Air Is Stuff
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Many of us hardly think of air as being something that has mass and takes up space, but air fills the space all around us; in that sense, air is indeed “stuff.” Air may be invisible and may not be felt, but it has the characteristics of matter and is, in fact, matter.
Once we recognize air as matter, we can start to understand air pressure. Air has weight due to gravity, and gravity can only act on air if it is matter. To demonstrate air pressure and air acting on the environment around it, Austin set up an experiment with fire, a bottle, and a balloon. Austin filled a balloon with air and tied it off. He then set a piece of tissue paper on fire and put it in a bottle, and he covered that bottle with the balloon. As time went by, the balloon started to slip into the bottle.
Dalton’s law of partial pressures can explain what happened; each element in air has a partial pressure, and they all add up to the total pressure of air. The ideal gas law also explains the relationship between pressure, temperature, and volume. When the fire is in the bottle, the air in the balloon is at a high pressure, and the temperature of the fire in the bottle is high; the pressure increases as the temperature increases. However, when the fire blows out, the air inside the bottle decreases in temperature (and pressure), and the high-pressure air in the balloon is drawn to that area of low pressure, so the balloon sags into the bottle as the air attempts to seek equilibrium.
Since we know that high pressure goes to low pressure, we can understand how the blower wheel creates a pressure differential that brings air into and out of the HVAC system.