Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-L] heat content

The parable of the two boxes is illuminating. I think we all agree that energy of a system is conserved in the sense that the change in the overall energy of a system is the sum of the transfers of energy through its boundary. However, when we think of temperature, we mostly think of a substance. Then T = dU/dS has a meaning to us mostly if we think of U as the "thermal" energy of a substance - not the various possible energies of a complicated mechanical system. We may use the words internal energy, but for the most part, our brains are envisioning random molecular motion in a substance.

Bob at PC

-----Original Message-----
From: Phys-l [mailto:phys-l-bounces@phys-l.org] On Behalf Of Jeffrey
Schnick
Sent: Friday, February 14, 2014 2:54 PM
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] heat content

Suppose I have two black boxes that look and feel the same to me. I push
down on the top of each one and it goes down. Let each black box and its
contents be a system, more specifically, a control mass. In each case, I have
increased the energy of the system. By mechanical means, I have caused
energy to flow into each system. I say I did work on the system. Next I bring
a hot object in contact with each black box. In each case the hot object is at a
higher temperature than the black box. In each case, some energy
spontaneously flows from the hot object into the system. I say I have caused
heat to flow into the system. In either case, the energy of the system, the
black box and its contents increases. To exclude the possibility that someone
thinks that some of the energy of the system might have to do with the
translational motion of the center of mass of the system, I tend to say that
the internal energy of the system increases, rather than just saying that the
energy of th e system increases. I don't think I should have to include the
adjective "internal" but it helps clarify things.

Up until a few years ago, I considered the internal energy to be the energy
associated with unorganized molecular interactions and random molecular
motion. Then John Denker brought up a situation along the lines of the one I
am in the process of describing here. I open up the boxes. I find that one of
them contains a gas. When I pushed down on the lid of the black box I
compressed the gas. When I brought the hot object in contact with the black
box heat flowed into the gas. But in the other box I find a bunch of gadgets
and a cold potato. When I pushed down on the lid I pushed a gear rack down
which caused a flywheel to spin. When I brought the hot object into contact
with the black box a heat engine inside the black box caused the flywheel to
spin faster and warmed up the potato. John's discussion of a similar situation
convinced me that the internal energy of a system is the energy of the
system whether it has to do with random interactions and motion at the
atomic/molecul ar level or not. I started using the expression thermal
energy for what I had been calling internal energy. The internal energy of a
system became the energy of the system in the center-of-mass reference
frame of the system.

Despite the many different ways that heat is defined, I have the impression
that physicists have pretty much reached a consensus on a formal definition
of heat as energy in transit, in particular the energy that spontaneously flows
from an object to a colder object when the two are brought into thermal
contact. Once it gets there it is not heat, nothing can contain heat, once it
gets there it is part of the colder object's internal energy. ( I understand that
John's position is that there is no such consensus. I think I have read all your
arguments over the last few years about it, John; I still disagree.) So talking
about the "heat content" of the ocean strikes me as wrong. It never
occurred to me that a physicist might think that once that energy gets into
the cold object it can be considered separately from energy that might enter
the object by mechanical means and that the energy that got into the object
because the object came into contact with a hotter object might be called
the heat or thermal energy of the object in contrast with the energy that
came in by mechanical means, and, it never occurred to me that a physicist
might call the latter the work of the object. I thought John had constructed a
straw man and was doing a good job of bashing it. Now I'm not so sure. I
perused the Wikipedia entry on thermal energy and it is horrid. I'm still not
convinced that most physicists who use the expression thermal energy think
of it as the energy contained by an object that flowed into an object because
of a temperature difference--perhaps none of the contributors to the
Wikipedia article are physicists--but like I say, I'm not so sure. I am inclined to
try to stop using the expression "thermal energy" because people might
think I mean "heat content".

As regards the ocean, it has some energy associated with intermolecular
interactions and random molecular motion and some energy associated with
the bulk motion of water. They are both part of the total energy of the
ocean. I think they are both part of the internal energy of the ocean. I think
it would be very wrong to think that all the random molecular energy
entered as a result of energy transfer owing to a temperature difference and
all the energy associated with the bulk motion of the water entered by
mechanical means. And yet, up until recently, I have been comfortable with
calling one of them thermal energy and the other mechanical energy. There
has, historically, been quite a bit of discussion on this list of thermalization.
Stir some water in a bowl so that it has a lot of macroscopic angular
momentum and wait a while. Wait until the water has stopped spinning. The
water is said to have been thermalized. To me the verb implies that
mechanical energy has been conver ted into thermal energy.

My current inclination is to go back to internal energy for the energy
associated with unorganized molecular interactions and motion, use energy
in the center of mass frame to include that and the energy due to bulk
motion and arrangement.

The language is not precise. The water swirling around in the bowl helps to
highlight that. There is a gray area between when the energy is kinetic
energy of bulk motion and when it is energy of unorganized molecular
interactions and motion. There is a gray area between when the things that
change rapidly are done changing and when they are still changing. I don't
think that the fact that boundaries are indistinct is a reason to stop using the
terminology to talk about the things on either side of that boundary. John
Denker has argued that as the difference between infrared radiation and
microwaves is indistinct, the difference between heat and work as ways in
which energy is transferred is indistinct. I agree but I don't think that that is a
reason to stop using the words. In analyzing a turbine, it is useful to
distinguish energy flow out of the turbine that occurs spontaneously because
of a temperature difference and energy that flows out through the shaft
mechanically. Th e words heat and work make communication easier.


_______________________________________________
Forum for Physics Educators
Phys-l@phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l