Lesson 2 of 6 · Pressure differences & the ideal gas model
Pressure comes from particle motion
9 minute lesson · 3 checks along the way
Your goal for this lessonUse absolute pressure and kelvin temperature in a bounded gas model, and distinguish pressure loading from temperature.
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A gas pushes in more than one direction
Gas particles continually collide with a container’s walls, transferring momentum. The collective effect produces : normal force per unit area. A pascal is one newton per square meter. In an ordinary gas at equilibrium, pressure acts in every direction, not just downward. A pressurized spacecraft cabin therefore pushes on its roof, floor, and side walls even when the vehicle and crew are in free fall.
A wall responds to the pressure difference between its two sides. For an ideal flat panel with uniform pressures, multiply that difference by area to obtain the net pressure force. The structure supplies opposing forces if the panel remains still. This relationship does not supply a complete structural design: shape, joints, material strength, fatigue, and safety margins matter alongside the simple load calculation.
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State what stays fixed
The model relates P, volume V, particle count N, and absolute temperature T through PV = NkT. Here k is a physical constant. Use kelvin for T and pressure measured relative to vacuum for P. A gauge reading relative to surrounding air must be converted before it is used in this equation. The model works best when particle volume and interactions are small corrections.
At fixed particle count and volume, pressure is proportional to . At fixed count and temperature, doubling volume halves pressure. A real process may change several quantities at once: gas can escape, the container can expand, or compression can do work that raises temperature. Saying that compression doubles pressure is incomplete unless the temperature and amount of gas are specified too.
Work through a decision
Read a warming-tank report
In a fictional sealed rigid tank, dilute gas has an absolute pressure of 80 kilopascals at 250 kelvin. It warms uniformly to 300 kelvin without leaking or reacting. Fixed N and V give a pressure multiplier of 300/250 = 1.2. The new absolute pressure is 96 kilopascals. The increase is 16 kilopascals, not a doubling based on a Celsius scale or a change in the tank’s mass.
For a separate invented panel, take an internal pressure of 100 kilopascals, negligible external pressure, and an area of 0.02 square meters. The outward pressure force is 100,000 × 0.02 = 2,000 newtons. These two examples answer different questions. One predicts a gas state; the other predicts a load from a specified state. Neither number is a measurement or a design specification for the pictured spacecraft.
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Make vacuum a measurable condition
The TEMPO laboratory photograph shows an instrument being installed for thermal-vacuum testing. Pumping a chamber removes much of its gas, reducing pressure and changing transfer. It does not guarantee that every particle is gone or that every component reaches one particular temperature. Engineers record pressure and temperature separately because an apparently cold-looking metal surface is not a pressure gauge.
A gas can be sparse yet have energetic particles, and a dense gas can be comparatively cool. Pressure depends on both and temperature in the ideal model. Near condensation or under sufficiently strong compression, a real-gas description becomes important. The useful habit is to identify the gas, its state, and the variables held fixed before applying a proportionality that only works under those conditions.
TEMPO is being installed for thermal-vacuum testing at Ball Aerospace in Boulder, Colorado, in this NASA photograph published in June 2018. The chamber lets a test team control and measure a low-pressure environment. Its dark interior and the instrument’s coverings do not reveal the chamber pressure or any component’s temperature.
Look closely
Which measurements would establish the test conditions more directly than the visible darkness of the chamber?
See what to notice
A calibrated pressure measurement and temperature sensors at relevant locations would establish different parts of the test state. A photograph shows the arrangement of hardware. It cannot substitute for pressure, temperature, or residual-gas measurements.
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Put it together
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