Re: COLLISION 2

[Jim Green <JMGreen@SISNA.COM>]

> This is the clearest statement of Jim's position on heat vs work that I
> have seen.

Thank you. I  keep striving for clarity.

> In considering the various gradations between conduction, convection and
> radiation, I fancy Jim would see a hot plate in contact with a water
> container as heating the water.

Any action which gets the water hotter, I would call "heating".  But not
necessarily connected with a DQ.

> I reckon Jim might allow that if the contact method were to be replaced
> by a gas flame playing hot air on the water pot, it would yet be heating
> the water.

Yes

> But if a 'heat' lamp replaces the gas flame or the hotplate, then it
> seems this is not heating but working the water into boiling...
> ...because there is no physical contact with a higher temperature
> reservoir.

No. Any action which gets that water hotter I would call "heating" BUT at
the same time I would say that the action did a kind of WORK on the system
-- even if the action consists only of molecules bashing other molecules --
and changing their average KE.

>  If this same heat lamp were placed beneath the water pot, and played
> on a plate in contact with the pot, then the pot would be heated
> even though the (radiatively heated) plate was worked.

In the case of radiative heating I am lost, except to say that "something"
must do work on the water/system.  I would only very cautiously talk about
"photons" in the case.

Remember that ALL heating (dE>0) is by doing WORK.  What remains is the
partitioning of that "work" into DQ and DW -- in cases where it
matters.  In the case of the piston/cylinder this is easy.  For other
processes this is NOT so easy.

I am still looking for another piston/cylinder-like example BTW.  I don't
see that the spring will satisfy - at least until I can calculate DS for a
spring-like process.  Now someone will repeat the suggestion that I use
DS=IntDQ/T   THAT WON'T WORK!

What is clear is that DQ is probably NOT always due to a temperature
difference between a thermal reservoir and the system despite the common
belief.  If we want dS to be DQ/T, it surely is not!

Jim Green
mailto:JMGreen@sisna.com
http://users.sisna.com/jmgreen