Re: ENERGY WITH Q

[Ludwik Kowalski <kowalskiL@MAIL.MONTCLAIR.EDU>]

Carl's message prompted me to share this speculation.
I was thinking about the First Law in connection with
another draft of the "acceptable first energy sequence"
outline. Comments and criticism will be appreciated.
Please try to address the issues at the level of an
introductory physics course, if possible.

Carl E. Mungan wrote:

> The standard First Law is:
> W + Q = delta (E_mech) + delta (E_internal). Note
> that E_mech + E_internal is the total energy of the system.
************************************************

According to the First Law every system is characterized by a
physical quantity called its total energy, Esys. It is a generalization
of what students learn in Models 1 (world without friction) and
Model 2 (word with friction but Q is always positive). For a
closed system that quantity remains constant, for an open system
it can either increase or decrease, depending on what happens
in the surroundings. This is often described mathematically as:

       deltaU=Q+W            (The First Law)

where deltaU is a finite change in Esys while Q and W (with
appropriate signs, according to a convention) are heat and work.
By seeing Q and W together a student may be tempted to think,
incorrectly, that Q and W are forms of energy.

As discussed in Model 2, heat is a form of energy but work (done
by a force with which a rigid piston acts on its surroundings) is
not. The above formula suggests that the transfer of energy from
an open system to the surroundings, or vice versa, can occur in
direct and indirect ways. The direct way is when "energy travels
as if it were a fluid" the indirect way is when it "disappears here
and appears there at the same time." I am saying this because
work is only an indication that an energy transformation takes
place, it is not a localized form of energy.

Suppose I push a box along a floor with a rigid stick. Some
chemical energy is lost by my body while thermal energy is added
to the box and floor. The stick is not a thermal conductor, no
energy travels through the stick. This can be contrasted with the
situation in which work is zero but energy travels from my
body to the stick via its thermal conductivity.

On the other hand, one may say that a perfectly rigid stick
does not exist and that the transfer of energy via work is not
instantaneous, it is like transferring energy via a domino wave.
In this interpretation the mechanical energy does travel through
my non-conducting stick. If this is a correct then we should
replace W by mechanical energy X (deltaU=Q+X). In that
context both Q and X flow except that no randomness is
involved in the rapid flow of X.

The traditional First Law tells us that energy is conserved in
a closed system (U=constant). That is it. One may argue that
this is also a definition of a closed system and that deltaU=Q+W
is necessary to avoid an embarrassment (circual logic). So
instead of limiting the law to a closed system we are adding a
description of "two ways of transferring energy" from an open
system to its environment (or vice versa) . This is much more
than conservation of energy.  Why should the issues connected
with the path dependence, etc., be added to the Energy
Conservation Law?

In my opinion learning of physics (in an introductory course)
would be easier if the energy conservation idea was presented in
the spirit of Denis the Menace, as described by Feynman. We
keep inventing new forms of energy to preserve a philosophical
preconception (a generalization, if you prefer) that the energy
must be coserved in the universe. This approach turned out to
be very useful (for example, to predict neutrinos); we hope it
will continue to be useful in the future. Is this is not enough?
Why do need W in the First Law? The Second Law, I think,
does not need it either. We use W when we introduce different
forms of energy;  that is where it is really needed.
Ludwik Kowalski