### Author Topic: Has there ever been an experiment about mixing gases  (Read 1773 times)

#### jedisawyer

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##### Has there ever been an experiment about mixing gases
« on: 12/28/2015 08:15 pm »
If you mix gases of different temperatures if you conserve Kinetic Energy for the mixed gas you end up with a gas at a different temperature than you would get if you conserved momentum for the mixed gas.  For example if you mix equal amounts of Helium that are 500 degrees Kelvin with Helium that is 100 degrees Kelvin your mixed gas is 300 degrees Kelvin if you conserve KE and 261.7 degrees Kelvin if you conserve momentum.  Has there ever been an experiment to find out what temperature mixing gases of different temperatures will become?

#### gargoyle99

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##### Re: Has there ever been an experiment about mixing gases
« Reply #1 on: 12/29/2015 05:52 pm »
If you mix gases of different temperatures if you conserve Kinetic Energy for the mixed gas you end up with a gas at a different temperature than you would get if you conserved momentum for the mixed gas.  For example if you mix equal amounts of Helium that are 500 degrees Kelvin with Helium that is 100 degrees Kelvin your mixed gas is 300 degrees Kelvin if you conserve KE and 261.7 degrees Kelvin if you conserve momentum.  Has there ever been an experiment to find out what temperature mixing gases of different temperatures will become?

First, your question as posed isn't answerable because you haven't specified the volumes and pressures of the gas mixing.  Is it constant volume?  Constant pressure?  It isn't constant temperature, so I assume you're talking about an adiabatic process.  The final temperature (and kinetic energy) depends on those things.  The gases will do work and change temperature (and increase in entropy) as they expand (and compress) into each other's volumes.

In addition, I don't think your equation for total momentum is valid, as you noted.  Even to get a total scalar value, you would need to add up the individual momenta of all the particles, which have velocities in a Boltzmann distribution.

To summarize, both total kinetic energy and total momentum at the macroscopic and atomic levels are conserved for all types of gas mixing and this has been very well described both experimentally and theoretically for many many years since the time of Joule, Boyle, Charles and Avogadro.  This is the field of thermodynamics, which is one part of physical chemistry.  It is complicated, but very important for all rocket science.  If you get an answer where they aren't conserved, then you've made a mistake somewhere.

Here's a place to start.  Further on, a college course in thermodynamics will go through all of this (and a lot more) in great and sometimes very painful detail.  There's a reason it's sometimes referred to as "thermogoddamnics."

Good luck!

#### jedisawyer

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##### Re: Has there ever been an experiment about mixing gases
« Reply #2 on: 12/29/2015 07:07 pm »
I feel the question was framed well enough by specifying the Molar amount of Helium and Temperatures there is enough information for what I am curious about.  As far as being adiabatic or not, that is what the question is all about.  Wouldn't it be easier to give me a named and searchable experiment that deals with the question rather than trying to send me back to school.

There is a theory of kinetics for matter in the gas state, (free to rotate and vibrate, with large space separation of nucleuses), that such molecules do not need to push off anything to gain momentum, that just adding energy will cause them to propagate faster and thereby pick up momentum, and the momentum of photons can be neglected.

#### gospacex

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##### Re: Has there ever been an experiment about mixing gases
« Reply #3 on: 12/29/2015 07:19 pm »
If you mix gases of different temperatures if you conserve Kinetic Energy for the mixed gas you end up with a gas at a different temperature than you would get if you conserved momentum for the mixed gas.  For example if you mix equal amounts of Helium that are 500 degrees Kelvin with Helium that is 100 degrees Kelvin your mixed gas is 300 degrees Kelvin if you conserve KE and 261.7 degrees Kelvin if you conserve momentum.  Has there ever been an experiment to find out what temperature mixing gases of different temperatures will become?

Motionless volume of gas has minuscule total momentum - momentum of individual molecules is randomly directed, so it cancels out.

Of course, both momentum and energy is conserved during mixing. Did you expect any other answer?
« Last Edit: 12/29/2015 07:21 pm by gospacex »

#### jedisawyer

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##### Re: Has there ever been an experiment about mixing gases
« Reply #4 on: 12/29/2015 08:04 pm »
Seeing as how it has been a confusion point for people I will explain a little more about my thinking about kinetic energy and momentum.  To me for a gas's KE is potential momentum, that is if a contained gas is allowed to take a preferred direction by letting it pass through a orifice such as a rocket nozzle it becomes directional KE. Gas molecules in the container will bounce all around the inside of the container until they find the way out of the container for a rocket moving through a vacuum.

So if no one can actually direct me to an experiment that answered the question about what temperature gases of different temperatures will stabilize at.  Let's suggest that somebody should do the following experiment:

Have equal amounts of a low temperature gas and high temperature gas in two different containers, measure how much total impulse you can get from them separately, and then mix the gases and see how much total impulse you can get.  Will they be the same?  I am unsure.

#### Carl G

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##### Re: Has there ever been an experiment about mixing gases
« Reply #5 on: 12/29/2015 08:21 pm »
This has nothing to do with spaceflight. Locked.

Don't blame the mods, its the members who complain over and over again about such things.
« Last Edit: 12/29/2015 08:22 pm by Carl G »

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