# Do gasses ever move via anything other than a pressure differential?

Asked by Poser (7800) September 19th, 2008

Every example I can think of involves some sort of pressure differential—wind, balloons, breathing, exhaust, jet blast, rotor wash—all involve movement of air due to pressure differentials. Is there any other mechanism by which gasses can move?

Observing members: 0 Composing members: 0

Gases (sp!!) are always in motion.

SRM

srmorgan (6740)

They can move via physical force.

Believe me, I know!

PupnTaco (13850)

Gravity keeps the atmosphere close to the Earth, but i guess that is still a pressure differential.

if you consider the definition of “Pressure Differential” – The difference in pressure between two points of a system – then I guess any time you are talking about movement of any kind, there has to be a pressure differential.

Once something moves, the density (a measurement of pressure) of the space it occupied changes (or is made different).

fireside (12302)

Sometimes when I eat beans I create gasses in my body that I am able to move out of my body with extreme force. Sometimes it even makes noise and smells. :P

big3625 (41)

Sometimes the pressure differential involved is the effect not the cause, like when a physical force is applied.

two gasses placed next to each other with each under the same pressure will mix with each other by diffusion

eyeguy (25)

1. Get jam jar.
2. Put gas in jam jar.
3. Close jam jar.
4. Move jam jar.
5. Open jam jar.

But I agree with what lightminutesaway, but also as others have said particles are always moving in anything. Absolutely anything. Therefore diffusion is possible from what eyeguy said. Although actual movement of a gas is always to do with pressure. The gas particles will always move into a space where there are none, therefore moving from high pressure to low pressure, even if the difference is minute.

Skyrail (1695)

Density variations will cause movement that’s not related to pressure. If I open a plastic bag containing argon (a dense gas) up near the top of a chamber containing helium (a light gas), the argon will spill downward and pool at the bottom of the chamber. There would be no pressure difference involved and no mechanical impetus needed.

Harp (19105)

@Harp—but wouldn’t there be a pressure differential when a denser gas tries to fill the same space as a less dense one?

Poser (7800)

Think of it this way:
I have a chamber filled with helium. That chamber will be at a certain pressure. If through some mechanical slight of hand I introduce my bag of argon (assume that the bag is only loosely filled, not “pressurized”) into the chamber, I will have slightly increased the overall pressure in the chamber by virtue of adding this extra volume of the bag. But before opening the bag, the pressure inside and outside the bag would be identical, right? The fact that outside the bag is helium and inside the bag is argon doesn’t change the fact that they’re both under the same pressure. If the argon were under higher pressure, it would simply expand the bag until equilibrium was reached.

Now, when the bag is opened, the downward flow of the argon can’t be ascribed to a pressure differential since none exists. It’s purely gravity at work

Harp (19105)

Ah. I see.

Poser (7800)

I’m wondering if temperature changes would .Heating a gas makes it expand as the molecules move around more.

Bioplasmic (123)

when a gas is heated, the molecules move faster, creating higher pressure. this is why you do not throw pressurized cans in a fire… they will explode :)

I don’t know Harp leaving the assumptions behind, it still sounds like a pressure differential to me.

The combination of gravity and the density of the argon creates more downward pressure than the upward pressure created by the combination of gravity and the density of helium.

That said, I was a video production major, so i’m about as qualified to answer this as I am to answer one about a personal experience with breast augmentation.

fireside (12302)

Well Harp already explained the idea behind body forces fairly well (especially the equilibrium pressure at release of the gas), but technically buoyancy could probably be considered a pressure differential though. A lower density gas or object within a higher density gas or liquid moves because of a force imbalance on it. One force is the force of gravity acting on the volume of the immersed object and the immersion fluid, the other is the pressure force of the immersion fluid acting on the surface of the immersed object. So fireside is technically correct in this case.

Shuttle128 (2979)

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