The Ideal Atmosphere
A Virtual Laboratory

The atmosphere is made up of an incredible number of molecules whose basic interactions obey Newton's laws of motion and Newton's law of universal gravitation with the Earth. These simple rules, applied to all those molecules, produces an atmosphere whose density decreases with altitude. If one kind of molecule is more massive than another, we also see that the more massive molecules stays closer to the ground (on average) than the less massive molecules. The work done by gravity sees to this.

The simulation running below shows an ideal gas of non-interacting particles. The particles obey Newton's laws of motion, with the ground temperature determining their average kinetic energy through thermodynamics. They start off at ground level with random placement and velocity direction, and undergo a series of elastic collisions with the ground as they climb upwards under the influence of gravity.

You can control several of the simulation paramters and see what changes they make in the "atmosphere's" behavior:

  1. The slide control on the left, labeled N, sets the number of molecules bouncing around. It lets you change the overall gas "density."
  2. The middle slide control, labeled M, lets you make half of the molecules more massive than the other half. It sets the ratio of the masses of the two types of molecules. (When the simulation first starts, the mass ratio is set to 1, i.e., all the molecules have the same mass.) How do the relative altitudes reached by the different kinds of molecules change with mass?
  3. The right slide control, labeled T, sets the ground temperature. The warmer the ground is, the bigger the "kick" it can give to a molecule that hits the ground. This will change the altitudes reached, also.

Take control of your atmosphere, and see what happens!

Applet by Christopher J. Grayce, UC Irvine (