MadSci Network: Physics
Query:

Re: Can heat flow better downwards than upwards in a solid?

Date: Tue Aug 3 15:39:17 1999
Posted By: John Pojman, Faculty, Chemistry, University of Southern Mississippi
Area of science: Physics
ID: 933172535.Ph
Message:

You performed a very interesting experiment!  

To think about this problem we need to consider what is "heat".  We use the 
term as if it were really something, like a fluid (e.g., heat capacity) but 
this is a vestige of the caloric theory.  It is better to think of heat as 
a metaphor for energy transferred through random molecular motion.  

When we casually speak of "heat rising" what we really mean is "hot air 
rises."  This is so because the density of air, and most everything, 
decreases with increasing temperature.  Following Archimedes' Principle, 
the lower density object experiences a buoyant force when surrounded by a 
more dense fluid, i.e, it floats.

There are two relevant mechanisms for thermal energy transfer - conduction 
and convection.  Conduction occurs through the local, random molecular 
motion.  Convection occurs through fluid flow, which can be forced by a 
fan, for example, or occur from buoyancy (natural convection).

If you heat a piece of metal from below, hot air will flow up around the 
object and transfer the heat more rapidly.  Likewise, if you cool from 
above, the cool air sinks and cools the object faster than from conduction 
alone.

How could you test this?  Perform the experiment in weightlessness where 
there is no buoyant force.   (In our research, we study chemical reactions 
in weightlessness, and the reduction in heat transfer is enormous)  Or, do 
the experiment in a vacuum.  Without air, no convection.  This is the 
principle of the Thermos, in which an evacuated sleeve surrounds the 
liquid.  Energy can only be lost through conduction through the ca


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