Convection is the movement of molecules within fluids (i.e. liquids, gases and rheids). It cannot take place in solids, since neither bulk current flows or significant diffusion can take place in solids.
Convection is one of the major modes of heat transfer and mass transfer. Convective heat and mass transfer take place through both diffusion – the random Brownian motion of individual particles in the fluid – and by advection, in which matter or heat is transported by the larger-scale motion of currents in the fluid. In the context of heat and mass transfer, the term "convection" is used to refer to the sum of adjective and diffusive transfer.[1]
Natural convective heat transfer
When heat is transferred by the circulation of fluids due to buoyancy from the density changes induced by heating itself, then the process is known as natural convection or free convection.
Familiar examples are the upward flow of air due to a fire or hot object and the circulation of water in a pot that is heated from below.
For a visual experience of natural convection, a glass that is full of hot water filled with red food dye may be placed inside a fish tank with cold, clear water. The convection currents of the red liquid will be seen to rise and also fall, then eventually settle, illustrating the process as heat gradients are dissipated.
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