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Physical Properties of Air
Air, in an of itself, is a gas which displays the same basic characteristics of any gaseous substance; a series of molecules floating around with spaces in between. Like any other material, air molecules possess corresponding properties of mass, momentum and energy. Gases, in particular, are characterised by how their individual molecules behave, as well as by how the gas as a whole behaves. As a result, any force or pressure applied will create a molecular effect, as well as a counter-effect of the whole mass upon its individual particles. Even in a so-called static state, individual gas molecules are in constant motion, moving about and colliding with one another at will. In essence, applying heat to anything is a way of adding energy to whatever molecules are present. Just like most every material in nature, when heat (or energy) is applied to air, the individual molecules begin to move faster.
Effects of Heat
Even though the molecules floating around in the air are small, they do exert a certain amount of pressure on objects within their reach. The temperature of the air plays a big part in how much pressure this is. The amount of pressure depends on how densely packed these moving molecules are. Since movement increases when heat is applied, air molecules become less densely packed, and begin to spread out. This is actually a cumulative effect in that these faster moving molecules are now generating heat, or energy of their own. The more heat that's applied, the faster they move, and the further apart they spread. This accounts for why cold winter air can feel heavier than the dry airs of summer, and why the heated air in a home will rise while the cold air settles.
Effects of Gravity
Just like all materials in the physical world, air molecules are affected by the force of gravity. And, as with all objects, the heavier or denser an object is, the greater gravity's pull on it. As far as air is concerned, a phenomena called "convection" is used to describe gravity's effects on heated air, and how it circulates. If air is contained in a space, like an oven, and heat is applied to the bottom layer, the air molecules on the bottom become highly agitated, and begin to spread apart. When this happens, this bottom layer begins to rise until it reaches the top of the oven. In the process, the cooler air molecules are pushed downward. The molecules that are now at the bottom become heated, and start to rise to the top as well. This cycle keeps repeating for as long as heat is applied, which is how convection ovens work. Gravity continues to pull down the heavier molecules as the lighter ones rise to the top.
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