Re: dW turns into dQ

[Joel Rauber <Joel_Rauber@SDSTATE.EDU>]

Bob wrote:

> Joel Rauber wrote:
> > You have made your point clearer; I'm not sure where the differences
> lie,
> > but there are some, perhaps they are mostly philosophical.
> > . . .
>
> Hi Joel,
>
> I think that we two are very close in our thinking.  Let me
> spin a little.

I think so, but I still have a little voice that there is a significant
difference in there somewhere.

> In the (very common) case of a current carrying resistor immersed in a
> fluid, some would choose to exclude the resistor from the
> defined system
> and then call the energy transfer into the system dQ (Heat);
> others would
> include the resistor in the defined system and call the
> energy transfer dW
> (work).

This matches my thinking exactly, if I exclude the resistor I'd call it DQ,
but if I include the resistor I'd call it DW.  But I'd never exclude the
resistor and call it DW in the first law sense.

Of course, if Jim Green requires me to look at the microscopic details of
exactly how the energy got from the resistor (outside the system boundary)
to the system (inside the system boundary); I'm going to start talking about
work done during the collision of "resistor molecules" with "system"
molecules.  It would also be interesting to consider this in light of JM's
different definitions of work and decide which ones are being used.  A whole
nother can of worms.

We are in definite agreement as to the importance of where you put the
system boundary.

> Note that even the one who would exclude the resistor, and so call the
> energy transfer dQ,  would in practice not measure this dQ at
> the defined
> system's boundary, but far removed therefrom - at the EMF, as an
> electrical quantity.

That would be one way, but also you might stick a thermometer into the water
and measure its temperature rise.

>  In practice, difficulties arise only when a teacher forces a
> student to
> choose among the "purely philosophical" interpretations.

unfortunately I am both the teacher and student to myself and I force myself
to choos between purely philosphical interpretations.  I don't really know
any other way to proceed.  I try to understand a current paradigm by trying
to understand the definitions and epistemology implicit in the paradigm and
trying to make sense of it; relating it of course to reality which is the
final arbiter; this often causes cognitive dissonance.  Rousing discussions,
of the type found here, help the process immensely.

Joel