The Motion of Gas Particles, and Transport Processes

Abstract

Why does the air around us cool when we climb a mountain? Climbing in the mountains does, of course, take us a minuscule distance closer to the Sun; but air derives almost no direct warmth from the Sun: the interaction between solar photons and air molecules is very weak. Rather, the Sun’s radiation interacts strongly with the ground, and the warmed ground then heats the air. With mountain peaks all around, the Sun can rise later or set earlier in high country.

In which we study the velocity and speed distributions of particles in a gas. We use these to examine the temperature gradient in our atmosphere, and the composition of planetary atmospheres. We find out how to relate viscosity, thermal conductivity, and heat capacity using an atomic view of matter. We finish by describing the energy–momentum tensor that plays a key role in Einstein’s theory of gravity.