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Re: First Law



Jim,
I read your web page recently and thought it good. I firmly
belong to your school of thought regarding the use and
misuse of the term heat.


To your present request.

You wrote
I would like to update my First Law web page.


If you are going to confine your system to a gas there
doesn't seem to me to be any substantial alternative to the
standard gas in a cylinder fitted with a piston. I like to
elaborate my cylinder with stops so I can set the volume and
weights I can place on the piston to set the pressure and
adiabatic walls except at the base where the wall can be
either adiabatic or diathermal for contact with an infinite
collection of hot plates at infinitesimally different
temperatures. As well I need measuring devices to tell me
the volume, pressure and temperature. The basic point is
that the variables of state for a gas system are volume,
pressure and temperature and so you are stuck with the
standard cylinder setup or some trivial variation of this.

What about changing the system. Suppose we take a wire.
The thermodynamic variables are length, tension and
temperature. So we can arrange for fixed tension by varying
the load hanging from the wire or fixed length by clamps
and/or fixed temperature by placing the wire in one of a
series of chambers of fixed temperature. Measuring devices
can (should) be added to the system. The system (the wire)
can then undergo combinations of isothermal,constant-length,
constant-tension and/ or adiabatic reversible processes.

Similarly it should be possible, in principle, to devise
sets of isoparametric reversible changes for a liquid
surface (area, surface tension and temperature as
thermodynamic variables) or an electric cell (charge, emf
and temperature as variables).

Brian McInnes