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] work versus mechanical transfer of energy

A colleague posted...

In the context of
dE = T ds - P dV [1a]

Another posted...
I use in that eq dU as internal energy not dE or energy
gravitational energy is outside dU

Then..
To me that seems like one step forward and two steps back. For
one thing, U is not conserved, so to do anything useful you need
to constantly switch back and forth between U and E.

???
I have a concern about the above comment. If U is internal energy, or the
energy if the constituents if the system, can we still think of it as being
conserved? It may or may not be constant, but energy that enters or leaves
the system can be accounted for. This internal energy would not be
constant, but it would be conserved.
It may be cleaner to say 'energy' than 'internal energy', but it seems that
we whenever we use equation 1a we are using E as the Energy that is within
the system.
???

Secondly,
the definition of E is reasonably well known, whereas it’s hard
to learn and hard to remember what’s supposed to be included in
U. In situations where the height h is relevant, you will have
to keep track of it one way or another, so the only question is
whether you will need one new variable or two (h alone versus
h and U both).

???
I am probably missing something obvious, so I apologize...but I've always
considered all energy within the system to be included as some factor of U.
Kinetic energy of all constituent particles, any form of potential energy
between the particles within the system, etc... What am I missing that
makes it difficult to keep track of these things.

Thanks for your time.
Paul