Page 38 - Curriculum Visions Dynamic Book

A coffee percolator is an example of where convection is used. The water is heated from below so that convection will carry the water around and around, and the water will get evenly hot.
In a hot water tank, on the other hand, the heating element may be near the top, with cold water entering at the bottom. When a faucet is turned on, the hot water flows out of the top of the tank, pushed out by the inflowing cold. Here the objective is to keep hot water for the faucets at the top of the tank without it mixing with cold water that has just entered at the bottom of the tank. If this happened, the water reaching the faucet would be cooler.
As you can see, by careful manipulation of the properties of convection, quite different results can be achieved.
Buoyancy
When you drop a log onto the surface of water, it floats. For this to happen, the weight (downwards force) of the log must be balanced by an equal upwards force produced by the water.
The upwards force is called buoyancy. It was first discovered by the Greek scientist Archimedes, and the principle of buoyancy is called Archimedes’ principle.
Archimedes’ principle is often thought of as applying to floating objects, but it also applies equally well to submerged objects. Thus a stone that feels heavy on the ground in air will be much lighter in water. This important fact helps explain why ships float, why rivers can carry boulders, as well as why submarines can remain underwater without having to balance their entire weight
(Right) Buoyancy occurs because the upthrust on the bottom of a floating object balances the weight of the object.
Hot water out
Coldwaterin
(Above) Water heaters have elements in the top of the tanks so that the upper part of the cylinder gets hot. This reduces convection and prevents the hot water mixing with the cold. Hot water can then be drawn from the top of the tank and cold water introduced at the bottom.
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