Physics

Ideal Gas

A few hundred argon atoms in a glass cylinder under a piston, and nothing else. Pressure is the drumming of their impacts, temperature is how fast they fly, and PV = NkT is not a law you have to be told: it is what the counting adds up to.

slow atomfast atomwall hitNkT/V

Drag the piston to squeeze or expand · drag anywhere else to orbit · scroll to zoom · click the plot to switch it. The atoms are hard spheres 0.5 nm across with the mass of argon, in a cylinder 24 nm wide. Because they take up a little room themselves, a hard squeeze reads a few percent above NkT/V.

Pressure, measured—

—

Heater plate

Plate temperature 300 K

coldhot

Piston height 30.0 nm

squeezedroomy

Atoms N 300

Side plot

Time rate 1.00× (100 ps/s)

Spark at every wall hitTrace one atom

Numbers

Pressure, measured—

NkT/V—

Ratio (10 s average)—

Gas temperature—

Plate temperature—

Volume V—

Speed v_rms—

Wall hits per ns—

What to observe

With the plate held at T, push the piston from 30 nm down to 15. Once the plate has taken back the heat of squeezing, the atoms fly as fast as before but hit a box half the size twice as often: P doubles when V halves, back on the isotherm.
Insulate the plate and squeeze to a third. Atoms bounce off the advancing piston faster, like a ball off a swung bat: the gas heats up by about 2× and glows hotter, and its path climbs close to the steeper adiabat.
Hold the piston still and turn the plate from 300 K up to 900 K. Faster atoms hit harder and more often, so the pressure triples with the temperature.
Reset starts every atom at the same speed. Collisions alone spread the histogram onto the Maxwell-Boltzmann curve within seconds; the traced atom shows the zigzag of collisions doing it.

Shortcuts:space run/pause · s step · r reset · f fullscreen