Upthrust and particles

QUESTION:

How can upthrust be explained to a Year 12 set in terms of the particle model. They are comfortable with the idea that an increase in pressure can be explained by an increase in the number of particles colliding with a surface, or an increase in the temperature/energy of these particles, or a combination of both. But we struggled to use this concept to explain the pressure differential between the upper and lower surfaces of a submerged object.

Answer

To begin with I think it is important to distinguish between the behaviour of particles in a real gas and between those in a liquid. I do not really think it is helpful to try and explain upthrust and pressure differential in terms of particle collision. So I will consider upthrust from a particle point of view but not from a particle collision standpoint.

It seems to me that if we can explain why liquids have a weigh in a gravitational field we can justify upthrust since Archimedes principle talks about the weight of fluid displaced.
In liquids there are intermolecular forces and if you attempt to compress a liquid these intermolecular forces rise rapidly.

Every molecule has a weight due to the gravitational attraction the Earth. If you consider a liquid to be made of very thin horizontal layers of molecules then each layer will have weight and the layers near the bottom will have the weight of the layers of particle above pressing down on them. This will tend to increase the forces between the molecules and this increase in force will be transmitted to the base of the container as weight by the lowest layer of molecules. Hence the liquid has weight.

The liquid will be in dynamic equilibrium, molecules moving between one layer and another but since the molecules are indistinguishable from one another this should not affect the above argument. The molecules will collide with the walls of the container but I do not believe that these collisions contribute to the weight of the liquid.

If you now agree that a liquid has weight from a particle point of view then Archimedes follows directly.