Question: What are the differences between radiation, conduction, and convection?
1. Simple definitions.
CONVECTION: the transfer of heat by currents moving through a flowing material (liquid or gas)
RADIATION: the transfer of heat by light (photons)
CONDUCTION: the transfer of heat by the collision of molecules
"Convection" only works in a material that can flow, like liquid or gas. If part of the fluid is hot and it flows into another part of the fluid that is cold, the hot fluid dissipates into the cold fluid (at least partially) leading to an overall temperature somewhere in the middle of the two. The key idea is that material is physically transferred from one region to another, and that it is the SAME material (or same type of material) only at different temperatures. Because it is the same material, heat gets distributed as the material naturally blends with itself.
"Conduction" involves two pieces of matter that are in direct physical contact. The piece that is hotter will have molecules that are vibrating faster and more energetically. These vigorously vibrating molecules bump against nearby molecules causing them to also vibrate faster. The spread of vibrations causes heat energy in the hotter piece of matter to get transferred into the colder piece. The key here is that the two pieces of matter must be in physical contact so that the molecules can bump against each other.
With "radiation," heat is transferred without the movement of matter and without two regions being in direct physical contract. The heat energy is transferred via little electromagnetic wave packets called "photons" which collectively we refer to as "light." It is by this method of heat transfer that energy from the sun reaches our planet through the emptiness of space. Furthermore, it is by this method that our planet releases heat into outer space. Billions of years ago, the Earth was a molten ball of magma. It cooled off by releasing heat into outer space, and the only way this was possible was by radiation -- by radiating heat off in the form of photons.
convection: currents in the ocean; an air conditioner spreading cool air through a building
radiation: heat from the sun reaching Earth; a microwave oven heating food
conduction: a burner on an electric stove heating a pot of water; whenever two solid objects at different temperatures meet
by Dr. Noh
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