what is convection what what what
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Answer:
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Convection is the heat transfer due to the bulk movement of molecules within fluids such as gases and liquids, including molten rock (rheid). Convection includes sub-mechanisms of advection (directional bulk-flow transfer of heat), and diffusion (non-directional transfer of energy or mass particles along a concentration gradient).
Explanation:
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Answer:
the movement caused within a fluid by the tendency of hotter and therefore less dense material to rise, and colder, denser material to sink under the influence of gravity, which consequently results in transfer of heat:
Explanation:
Convection cannot take place in most solids because neither bulk current flows nor significant diffusion of matter can take place. Diffusion of heat takes place in rigid solids, but that is called heat conduction. Convection, additionally may take place in soft solids or mixtures where solid particles can move past each other.
Thermal convection can be demonstrated by placing a heat source (e.g. a Bunsen burner) at the side of a glass filled with a liquid, and observing the changes in temperature in the glass caused by the warmer fluid circulating into cooler areas.
Convective heat transfer is one of the major types of heat transfer, and convection is also a major mode of mass transfer in fluids. Convective heat and mass transfer takes place both by 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 combined effects of advective and diffusive transfer.[1] Sometimes the term "convection" is used to refer specifically to "free heat convection" (natural heat convection) where bulk-flow in a fluid is due to temperature-induced differences in buoyancy, as opposed to "forced heat convection" where forces other than buoyancy (such as pump or fan) move the fluid. However, in mechanics, the correct use of the word "convection" is the more general sense, and different types of convection should be further qualified, for clarity.
Convection can be qualified in terms of being natural, forced, gravitational, granular, or thermomagnetic. It may also be said to be due to combustion, capillary action, or Marangoni and Weissenberg effects. Heat transfer by natural convection plays a role in the structure of Earth's atmosphere, its oceans, and its mantle. Discrete convective cells in the atmosphere can be seen as clouds, with stronger convection resulting in thunderstorms. Natural convection also plays a role in stellar physics.
The convection mechanism is also used in cooking, when using a convection oven, which uses fans to circulate hot air around food in order to cook the food faster than a conventional oven.
Example
1. Convection in water
Fill a beaker with cold water and then carefully drop a few crystals of potassium permanganate into it so that they fall close to one side of the beaker. Now heat the base of the beaker just under where the crystals have fallen.
You should see the colour rise up this side, go across the top and then fall down the other side of the beaker — this is a convection current.
You can also use the special piece of apparatus shown in the diagram. It is a "square " glass tube filled with water. Drop a crystal of potassium permanganate into the top and then heat one of the bottom corners gently. You will see the colour begin to move round the tube going down the limb opposite the heating and then rising up the other side above the Bunsen due to convection currents in the water.
2. Convection in air
(a) simply hold your hand above the bunsen flame. You can easily feel the hot air rising.
(b) Use the apparatus shown in the diagram. Light the candle and then hold a piece of smouldering paper or string over the top of the other chimney.
The smoke should be pulled down that chimney and rise up the other chimney with the hot air above the candle.
3. Make a small rotor out of aluminium foil and hold it above a bunsen burner to test the effect of the convection currents in the air.
Convection occurs because the air (or other fluid) is heated, expands, becomes less dense and so rises through the more dense colder air
Land and sea breezes
In the daytime the land heats up quickly, the air above it rises and cool air is drawn in from the sea.
At nighttime the land cools quickly while the sea stays warm. Air rises above the sea and a breeze blows off the coast onto the sea.