What is pressure when there are no container walls?

[jbriggs444]

Isn't there a factor of 2, because elastic collisions would reverse the momentum?

You have stuff flowing from one side to the other. That's a factor of one. But you also have stuff flowing from the other side to the one. There's your factor of two.

From the point of view of side A, both flows amount to a momentum transfer in the same direction. You have a loss of material with a toward-the-boundary momentum and a gain of material with a from-the-boundary momentum. Same for side B, of course.

 

[Let'sthink]

Even without a wall and without collisions, there is a rate at which photons (or better, just "stuff") passes through any surface that you choose to define. The rate at which momentum is transferred through that surface per unit area per unit time amounts to a pressure.

You do not have to have a force or collisions to transfer momentum.

If they just pass through there is no change of momentum so they transfer nothing. You need to have a hard surface or you need to imagine it there is no escape if you wish to apply Newton's Laws. It is another matter if photons get reflected from an imaginary mirror how much momentum they transfer. I think they transfer nothing as they lose no energy. In certain sense photons are unlike classical particle we have long chain of replies on this aspect of photon on Physics Forum

 

[Chestermiller]

How did photons get into this discussion?

 

[jbriggs444]

If they just pass through there is no change of momentum so they transfer nothing.

If moving material leaves a system, it carries momentum away with it. If moving material enters a system, it carries momentum with it. A transfer of material transfers momentum.

 

[jbriggs444]

How did photons get into this discussion?

A photon gas was mentioned at least once as a situation with pressure but without collisions.

 

[Chestermiller]

A photon gas was mentioned at least once as a situation with pressure but without collisions.

This was the OP's original intent?

 

[jbriggs444]

This was the OP's original intent?

It seems very much on-point. The original post ponders pressure without a change of particle momentum.

 

[sophiecentaur]

67 posts, all trying to nail down what is only an idealised behaviour of an idealise model.
Pressure is 'the force per unit area that would be there IF a wall was in position. Just 'this side' of the virtual wall, there would be no way of telling if it were there (by looking at the passing molecules) or not. It's just an abstraction that's useful for deriving gas laws from some simple basic kinetics - that's all. Physics is full of such very basic models and people keep believing they're real life. Some things you just have to suck up and go along with them. If you do that, you have a chance of understanding what it's all about.