Wind and Air: Compressible Gas Models

Use absolute pressure and temperature in ideal-gas comparisons.

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For a fixed amount of ideal gas, pV is proportional to absolute temperature T. At constant T, p1V1 = p2V2. Use absolute pressure, not gauge pressure, and kelvin temperature, not Celsius ratios. These are calculation models, not home pressure experiments.

Worked example

At fixed temperature, 2 L at 100 kPa absolute becomes 1 L at 200 kPa absolute. Halving the volume doubles pressure. Heating a rigid container from 300 K to 600 K also doubles absolute pressure if the amount of gas stays fixed.

The same amount of ideal gas at the same temperature is shown at 2 liters and 100 kilopascals, then at 1 liter and 200 kilopascals. Boyle law keeps absolute pressure times volume constant.

Would you believe it?

Invisible air presses on surfaces in many directions, not only downward. A difference in pressure on opposite sides of a surface can produce a net push.

NASA: air pressure

Question 1 At constant temperature, 2 L at 100 kPa is compressed to 1 L. Final pressure is what? 200 kPa 50 kPa 100 kPa

Question 2 Which pressure should be used in pV = nRT? Gauge pressure without correction Wind speed in m/s Absolute pressure

Question 3 At fixed volume and gas amount, 300 K becomes 600 K. Pressure changes by what factor? 4 2 1/2

Question 4 Why not substitute Celsius temperature ratios directly? The gas law requires an absolute temperature scale Celsius numbers are always negative Kelvin has no units

Question 5 What could invalidate the fixed-amount comparison? Keeping temperature constant Writing pressure in pascals Gas leaking out

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