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Basic State Variables
Basic State Variables
Temperature (T) / Density (ρ) / Pressure (P)
Pressure
Pressure
Air pressure can be thought of as the weight of a column of air extending from the Earth's surface to the top of the atmosphere.
Why does pressure decrease as altitude increases? less of the atmosphere above you = less air pressure
How do we measure pressure?
How do we measure pressure?
Mercury Barometer
Mercury Barometer
The weight of mercury is balanced by the force of the air (pressure) on the mercury in the dish.
Aneroid Barometer
Aneroid Barometer
Works under the principle of deformation. Changes in external air pressure cause a small, flexible metal box to expand or contract and move mechanical levers. (Used in altimeters)
Ideal Gas Law
Ideal Gas Law
P=ρRT
P is pressure (units: Pa)
ρ is density (units: kg/m3 )
R is the gas constant for dry air (units: J/kg*K)
T is temperature (units: K)
We can substitute the definition for density (ρ= m/V) to put the ideal gas law into terms that we can easily measure:
PV=mRT
Where P, R, and T are the same as before, and
V is volume (units: m3 )
m is mass (units: kg)
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