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: CONSERVATION OF ENERGY



On Sun, 13 Jul 1997 Leigh Palmer <palmer@sfu.ca> wrote:

As you state, it should properly be called heat, since a process, though
not a uniquely specified one, is implicit. It is certainly not an energy.
Use of the symbol dU (by which I infer you mean the change in U) is most
improper; heat is not the change in anything. You should use Q instead:

Q = m c dT

I did not mean to say that Q is a state function and something better than
"thermal energy" would be appropriate because energy is a state variable.
I did not invent "thermal energy"; many authors are now using this term
where older textbooks used to say "heat".

Well (m c dT), whatever you call it, does not represent a change in a state
function *unless* c is further qualified *and* there are no other degrees
of freedom which could contribute to the internal energy that need to be
specified. For example, if the system is a coil spring then part of its
internal energy may be in the form of elastic potential energy. That part
must be accounted for.

It is my understanding that heat, in the first law, is defined as that
part of internal energy which is transfered (driven by?) a difference
in temetratures. The dT, in the formula above, is a difference between
the initial and final temperature of a body (in a process). In the
example I used dQ is due to friction. If I am correct (that c*m*dT is not
heat, as defined in thermodynamics) then what name should be given to it?
We need distinct names for distinct physical quantities.

Heat is not energy any more than work is. Therefor it cannot be "part of"
any energy. One cannot examine the energy of a system and identify part
of it as heat energy.

Treating heat, work, and energy as though they are all the same thing
is [wrong].

I agree. And that is why I was asking how a common statemet "energy =
ability to do work" should be interpreted. Or what does it mean that
"work by friction is done at the expense of kinetic energy"? If these
questions can be clearly answerd (for a specific situation invented
to discuss the issue) then some progress will be made.

The statement "energy = ability to do work" is at best meaningless.
The second statement is not useful; perhaps it is wrong. It is unlikely
to enlighten a student since I can't understand it.

1) Can somebody describe the process under consideration using the terms
on which we can agree?

I can describe it in terms I understand. Since we don't agree now, it
is unlikely that we shall do so after I describe it. Certainly heat
plays no role in the process; no energy is transferred between the
blocks impelled by temperature differences. If you say "but heat must
be generated by the process of friction" you will be using the term
"heat" with its vulgar meaning, not the technical meaning we employ
in thermodynamics.

2) Can somebody explain the meaning of "work at the expense of energy"
and "energy is an ability to do work"? Are these phrases acceptable?
Why yes? why no?

No. As to why, answer the question "Why is it unacceptable to spit on
the floor?" Acceptability is a matter of convention in both cases.

I an not against generalizations but I suggest we begin with the concrete
situation at hand. This will help us to understand each other better.

You have defined the problem almost well enough. Please reformulate
it carefully and ask a physical rather than a linguistic question
(e.g. "What temperature rise is to be expected?" rather than "Can we
say that...?").

Leigh