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Molecular diffusion, often simply called diffusion, is the thermal motion of all (liquid or gas) particles at temperatures above absolute zero

As I quoted through the Wikipedia,

The diffusion should be affect of the thermal motion. As on it my first feeling was what is the fundermental force acts on it with the main 4 fundermental forces. I found much details from Force causing diffusion but arise a question considering a single particle.

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As this ,when we consider a set of particles, each can collide with the other particle, the particles are moving from the more concentrated place to the lower one.

If we take a vacuum tube and keep a particle there, in the temperature of absolute zero, and then slowly increasing the temperature, the particle starts to move with the thermal motion.The is no way for any collision or way to vary it's momentum which force affects for this motion by the main 4 fundamental forces?

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  • $\begingroup$ I'm not sure what the question is here? $\endgroup$
    – Gert
    Commented Sep 10, 2019 at 15:34

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If the single particle is in a vacuum tube, then there really isn't a way to define the temperature. Temperature is a concept that only applies to collections of a very large number of particles, since temperature is somewhat a measure of the average kinetic energy of a system.$^*$ But for your single particle it has a well defined kinetic energy already. We don't (can't?) apply the ideas of statistical mechanics to it.

Therefore, a particle in a tube that is otherwise empty will just move in straight-line trajectories until it bounces off of a wall and changes direction. There will be absolutely no diffusion in this case, and its motion can easily be described using Newton's laws.

However, if you have your single particle in a tube filled with some medium, then you could observe the particle to be "diffusing", and this is what Brownian motion is. This is what your first animation is showing. The single particle that is shown is obviously not the only thing in the tube. There is most likely smaller particles present such that thermal fluctuations case the shown particle to jostle around in the way it does.

$^*$ I say somewhat because other degrees of freedom not related to the kinetic energy can contribute to the temperature of a system, but for a system of non-interacting, single particles (like an ideal gas) then it is fine.

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  • $\begingroup$ Thanks for giving attention to my post. Couldn't we take a vacuum tube and keep a metal tray which can heat by electric force and keep a Iodine particle on that and change the temperature of the single particle by heating through the electricity. As there should be no motion due that not having a external force in the vacuum tube. $\endgroup$
    – Osal Thuduwage
    Commented Sep 10, 2019 at 15:53
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    $\begingroup$ @OsalThuduwage If you want to devise a way to add energy to the particle that is fine. It just won't undergo Brownian motion if it is the only thing in the container. You could change its trajectory with E&M radiation too, but that still isn't diffusion. A single particle does not have a temperature. Just energy. Temperature applies to large ensembles of particles. $\endgroup$
    – BioPhysicist
    Commented Sep 10, 2019 at 15:59

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