| Topic: |
Science > Physics |
| User: |
"Roland Paterson-Jones" |
| Date: |
07 Aug 2003 05:28:12 PM |
| Object: |
First Law revisited |
My tome, "Thermodynamics: an engineering approach, fourth edition", Yunus A.
Cengel & Micheal A. Boles, ISBN 0-07-238332-1, pg. 166, introduces the 1st
law of thermodynamics as follows:
'For all adiabatic processes between two specified states of a closed
system, the net work done is the same regardless of the closed system and
the details of the process'
This is motivated by Joule's experiments, and used to define the 'First Law
of Thermodynamics'. Thence, the concept of the 'total energy' property, and
thence, the conservation of energy principle.
Is the 'conservation of energy' principle really on such shaky foundations?
Surely, adiabatic processes can't, on their own, imply such a powerful
statement as the conservation of energy? Surely, you'd have to consider heat
exchange, as well?
Please help me to drop the penny.
Thanks
Roland
p.s. flames not appreciated, go to sci.physics.research instead
--
Roland and Lisa Paterson-Jones
Forest Lodge, Stirrup Lane, Hout Bay
http://www.rolandpj.com/forest-lodge
mobile: +27 72 386 8045
e-mail:
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| User: "Martin Hogbin" |
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| Title: Re: First Law revisited |
09 Aug 2003 03:37:45 AM |
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"Roland Paterson-Jones" <roland@rolandpj.com> wrote in message news:3f32d273$0$226@hades.is.co.za...
My tome, "Thermodynamics: an engineering approach, fourth edition", Yunus A.
Cengel & Micheal A. Boles, ISBN 0-07-238332-1, pg. 166, introduces the 1st
law of thermodynamics as follows:
'For all adiabatic processes between two specified states of a closed
system, the net work done is the same regardless of the closed system and
the details of the process'
This is motivated by Joule's experiments, and used to define the 'First Law
of Thermodynamics'. Thence, the concept of the 'total energy' property, and
thence, the conservation of energy principle.
Is the 'conservation of energy' principle really on such shaky foundations?
No law of physics is proven by experiment. Thermodynamics is
just one area of physics where it is confirmed.
Surely, adiabatic processes can't, on their own, imply such a powerful
statement as the conservation of energy? Surely, you'd have to consider heat
exchange, as well?
Making the process adiabatic simply makes the accounting easier since
there is no flow of heat energy into, or out of , the system. Energy is
always conserved globally in thermodynamics.
Martin Hogbin
.
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| User: "Martin Hogbin" |
|
| Title: Re: First Law revisited |
10 Aug 2003 05:30:29 PM |
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"Roland Paterson-Jones" <roland@rolandpj.com> wrote in message news:3f356f14$0$236@hades.is.co.za...
"Martin Hogbin" <spam3@hogbin.org> wrote in message
news:bh2bsp$hhb$1@sparta.btinternet.com...
"Roland Paterson-Jones" <roland@rolandpj.com> wrote in message
news:3f32d273$0$226@hades.is.co.za...
No law of physics is proven by experiment. Thermodynamics is
just one area of physics where it is confirmed.
Thanks for the bite. What I was trying to ask is the following. Given that
all adiabatic processes demonstrate the same net work, is there a purely
logical deduction that extends this to the domain of processes that exchange
heat with the environment.
The logic is, I guess, that with a suitable choice of system
you can always make it adiabatic.
The book that I am reading (see above) seemed to imply that the adiabatic
case was good enough to motivate the entire concept of energy conservation.
Motivate, yes, but the principle is also observed with
other forms of energy.
In fact, the same book essentially defines the First Law of Thermodynamics
as 'For all adiabatic processes between two specific states of a closed
system, the net work done is the same regardless of the nature of the closed
system and the details of the process'.
That is a normal definition of the law.
On the other hand, I understand the First Law to be the global conservation
of energy, which, I think, is the common view.
The extension of any law of physics to the entire universe
is always somewhat speculative.
Is this an oddment of the particular text-book, or am I missing something?
The law as stated in your text book is the normal
thermodynamic formulation. Global conservation of
energy is an extension of this.
Martin Hogbin
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