# The Naked Scientists Forum

### Author Topic: What forces are acting on a gas in a sealed box?  (Read 2858 times)

#### McQueen

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##### What forces are acting on a gas in a sealed box?
« on: 31/07/2012 13:03:09 »
Here is an experiment, imagine a sealed box with sides of 10 cms, filled with air at a pressure of 3 Kg/cm2  .  What is in the box ? It is sealed from atmospheric pressure, yet although the pressure inside the box is at 3 Kg/cm2  meaning that the force acting on each side is 300 Kgf ( i.e., 2940 N),  gravity hardly effects the contents of the box since the air inside it weighs only about 3.75 gms and   therefore the box can easily be lifted.  So what is the force acting inside the box, is it the sum of the kinetic energy of the individual molecules each striking the inner surface of the box ? If so would it be right to say that the energy inside the box is the kinetic energy of the gas (air) in the box?
« Last Edit: 06/08/2012 09:28:40 by chris »

#### McQueen

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##### Re: Pressure in a gas
« Reply #1 on: 01/08/2012 03:04:02 »
Error here, is should be ............ air inside it weighs only 3.75 gms and not Kgms !

#### evan_au

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##### Re: Pressure in a gas
« Reply #2 on: 05/08/2012 12:27:41 »
The force acting in the upwards direction (on the top surface) is exactly balanced by the force acting on the bottom surface. Therefore it is easy to lift. The same happens in the left/right direction.

The force on the walls comes from the change in momentum as the moving molecules bounce off the wall.

However, assuming that the walls are perfectly rigid, so they don't move (or transfer heat), there is no kinetic energy transferred through the walls. Energy is only extracted from the system if the wall can move, and the gas is allowed to expand into a space which has a lower pressure (like the cylinder in an internal combustion engine or steam engine).

#### McQueen

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##### Re: What forces are acting on a gas in a sealed box?
« Reply #3 on: 12/08/2012 07:49:53 »
Hi Evan Au,
It sounds simple but in actual fact this phenomenon is almost as amazing as the Higgs boson. Why ? Because pressure in a gas is absolute ! Considering that , at least superficially, what we are looking at is the average kinetic velocity of the gas molecules, it seems to make no sense at all that pressure in a gas is absolute and has [seemingly]little to do with kinetic energy.

#### William McCormick

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##### Re: What forces are acting on a gas in a sealed box?
« Reply #4 on: 13/08/2012 10:15:26 »
The bottom panel of the box, has all the force, created by the pressure upon it, plus the weight of the gas.

If it did not, then the gas would not be able to add weight to the box, by actual experience we know it does add weight to the box. The only place the gas could be applying downward force, would be to the bottom of the box.

We use large gas cylinders of argon that hold about 300 cubic feet of gas, the cylinders increase in weight by about 40 pounds when full, as compared to when they are empty.

Sincerely,

William McCormick

#### William McCormick

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• Posts: 153
##### Re: What forces are acting on a gas in a sealed box?
« Reply #5 on: 13/08/2012 10:17:58 »
Hi Evan Au,
It sounds simple but in actual fact this phenomenon is almost as amazing as the Higgs boson. Why ? Because pressure in a gas is absolute ! Considering that , at least superficially, what we are looking at is the average kinetic velocity of the gas molecules, it seems to make no sense at all that pressure in a gas is absolute and has [seemingly]little to do with kinetic energy.

There will be a small pressure differential between the top and bottom of the box. Just like there is a pressure differential between sea level and the top of Mountain.

Sincerely,

William McCormick

#### The Naked Scientists Forum

##### Re: What forces are acting on a gas in a sealed box?
« Reply #5 on: 13/08/2012 10:17:58 »