What is convection?

Answer 1

Convection is the transfer of thermal energy (i.e. heat) due to the bulk motion of a material. This contrasts with conduction, the transfer of thermal energy without the bulk motion of material.

For example, if you put a pan on a conventional stove burner, the entire pan will eventually gain thermal energy, as the energy from the burner is transferred into the pan by first causing vibrations in the molecules of the pan's crystal structure next to the burner. Those vibrations cause the adjacent molecules to vibrate, then the ones adjacent to those, and so on. Eventually, the entire pan has increased thermal energy. This is conduction.

What does NOT happen is that a piece of the pan near the burner gains thermal energy, then breaks off and moves to another part of the pan to heat up that other part. Solids don't generally have sections break off and move to another part of the structure.

Fluids, on the other hand, can do that. If there is water in the pan, the molecules of water near the pan's surface can heat up by conduction, as the pan transfers thermal energy to the water. But if some amount of water absorbs enough thermal energy, it can buoy up and float to the top, carrying the thermal energy from one part of the water to another by bulk motion. This occurs in what is commonly called a "rolling boil," and is called convection (since the thermal energy is conveyed (i.e. transported) by the moving fluid.