Heat energy is described as the kinetic energy of a molecule or atom, but how does that energy present itself?

I keep hearing that it's "random", akin to a randomly jittering atom in space, but I'd like to understand specifically how this energy presents itself in comparison to macroscopic scale things we can see.

If I throw a ball in zero gravity, it will travel along a direction vector at a given velocity, of which you can calculate its kinetic energy and momentum, and everything is predictable enough.

If you had a lone atom at a certain temperature (heat/kinetic energy), is it moving in a relatively consistent manner like the ball, such as a translation along some vector?

If so, are the interactions with trillions of other atoms in dense surroundings that give rise to the "randomness" of its movements?

  • $\begingroup$ The concept of energy is really only defined in terms of 'bulk matter' according to the premises of statistical thermodynamics, so to talk about the 'temperature of a single atom' does not make sense. $\endgroup$
    – theo
    Commented Dec 13, 2014 at 13:03
  • $\begingroup$ @theo that's what I would like explained. I do understand that, but what I want to know is if a bulk of gas has a temperature, what is the individual molecule doing if it wasn't being bumped around by all the other molecules. Is it simply moving in a predictable manner like an asteroid moving through space? $\endgroup$ Commented Dec 13, 2014 at 23:20

1 Answer 1


yes, the collisions between molecules in the gas and or walls in the container can change both the kinetic energy and the direction of motion, like millions of billiard balls moving in a huge pool table. The analogy with the billiard ball is not that good because the billiard balls will eventually end up at rest, as many collisions are inelastic and there is friction from the table. So, energy is dissipated. In a gas instead, energy is not generally lost, but redistributed by the collisions. The solutions to the equations of motion for that are chaotic that is why , for any practical purposes, the collisions and velocities will look random to us.


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