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Pressure from molecular collisions
Intermediate
Gas pressure arises from momentum transfer to walls.
P = ⅓ρ⟨v²⟩Each molecule hitting a wall transfers momentum 2mv_x. Averaging over huge numbers gives P = ⅓ρ⟨v²⟩ = ⅓(N/V)m⟨v²⟩.
Air at 1 atm: about 10²⁷ collisions per m² each second.
Pressure is from the average, not a single hit.
In more detail
Gas pressure arises from molecules colliding with container walls and changing their momentum. Averaging over many collisions gives a steady pressure P = ⅓ρ⟨v²⟩.
Real-world uses
- Tyre pressure comes from countless molecular impacts on the rubber.
- Pressure gauges measure the average effect of molecular collisions.
- A balloon stays inflated because billions of molecules hit its inner surface every second and push outward.
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Frequently asked questions
Why is pressure steady if collisions are random?
Because there are enormous numbers of collisions every second.
Does pressure depend on temperature?
Yes, because faster molecules hit the walls harder and more often.
Why does gas pressure increase when I heat it?
The molecules move faster, so they hit the walls harder and more often, raising the pressure.
Related topics in Kinetic Theory & Statistical Physics
Molecular picture of matter · Brownian motion · Ideal gas assumptions · Kinetic interpretation of temperature · Boltzmann constant · RMS speed