Why convection happens when concentration gradients exist?

[lionelwang]

Hello, everyone!

I could not imagine what force makes this convection under concentration gradients , no matter how hard I thought.
Maybe we can use principle of entropy increase to explain it, but that is not vivid to me.

anyone can help me? Thans!

 

[jVincent]

Consider a closed container which we mentally divide into two equally shaped and sized sections, denoted the left and right sections.

If at zero time there are a million particles in the left side, and no particles in the right, and all these particles are bouncing around randomly, how many particles would you expect to end up in each section if we wait long enough for the average number of particles in each to be constant?

Naturally half will end up in each section. So during the waiting period, half the particles moved to the right, due to the concentration difference. This is the diffussive flow, which as you suggest is due to the entropy increase. There is only one way to have all particles in the left compartment but very many ways to have half the particles in each compartment.

 

[lionelwang]

Thank you for your explanation! It is vey clear and easy to understand, maybe it is a problem of statistics as you explained, the force which result in the uniform distribution of the molecules is not important.

Consider a closed container which we mentally divide into two equally shaped and sized sections, denoted the left and right sections.

If at zero time there are a million particles in the left side, and no particles in the right, and all these particles are bouncing around randomly, how many particles would you expect to end up in each section if we wait long enough for the average number of particles in each to be constant?

Naturally half will end up in each section. So during the waiting period, half the particles moved to the right, due to the concentration difference. This is the diffussive flow, which as you suggest is due to the entropy increase. There is only one way to have all particles in the left compartment but very many ways to have half the particles in each compartment.