Page 37 - Curriculum Visions Dynamic Book

burning material. Furthermore, water smothers the burning substance, preventing oxygen from getting to it and so cutting off the cause of combustion.
Water containing wetting agents to reduce surface tension can also be applied to structures near a fire. This so-called “wet” water, when applied with foaming agents, clings to surfaces better than ordinary water.
Convection of heat
When a solid is heated, the heat moves from one molecule to another. This process, conduction, was described above. The solid swells a little and become less dense as it warms because it has more energy in it, and the molecules begin to vibrate more vigorously. However, the molecules never change position.
When a liquid such as water is heated, the heated molecules (for example, those touching the bottom of a saucepan on a hob) receive heat by conduction, and they expand and so become
less dense, just like a solid. But the molecules in a liquid are
not locked together in the same way as in a solid. Instead, they can move, and less dense molecules start to rise up through the colder, more dense molecules like corks bobbing to the surface. This, in turn, causes cold molecules to flow in to take the place of the rising hot molecules. In their turn the cold molecules are heated and begin to rise.
In this way an upwards flow of warmer molecules is balanced by a downwards flow of colder molecules. This causes a flow of water to begin, which is called convection.
Convection depends on heat being added to the water from below. If the source of heat is near the surface, then convection cannot happen. This has very important results. In nature most ocean
water is heated from above by the Sun. As
a result, the heated water remains on the
surface and does not mix with the cold
water below. That is why convection is
uncommon in the world’s oceans, and why
the heat that penetrates into the deeper
regions of the oceans is from conduction.
The effects of both conduction and convection are widely used in designs.
(Below) Convection in a saucepan of water heated from below.
37