Viscosity and Upthrust....


by Cheman
Tags: upthrust, viscosity
Cheman
Cheman is offline
#1
Apr16-05, 07:20 PM
P: 236
Viscosity and upthrust are both forces which occur in liquids, and must both rely on electrostatic effects. So what is there difference between them? What causes these 2 distinctly different effects?

Thanks in advance.
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shyboy
shyboy is offline
#2
Apr16-05, 10:13 PM
P: 138
none of the really rely on the electrostatic effects. That means that we do not need to know anything about electricity to explain them. What is important is that the liquid molecules may collide to each other.

The upthrust comes from the fact that liquid has no shape. So at the equilibrium the pressure on the piece of liquid is the same in all directions. That is pure mechanics, the potential, elastic and kinetic energy are players there.

The viscosity comes from thermodynamic. The important player in this game is thermal energy. Now the process becomes irreversible. That is kind of tricky and non-trivial- you need to use reversible mechanical equation to introduce irreversibility. Without going into deep, you may just use known thermodynamical equations. Friction heats.

Both viscousity and upthrust exist in gases, too.
Cheman
Cheman is offline
#3
Apr17-05, 05:52 AM
P: 236
But surely it must be an electrostatic effect if it relies on collisions? Afterall, particles when they collide do not actually touch as we often depict them classically ( unless travelling ridiculously fast, eg - after passing through a particle accelerator.) but simply experience repulsion effects.

shyboy
shyboy is offline
#4
Apr17-05, 12:17 PM
P: 138

Viscosity and Upthrust....


yes, of course it is electrostatic because we know that there is no other force responsible. But we don't need to know that to explain these effects. Archimedes knew nothing about the Coulomb. The problems are limited the mechanics and thermodynamics, no theory of electricity is needed (in a first approximation).

But if we are working with high energies, when atom and molecules can break down, then yes, we need to know the nature of the binding forces. At low energies only the intermolecular forces are important, and, in the first approximations, we can explain almost everything if we simply assume elastic collisions of solid structures.

I probably got your question.
The viscosity is an energy redistribution between constituents. It exists without any external force or field. The upthrust exist only if we have an external field- there is no upthrust without gravitation.

Viscosity appears because of the internal degrees of freedom of the macroscopic body. Upthrust is due to the interaction of the macroscopic body with the external field.


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