Message-Id: <v01540b13b12dfb05dcde@[192.174.2.173]>
Date: Thu, 12 Mar 1998 13:08:21 -0800
To: pinhole@exploratorium.edu
From: pauld@exploratorium.edu (Paul Doherty)
Subject: Re: temperature
>>From: Jmgoldberg <Jmgoldberg@aol.com>
>>Date: Wed, 11 Mar 1998 23:22:11 EST
>>To: pauld@exploratorium.edu
>>Mime-Version: 1.0
>>Subject: temperature
>>
>>Hi Paul,
>>I know temperature to be an measure of the average kinetic energy of the
>>particles that make up a substance. With a monatomic gas, piece of
>>cake--translational KE. How about a di/polyatomic gas? How do all modes of
>>motion (translational, rotational, vibrational) get in the act? What is
>>temperature a measure of when it comes to solids and liquids? And how about
>>the cosmic microwave background temperature of 2.7 Kelvins: is there stuff
>>out there that is at the same temperature and therefore at thermal
>>equilibrium? Is there any matter in space colder than 2.7 K ? Should I ask
>>about negative Kelvins now?
>>Jay Goldberg
>>
An object can change its temperature when it is in "contact" with another
object.
Physical contact allows heat to flow by conduction.
However, heat can also flow from one object to another by radiation.
So I can put an ideal gas in the vacuum of intergalactic space where it
will be bombarded with black body radiation from the big bang and
eventually the temperature of the gas will come to 2.7 K. (The vacuum is so
good that the temperature doesn'y change by conduction.)
The temperature of the gas that emitted the blackbody radiation was about
3000 K however the photons travelling from the big bang to our gas have
been travelling through an expanding universe and have themselves been
stretched. The universe is now about 1000 times bigger than when the
photons were emitted* and so the photon wavelength is 1000 times longer
than when they started, their energy is 1000 times less and so their
radiation temperature is 3000K/1000 about 3 K.
*The universe was born as a hot plasma which is opaque, when it finally
cooled to 3000K, 100,000 years after the big bang the plasma became
neutral molecules and atoms and the universe became transparent so that
radiation could travel from then until now. So the radiation was in
equilibrium with a 3000K plasma until the plasma went transparent and the
radiation became free: "Be free my pretties"
By the way the atmosphere at the elevation of a high shuttle flight is a
good vacuum. The temperature of the near vacuum gas is about 1500K. The
temperature of the radiation from the big bang at night is 2.7 K.
An object in contact with the hot yet thin atmosphere and the cold
radiation will cool.
Paul D