Gas pressure as molecular momentum transfer
Molecular collisions link macroscopic pressure to random translational motion.
Key knowledge
For a dilute classical ideal gas with isotropic molecular velocities in its rest frame, elastic wall collisions transfer momentum and give pV=Nm⟨v²⟩/3. Combining this with pV=Nk_BT gives mean translational kinetic energy m⟨v²⟩/2=3k_BT/2 and rms speed √⟨v²⟩=√(3k_BT/m). The rms speed is distinct from the mean speed and the net drift velocity.
Values and equations
pV=Nm⟨v²⟩/3=Nk_BT; ⟨K_trans⟩=m⟨v²⟩/2=3k_BT/2; v_rms=√⟨v²⟩=√(3k_BT/m)
- p
- Equilibrium gas pressure Pa
- V
- Gas volume m³
- N
- Number of identical gas particles 1
- m
- Mass of one gas particle kg
- v,v_rms
- Particle speed relative to the gas rest frame and root-mean-square speed m s⁻¹
- ⟨v²⟩
- Ensemble mean squared particle speed m² s⁻²
- ⟨K_trans⟩
- Mean translational kinetic energy per particle J
- k_B
- Boltzmann constant J K⁻¹
- T
- Absolute equilibrium gas temperature K
Conditions and scope
- Use N>0 identical particles of mass m>0 in volume V>0, at uniform equilibrium temperature T>0; translational velocities are relative to the gas rest frame.
- The collision derivation assumes an ideal dilute gas, isotropic motion, elastic walls and negligible interparticle potential/volume corrections.
- The translational energy is not the complete internal energy of a molecule with accessible rotational, vibrational or electronic degrees of freedom.
Sources · 2
References
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Evidence and review
Elastic wall momentum transfer and an isotropic dilute ideal-gas velocity distribution give the pressure–mean-square-speed relation.
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Foundational review
2026-10-10 · Independent AI editorial inspection of this new compact reference entry, its equations, applicability, sources and knowledge connections.
Sources from other places
Agreement within this scope
The isotropic classical ideal gas connects pressure with mean squared random speed and translational energy with 3kT/2.
Root mean square speed differs from mean speed and drift velocity; molecular internal modes add separate degrees of freedom.
National Council of Educational Research and Training (NCERT)
- Kinetic Theory · Physics (2026–27 reprint) IndiaEnglish
12.4.1 Pressure; 12.4.2 Kinetic interpretation; 12.5 degrees of freedom
- Kinetic Theory · Physics (2026–27 reprint)
Entry details
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- foundation-physics-kinetic-gas-pressure
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- foundations-audit.1
- Structured reference record
- Foundational core completion