Go Back   Science Forums Biology Forum Molecular Biology Forum Physics Chemistry Forum > General Science Forums > Physics Forum
Register Search Today's Posts Mark Forums Read

Physics Forum Physics Forum. Discuss and ask physics questions, kinematics and other physics problems.


Zero gravity

Zero gravity - Physics Forum

Zero gravity - Physics Forum. Discuss and ask physics questions, kinematics and other physics problems.


Reply
 
LinkBack Thread Tools Display Modes
  #1  
Old 04-21-2008, 06:48 AM
Androcles
Guest
 
Posts: n/a
Default Zero gravity





--
This message is brought to you by Androcles
[Only registered users see links. ]

"Steve Bell" <[Only registered users see links. ]> wrote in message
news:556d3$480bf0e6$943f641c$[Only registered users see links. ]...
| Hi zzbunker,
|
| I apologize, I don't know what a "Maxwell's Demon" is.
|

Consider two chambers and a door between them, one of which
has nothing in it (a vacuum) and the other is filled with a gas such as
air. Opening the door lets the air into the vacuum chamber until
the pressure equalises in each chamber, as you would expect and
experience dictates.

Maxwell's mischievous imp reverses the process.
He stands at the door and let's a gas molecule randomly wander
out of the vacuum chamber, but if it tries to get back in again
he slams the door shut. Eventually all the air molecules will
randomly try to wander out of the vacuum chamber and when
the last one has passed the imp he slams the door shut for
the last time and has succeeded.
As a physicist/chemist/engineer, what do you learn from this
thought experiment?




Reply With Quote
  #2  
Old 04-21-2008, 01:26 PM
Steve Bell
Guest
 
Posts: n/a
Default Zero gravity


"Androcles" <Headmaster@Hogwarts.physics> wrote in message
news:dLWOj.96222$5o5.35352@newsfe15.ams2...


Reply With Quote
  #3  
Old 04-21-2008, 02:20 PM
Steve Bell
Guest
 
Posts: n/a
Default Zero gravity

Hi Androcles,

I guess by "succeed" you mean the imp has succeeded in moving all the air
molecules from one chamber to the other. If the door is just left open, an
overall equilibrium would be established, but one that would have a lower
average velocity in the combined larger volume of the two chambers compared
to the average particle velocity in the smaller volume of the original
smaller air-filled chamber. The original average particle velocity in the
filled-chamber would go down to one-half its original value, and the average
velocity in the original vacuum chamber (which is zero) would increase to
one-half. The total average particle velocity would be "conserved," though.
The sum of the two halved average particle velocities would equal the
average particle velocity in the original filled chamber. But if the imp
were to force this complete transference of particles, the original
filled-chamber would go down to zero average velocity (no particles would be
present at the end of the experiment) and the original vacuum chamber would
get all of the original average particle velocity. The total average
particle velocity would still be conserved, but it seems to me this would be
highly unlikely without the help of the imp!

Steve


"Androcles" <Headmaster@Hogwarts.physics> wrote in message
news:dLWOj.96222$5o5.35352@newsfe15.ams2...


Reply With Quote
  #4  
Old 04-21-2008, 06:16 PM
Androcles
Guest
 
Posts: n/a
Default Zero gravity

"Steve Bell" <[Only registered users see links. ]> wrote in message
news:74870$480ca133$943f641c$[Only registered users see links. ]...
| Hi Androcles,
|
| I guess by "succeed" you mean the imp has succeeded in moving all the air
| molecules from one chamber to the other. If the door is just left open, an
| overall equilibrium would be established, but one that would have a lower
| average velocity in the combined larger volume of the two chambers
compared
| to the average particle velocity in the smaller volume of the original
| smaller air-filled chamber. The original average particle velocity in the
| filled-chamber would go down to one-half its original value, and the
average
| velocity in the original vacuum chamber (which is zero) would increase to
| one-half. The total average particle velocity would be "conserved,"
though.
| The sum of the two halved average particle velocities would equal the
| average particle velocity in the original filled chamber. But if the imp
| were to force this complete transference of particles, the original
| filled-chamber would go down to zero average velocity (no particles would
be
| present at the end of the experiment) and the original vacuum chamber
would
| get all of the original average particle velocity. The total average
| particle velocity would still be conserved, but it seems to me this would
be
| highly unlikely without the help of the imp!
|
| Steve

That's Maxwell's demon.
In reality, of course, the door is a valve and the vacuum chamber
is a cylinder with a piston in it, but one of the effects is temperature.
The kinetic energy of the gas molecules is moved from one chamber
to the other and that moves the temperature too, and so your refrigerator
works and diesel engines get hot enough on the compression stroke
to ignite the fuel. Refrigerators take the heat out, they do not put
cold in. Maxwell created his demon to help students understand
the relationship between pressure, volume and temperature whether
in steam engines, diesel engines, gas turbines, refrigerators, vacuum pumps,
rockets, etc., etc.
Let the air out of a truck or bus tyre and you may notice frost
forms on the valve.
[Only registered users see links. ]


|
| "Androcles" <Headmaster@Hogwarts.physics> wrote in message
| news:dLWOj.96222$5o5.35352@newsfe15.ams2...
| >
| >
| > --
| > This message is brought to you by Androcles
| > [Only registered users see links. ]
| >
| > "Steve Bell" <[Only registered users see links. ]> wrote in message
| > news:556d3$480bf0e6$943f641c$[Only registered users see links. ]...
| > | Hi zzbunker,
| > |
| > | I apologize, I don't know what a "Maxwell's Demon" is.
| > |
| >
| > Consider two chambers and a door between them, one of which
| > has nothing in it (a vacuum) and the other is filled with a gas such as
| > air. Opening the door lets the air into the vacuum chamber until
| > the pressure equalises in each chamber, as you would expect and
| > experience dictates.
| >
| > Maxwell's mischievous imp reverses the process.
| > He stands at the door and let's a gas molecule randomly wander
| > out of the vacuum chamber, but if it tries to get back in again
| > he slams the door shut. Eventually all the air molecules will
| > randomly try to wander out of the vacuum chamber and when
| > the last one has passed the imp he slams the door shut for
| > the last time and has succeeded.
| > As a physicist/chemist/engineer, what do you learn from this
| > thought experiment?
| >
| >
| >
| >
|
|


Reply With Quote
Reply

Tags
gravity


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
What is Gravity? sdr@sdrodrian.com Physics Forum 1 01-27-2008 02:53 PM
Gravity - Ideas James Allen Bressem Physics Forum 0 01-27-2007 05:30 AM
The mystery of gravity Henry Haapalainen Physics Forum 0 09-20-2006 09:11 PM
The Achilles Heel of String Theory. S D Rodrian Physics Forum 7 07-08-2006 02:40 PM
Question about gravity Greg Merideth Physics Forum 11 01-18-2005 09:15 PM


All times are GMT. The time now is 04:54 PM.


Powered by vBulletin® Version 3.8.4
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
Copyright 2005 - 2012 Molecular Station | All Rights Reserved
Page generated in 0.17682 seconds with 16 queries