What is the relationship of between radiation and conduction and convection?

Answer 1

All three are forms of energy transfer.

Radiation transfers energy by light waves (including visible light). Photons move through the medium and hit molecules (but the molecules stay in place).***

Conduction transfers energy between molecules, but the molecules stay in place. Since molecules stay in place, it often in times takes place in solids. In a more technical and simplify way: a molecule transfers its kinetic energy (vibrations) to other near by molecules. The original molecule looses kinetic energy, and the surrounding molecules gain energy, but the total energy is always conserved. If energy seems to be lost is most likely because of the air molecules which also surround the solid and could enter and escape the system. ***

Convection, in contrast to the other two, transfers its energy by moving its molecules (or even atoms in that matter) around the system. Convection takes place naturally in fluids (which include liquids and gases), and the whole purpose (in a physical-law point of view) is to bring balance to the system.

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Thermos apply these basis to minimize as muchenergytransfer as possible, meaning that it will keep your energy drink as cold as possible in a hot day, and at the same time it could keep your coffee drink as hot as possible in a cold day.

Radiation is reduced by having some sort of thick layer (like a metal as it is with aluminum alloy). The thicker (or denser) the better.

Because of the radiation that impacts the thick layer (relatively speaking), the layer will heat up, and the denser the material the faster it will heat up. And it so happens that most materials used are solids (like metal-alloys) and solids so happen to have the properties of conduction (because they are very dense). To minimize energy transfer for conduction, manufacturers use a second layer inside the first one, but contact between the two only happens at the top of the container (mostly in a form of circumference).

If between the two layers there's a fluid, convection will take place, and thus it won't be so good. Manufacturers then prefer to solve thisinconvenienceby eithercompletelyremoving the fluid (make a vacuum), or completely replacing it with an isolating-like material (like gel in case a vacuum is not possible), this way there would be little convection between the two layers.