Re: even more 'what flows'

["Roger A. Pruitt" <phrp@fhsuvm.fhsu.edu>]

Richard W. Tarara wrote:
> This thread won't die because Jim keeps bringing it up ;-)
>
> OK, the transfer of energy at the boundary between Jim's A and B containers
> IS due to intermolecular collisions, BUT viewed over time, the energy
> content of A decreases and B increases.  The 'common' name for this random
> KE of molecules is Heat Energy, and the imagery of that energy 'flowing'
> from A to B and therefore Heat flowing from A to B is not, IMO, harmful.
>
> The Heat Energy nomenclature is useful for beginning students because it
> helps in the process of identifying energy in complex systems.  Consider a
> block sliding along a rough surface.  Sure you can say that the ordered KE
> of the block molecules becomes random KE of the block and surface molecules
> by a series of interactions between molecules, but when I get to this
> problem, I simply have the students rub their hands together.  Where does
> the energy go--they answer as one--HEAT.  OK, later on, those who go on

I certainally wouldn't call this HEAT. The hands got warm because of 
mechanical work being done as they were rubbed together. The temperature of 
the hands increased indicating an increase in internal energy. 

If a bunsen burner is on the demo table you can carefully hold your hand 
above the flame or at the side to demonstrate the transfer of energy as heat. 
Zemansky says "Heat is energy in transit. It flows from one point to 
another." (Heat and Thermodynamics) Sears says that "To distinguish energy in 
any form, including that flowing across the boundary of a system by 
conduction or radiation, from work, we shall speak of it as 'a flow of heat,' 
and represent the energy by the symbol Q." (Thermodynamics) Quoting Fermi 
from his book <Thermodynamics>: "Q can be interpreted physically as the 
amount of energy that is received by the system in forms other than work." I 
could quote from a number of other authors on my shelves, but they all say 
about the same about heat. I personally like Zemansky's "heat is energy in 
transit." I then point out that conduction, convection and radaiation are the 
mechanisms of transit. Of course, there are demonstrations for beginning 
classes of each of these.

One thing to remind students is that once the energy has been received by the 
system, we can't tell if it was Q or W that contributed to the increase in 
internal energy U.


Roger
> need a more sophisticated understanding of all this, BUT those students
> SHOULD be able to make that jump without too much confusion.
>
> BTW:  Jim, what do you call the quantity calculated  by   kA(delta)T/d  in
> conduction?  Many of us call this the heat flow across a boundary.
>
> This crusade will certainly go the same way as the Bernoulli and
> centripetal/centrifugal wars--to be resurrected every 6 months without ever
> any resolution.
>
> *****************************************************
> Richard W. Tarara
> Department of Chemistry & Physics
> Saint Mary's College
> Notre Dame, IN 46556
> 219-284-4664
> rtarara@saintmarys.edu
>
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