Pressure gradients
Go to the experiment ↓Pressure at one location is only part of the story. To predict the direction of gas flow, compare the pressures at two connected locations. Their difference is the pressure gradient.
Net flow follows the gradient, from higher pressure to lower pressure. With the same resistance, a larger difference produces a larger flow.
Flow = (PA − PB) / R
Positive flow means A to B. R is the fixed resistance of the connecting tube in this model.
Two chambers
Two rigid, equal-volume chambers are joined by a tube with a closed valve. Set the initial pressures, then open the valve to connect them.
Before you start: If both chambers are above atmospheric pressure, can gas still move from one to the other?
- Connect
- Choose Big gradient, then Open valve. Watch the direction of flow and the two pressure traces.
- Compare
- Choose Both high, then open the valve again. Which chamber has the higher pressure?
- Remove the difference
- Try Equal, then Reversed. Predict the flow before opening the valve each time.
About this model. Equal 1 L chambers, constant temperature, and a fixed linear tube resistance. Particle density differences are exaggerated for visibility. These are finite chambers; the surrounding atmosphere is a much larger reservoir.
What happened?
With the Big gradient preset, A begins at +15 and B at −5 cmH₂O relative to atmosphere. Gas transfers from A to B, and their pressures approach +5 cmH₂O. Both high also produces flow, even though neither chamber starts below atmospheric pressure.
Equal pressure means no net flow, even when both chambers still contain gas.
Check your understanding
Once the pressures equalize, do the gas molecules stop moving?
Reveal explanation
No. Random molecular motion continues in both directions. What disappears is the imbalance between those motions: there is no net bulk flow through the tube.
Further reading: OpenStax, The Process of Breathing.