Molecular Motion & Temperature

Elastic collisions drive nonequilibrium velocity populations toward the 3D Maxwell–Boltzmann speed distribution.

300 K

Changes all molecular speeds while retaining the current distribution shape.

180
1.00×

Effective area σ = πd², relative to the baseline. Larger molecules collide more frequently; number density stays fixed.

Each preset is normalized to the same initial mean kinetic energy, and therefore the same temperature.

2.0×
Change the starting distribution and select Restart to observe relaxation. Particles interact by equal-mass, perfectly elastic 3D collisions. The display projects their 3D positions onto a 2D view.

Molecules in a 3D box

Collisions active
SlowSpeed relative to thermal RMSFast
Measured temperature—
Mean speed—
RMS speed—
Collisions0

Molecular speed distribution

Observed Theory (3D MB)
Histogram bars show a probability density (area ≈ 1), not raw counts. The orange curve is the theoretical 3D Maxwell–Boltzmann distribution at the measured kinetic temperature. Air is approximated as equal-mass molecules with molar mass 28.97 g/mol. Because the sample is finite, expect some fluctuations around theory.

Teaching model: hard-sphere ideal gas; translational energy only; no rotations, attractions, or chemistry. Cross-section controls effective hard-sphere diameter (σ = πd²), not physical mass density. Collision rate and box size are illustrative, not to scale. Individual collisions conserve total momentum and kinetic energy (within numerical precision).