Newsgroups: rec.arts.sf.science
Subject: Re: Superconductors
From: ecmtwhk@ccu1.aukuni.ac.nz (Thomas Koenig)
Date: Wed, 19 Aug 1992 11:26:29 GMT
Organization: University of Auckland, New Zealand.

neufeld@helios.physics.utoronto.ca (Christopher Neufeld) writes:

>In article <1992Aug19.061628.5126@ee.eng.ohio-state.edu> rob@guinness.eng.ohio-state.edu (Rob Carriere) writes:
>>
>>Err, that's the _magnetic_ energy, thank you.  I'm happy to be corrected, but
>>only when the correction is, well, correct.  Calling the energy stored in a
>>magnetic field `electrical' just because there happens to be a handy current
>>loop in the immediate vicinity is a bit Humpty-Dumpty-ish.
>>
>   Not at all. Look, I can take a superconducting inductor and apply a
>voltage to it. It draws current by the familiar V = L dI/dt, and hence
>absorbs power at the rate of V * I. Later I can extract the energy from
>the loop by putting the inductor across a resistance. I've stored
>electrical energy. I happen to have stored it in a magnetic field, but so
>what? Does a rechargeable battery store electrical energy or chemical
>energy?

Let us look at a closed system, which can be defined as a fixed amount of
matter, such as a piece of wire.

The first law of thermodynamics states that the change in energy in the
system equals the amount of work done on it plus the amound of heat it
receives from its surroundings. The work may be electrical or mechanical.
For differential amounts,

dE = dQ + dW_el + dW_mech

(I am using the convention here that everything which crosses the boundary
into the system counts positive).

There is an important difference between the energy of the system on
the one hand and the work done on it and the heat transferred to it
on the other.

Energy is a thermodynamic property. You can look at a certain system and
say "This system has an energy of 50 KJ"; in other words, you can
determine it without knowing the system's past history. Other properties
of a system are, for example, its enthalpy, its entropy, its pressure or
temperature.

Heat and work, however, are not properties of the system. They are
quantities which can be exchanged across the system boundaries. If, for
example, you wanted to increase a wire's temperature, you could either
run a current through it (electrical work), bend it fast several times
(mechanical work) or heat it from the outside (heat).

By examining the wire, it is impossible to say just how its energy
has been increased. You can, however, calculate the change in energy if
you know the amount of heat and work which has passed the system
boundaries.

To get back to the battery in the quoted example, electrical work is
converted to chemical energy there. This may then, at a later stage,
be converted either into work or (if the battery shorts out, for example)
into heat given off to the surroundings.

The clear distinction between heat, energy and work is something which
has only been developed in the last 20 years. Everyday language is much
more imprecise, because these fine distinctions do not really matter
that much there. Unfortunately, present - day scientific English has not
really kept up with these developments, either; you will still find
people using expressions like "latent heat", which are simply wrong,
even in publications. I suppose both they and the reviewers learnt it
that way, and can't be bothered to change.
-- 
Thomas Koenig, ecmtwhk@ccu1.aukuni.ac.nz, ib09@rz.uni-karlsruhe.de
The joy of engineering is to find a straight line on a double logarithmic
diagram.