- #1
Werg22
- 1,431
- 1
I have looked for the answer but I haven't found it. Why does a fluid ends up being pushed upward in the presence of a higher density fluid? How is it explained at the molecular level?
russ_watters said:If the fluids don't mix, then they work pretty much exactly like a solid in a fluid, and Archimedes principle can be applied directly at the macroscopic level. There is nothing relevant going on at the molecular level.
pivoxa15 said:The hotter gas will be less concentrated than the cooler one because the hotter gas will move more frequently to more space. So when you look at a given volume of space, there will be more cooler molecules than hotter molecules. So in that volume, cooler molecules are more dense. Now you can apply Archimedes principle - just like you can with fluids.
billiards said:No it doesn't work like that. The molecules themselves are not cool or hot, it is the combined effect of lots of molecules (and their kinetic energies) which makes a parcel of fluid cool or hot.
Agree. Pretty much what I said.billiards said:The molecules in a hotter gas are spaced further apart on average which makes a parcel less dense - once again it is not the molecules themselves which are less dense (unless of course you are considering gases of different compositions in which case the molecules will have different densities - nothing to do with kinetic energy though).
billiards said:it's just plain common sense that the denser one will be more stable at the bottom whether you're considering molecules or not.
russ_watters said:it isn't really relevant to why density affects buoyancy.
Werg22 said:Yes, I'd be interesting into knowing the cause of Archimedes principle.
Werg22 said:pivoxa15, I have to say I find your explanation to be porous. First, Archimedes principle applies to fluids of the same temperature also. Second, solubility is not affected by small temperature changes. The kinetic theory of gas shows us qualitatively that pressure is proportionate to density. That said, I am confused on how to use that information in order to deduce that the lower density gas will go on top.
Werg22 said:Also, a minor correction, pressure is directly proportional to density, not inversely.
One way to see that this must be the case is to imagine a fluid at rest. Would you agree that every "piece" of the fluid is in equilibrium--that the net force on it must be zero? (I hope so!) Now take a parcel of the fluid and picture an imaginary boundary around it. There are two forces acting on that fluid parcel that must be equal and opposite: The weight of the fluid within the boundary, and the force of the fluid outside the boundary pushing on the parcel. That last force is the buoyant force and it must be an upward force exactly equal to the weight of the fluid within the boundary.codec9 said:why is the buoyant force exactly equal to the weight of the fluid displaced?
Buoyancy is a force that acts on an object when it is submerged in a fluid. It is the upward force that opposes the weight of the object and allows it to float.
Archimedes's Principle states that the buoyant force on an object is equal to the weight of the fluid that the object displaces. This principle helps to explain why objects float or sink in a fluid.
Buoyancy is related to density because the buoyant force is dependent on the difference in density between the object and the fluid it is submerged in. Objects with a lower density than the fluid will float, while objects with a higher density will sink.
At the molecular level, buoyancy is explained by the behavior of individual molecules in a fluid. When an object is submerged, the molecules of the fluid exert a force on the object in all directions. This results in a net upward force, which is the buoyant force.
The buoyant force acting on an object is determined by the density of the object compared to the density of the fluid it is submerged in. If the object has a lower density than the fluid, the buoyant force will be greater than the object's weight, causing it to float. If the object has a higher density, the buoyant force will be less than the object's weight, causing it to sink.