What causes gas particles to accelerate so that P = F / A

[vanmaiden]

Dear Physics Forums,

Pressure is force / area, and force is mass * acceleration. When you have a gas in a container, it's said to exert a pressure on its container. Therefore, the particles are accelerating toward the container's walls. What's causing these gas particles to accelerate toward the container? Why can't pressure be momentum / area? Momentum, like force, has a tendency to transfer energy as well to a stationary object (the container's walls).

Thank you for addressing my confusion,

Vanmaiden

 

[Studiot]

You are nearly there.

Force = rate of change of momentum.

When a gas particle collides with a container wall it is deflected, suffering a change of momentum (vector change).

This is experienced as a force and a reaction.

The pressure is the aggregate of many such collisions, taken over area and time.

 

[Philip Wood]

Studiot has it. Where you were getting confused is in demanding that the molecule must be accelerating towards the wall. [For an ideal gas we ignore any such effect.] The acceleration occurs when the molecule hits the wall and changes its direction, and therefore its velocity.

 

[Studiot]

Where you were getting confused is in demanding that the molecule must be accelerating towards the wall.

Excellent point.

 

[PJC01]

Dear Vanmaiden,

The acceleration of gas particles towards the container's walls is due to the collisions between the particles themselves and with the walls of the container. Gas particles are in constant motion and collide with each other and the walls of the container, resulting in a net force acting on the walls. This net force, divided by the area of the container, is what we refer to as pressure.

The reason why pressure cannot be expressed as momentum / area is because momentum is a vector quantity, meaning it has both magnitude and direction. Pressure, on the other hand, is a scalar quantity and does not have a specific direction. Additionally, momentum is a property of an individual particle, while pressure is a collective property of the gas as a whole.

I hope this helps clarify your confusion. Feel free to ask any further questions.

Best,