Re: CONSERVATION OF ENERGY

[LUDWIK KOWALSKI <kowalskil@alpha.montclair.edu>]

Something must be wrong somewhere; my immediate reply to what Leigh wrote
did not come to me. I suspect it was lost. So let me answer again in a 
slightly different form:
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                              On Sun, 13 Ludwik asked:

> > Leigh, how should dU=c*m*dt be called? We used to call it "heat" but
> > this was in conflict with how heat is defined in thermodynamics. What
> > is wrong with the name "thermal energy"? It is now used by many authors.

                              The answer from Leigh was:

> 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
>
> where m is the mass of a homogeneous system having heat capacity per
> unit mass c (some modifier is appropriate here) and dt is the change in
> temperature that occurs in the process of heating.

Please note that c refers to a constant (zero) external pressure (vacuum). 
No mass is lost and T is a state variable. Thus the product m*c*dT is 
path independent. Following numerous authors I do not want to call this
quantity heat; a concept defined when we teach thermodynamics. Is it not
true that the accepted definition states that "heat is that part of internal 
energy which is transfered through a system boundary due to a difference 
of temperatures"? In the example used, dQ is due to friction, it is not
a difference of temperature responsible for the "injection of random
energy into the system". 

If I am correct (that c*m*dT is not heat, as defined in thermodynamics) 
then what name should be given to it? How can we teach without having 
distinct names for distinct physical quantities? You can invent another 
situation in which the cube (from our example) does not slide at all and 
dT is a result of an electric current (chemical reactions in a nearby 
battery). Or a radioactive iron isotope may be responsible for a slight
increase in temperature. These are different paths leading to the same
outcome. Let me repeat the original question; What is wrong with the name 
"thermal energy"? I did not invent it.

> 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 is
an ability to do work" should be interpreted. Or what does it mean that
"work by friction is done at the expense of kinetic energy"? I think that
I am not the only one who would benefit if these questions could be clearly 
answerd. Please help.

1) Can somebody describe the process under consideration using the first 
   law terms (work, heat, energy) properly? 
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 not?

I am not against generalizations but I suggest that we limit the initial 
discussion to the example of a block sliding on a horizontal surface. It 
has all the necessary ingredients and we are not distracted by changes in
gravitational energy.
                                                 Ludwik Kowalski