# Can pressure exist without a container?

I always hear pressure defined as the force exerted by particles on the walls of the container they're being held in. This makes sense since the mathematical definition of pressure is $p = \frac{F}{A}$. So, can pressure exist without walls to exert force on?

My understanding of pressure motivates me to think that even without a container, particles of gas in a vacuum could create pressure since their collisions with each other result in forces being exerted on areas (the areas being the surfaces of the particles being collided with).

On the other hand, a liquid in a vacuum could not exert pressure since on a microscopic level, the particles of liquid can't collide due to the cumulative strength of the bonds holding them together.

• "particles of gas in a vacuum" - if there's particles in it, it's not vacuum! Also, which "container" do you think the earth's atmosphere is in? – ACuriousMind Oct 4 '15 at 0:01
• an example would be a self gravitating ball of gas, such as a star. – user83548 Oct 4 '15 at 0:07
• Ah, yes you're correct @ACuriousMind. And thank you for the better example @brucesmitherson! – user94491 Oct 4 '15 at 0:13
• Yes, and you're almost right about how to define pressure at a point in a gas; rather than using the collisions of one particle with another, one uses an an imaginary plane at point. – Mozibur Ullah Oct 4 '15 at 0:13
• "an example would be a self gravitating ball of gas, such as a star." Another example of how an (enormous) pressure can exist even without containing walls is a 'dynamic' situation, such as the laser-driven implosion of a NIF capsule. In this case very large pressures are supported by the inertia of the mass of the capsule itself. Of course such a situation can only exist for a short time before the capsule flies apart. – Samuel Weir Oct 4 '15 at 1:11

The pressure of a gas is defined as the force the gas would exert upon a surface or container. However, there is no need for a container for pressure to exist. For instance, the air you're breathing right now (unless you're in an airplane or submarine) has pressure due to the column of atmosphere above you. Stars are balls of gas (plasma, actually) that are pressurized by gravity; no containers to be seen.

• Excellent answer, Mr. Griscom! Thank you very much. – user94491 Oct 4 '15 at 2:56