & Convection) require a medium. Conduction is seen in solids and convection is seen in liquids and gases
Radiation - All things above a very low temperature radiate heat
Transfer of heat through radiation does not require a medium. Radiation happens when there is medium(like air)and even if there is no medium
RADIATION - EXPERIMENTS
Hot utensil cools down because it emits more radiation than everything around it. Everything around the utensils gets heated a little due to radiation.
Similarly, ice cube on table melts. Since Ice cube is cold, everything around it gives more radiation. Ice cube receives this radiation and gets heated and melts.
When we sit in front of fire, the fire gives much more radiation than our bodies. So we receive this radiation heat and become warm
So we see that not just that all bodies give out heat radiation, all bodies also receive ( or absorb) heat radiation.
SUN-> Black & White utensil. Both get same heat radiation but black absorbs more.. It is the property of the object how much heat it absorbs. Not just absorb, the black object radiates more hear as well.
Radiation can be absorbed reflected ortransmitted.
Daigram of this.
How much will be reflected or absorbed depends on the object itself. Black reflects and absorbs more.
Dark clothes light clothes
RADIATION DOES NOT NEED A MEDIUM
Does not mean radiation does not work when there is medium :)
Earth - > atmosphere - > nothing -> Sun.
Radiation - by Soktaik
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