Re: ENERGY WITH Q

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

"John S. Denker" wrote:

> If U=constant is the first law, call it the first law.  Don't call the
> deprecated W+Q equation the first law, as was done above.

Before adding another good reason why deltaU=Q+W should
be decoupled from the First Law let me say what the law tells
us. The law is is a general statement "the total energy in the
universe remains constant." Suppose we find out that the sum
of all known forms of energies changed in a particular region
of the universe. It can mean only one of two things: (a) some
amount of Esys was transferred to the surroundings, (b) the
system contains a form of energy of which we are not aware.

A pure logician would add that a combination of (a) and (b) is
also possible. I agree. And why do I think that deltaU=Q+W
is not desirable in the first course (and perhaps it should be
replaced by deltaU=Q+X in more advanced courses).

We already accepted the idea that Q is a form of energy while
work is a dot product of two vectors, W is not energy. If so
then Q+W makes no sense, it is like adding proverbial apples
and oranges. Or like adding torque to kinetic energy (because
the units are identical).

As I indicated last night, the expression Q+W is likely to be
responsible for a misconception that work is a form of energy.
It leads to wordings which promotes this misconception. Even
the authors of leading textbooks can not avoid this trap. Here
is an illustration. "The thermal efficiency e represents the
fraction of Qh that is converted to work." (quoted from page
562 of "University Physics" by Sears, Zemansky, Young
and Freedman, 10 edition, 2000).

How can a form of energy be converted into something
that is not energy. Yes, it is converted into X, which is
a form of energy. I suspect that those who deal with heat
engines are well aware of this fact. The say "work" but
they do not mean it. That is why deltaU=Q+X is much
better. What is a good name for X?

Carl Mungan wrote:

> W + Q = delta (E_mech) + delta (E_internal)

Why not? However, to promote the interplanetary cooperation,
I would like to suggest an alternative. Use the Martian word
"duit" for X and our common word "heat" for Q. This would
be consistent with the common practice of thinking about Q
as a fraction of Eth traversing a system boundary.
Ludwik Kowalski
P.S.
"duit" is better than "doit", in my wife's opinion; she is a
language lover.