Kim Elmore wrote:
>
> Hi Eric,
>
> I understand your gripe, and agree with your sentiments. yet, I feel the
> need to weigh in. I have a fair bit of experience with numerical models,
> though not those involved in EE. Yet, the basic ideas behind numerical
> models are all quite similar. In particular, you say:
>
> >One statement by you and others ( in some of those OTHER armchairs) regards
> >the term 'computer modelling'. There is somewhat of a semantic problem here,
> >as follows. The computer models which we use are EXACT, precise physical
> >devices whose electronic equations we can write precisely.
>
> Can we really? I know that you're bright and that you do credible work.
> I'm not an EE, but, for example, are there *any* assumptions made in any of
> the models? I'll bet there are, because without assumptions of some type,
> it's nearly impossible to start the process. For example, unless each
> component invariably behaves in a way that can be represented by a
> closed-form solution of some type, an approximation is involved. The
> approximation could be linear (leading to errors that are O(2)), as in
> I*R=E. If there are assumptions of any kind, then the model is not exact.
> In this, I agree with Dave Kirby and Conrad (G0RUZ).
I'm glad some agrees with me on this.
> You proposed a straw-man: Where might the simple model I*R=E fail? A
> simple error occurs if the temperature coefficient of the device is
> ignored. Yet, the temperature coefficient is unlikely to be truly linear,
> or quadratic, or cubic, or quartic or to fit any order of polynomial. It's
> likely to have non-linear components, even though those may be small.
> What's worse, the nature of these will vary from one device to another with
> identical ratings. If any of those components are ignored, then the answer
> isn't exact: it's approximate. This isn't to say that the answer is *bad*,
> or useless: the answer we get may be supremely useful even if it isn't exact.
>
> Non-linear partial differential equations (which must appear abundantly in
> any EE model) almost never have a closed-form solution. So, some numerical
> methods must be derived to solve them. As you know, there are lots of
> them. A common, robust one is Runge-Kutta. But there are many different
> flavors of Runge-Kutta. For example, the order of the method (2nd, 3rd,
> etc.) helps define the errors in the converged solution. If we watch each
> iteration of the solution, we'll see the error term rapidly shrink, but, in
> the end, no matter how many iterations we make, we'll never see it simply
> cease to change: it will bounce around a small neighborhood. We stop
> iterating when the error term is "small," or "close enough," defined by our
> application.
>
> When viewed in this light, *none* of our numerical models of real-world
> devices or processes are exact. In fact, because we don't really know
> "everything" about any device, we wouldn't recognize an exact answer of it
> were given to us. Some might call knowing everything "divine knowledge"
> while a quantum physicist would simply say the concept of an exact answer
> has no meaning. Regardless, the answers we get are very, very good.
> Ideally, they are so good that we can't measure whatever errors they
> contain. Even short of that ideal, they allow us to design and build things
> undreamed of 25 years ago.
>
> There. Done. Said my peace. Back to RF!
>
> 73,
>
> Kim Elmore
> Kim Elmore, Ph.D.
> University of Oklahoma
> Cooperative Institute for Mesoscale Meteorological Studies
> "All of weather is divided into three parts: Yes, No, and Maybe. The
> greatest of these is Maybe" The original Latin appears to be garbled.
For those that don't know, any article submitted to a professional
journal (Institute of Physics, IEE, IEEE etc) are normally normally sent
to 2 experts in the field as referees, to ensure high standards. The
referees are asked to make comments on the paper, whether it should be
published without any correct, with minor corrects, major corrects or
whether they feel the paper is poor and should not be published at all.
Unlike the less professional amateur journals, a single editor will not
make a decision on the quality of a paper, although he/she will decide
whether it gets published at all.
As a scientist I'm often asked to referee papers in my field of
expertise. I personally would never recommend publication *without
changes* for any paper that claimed an exact model in this situation.
The equation for the time period T of a pendulum (as used in a clock) is
(if memory serves me correctly) T = 2 Pi*sqrt(L/g), where L is the
length and g the acceleration due to gravity. Even if the length could
be measured exactly (which it can't) and the acceleration due to gravity
measured exactly (which it can't), this apparently closed form solution
is based on approximations, so is not exact.
There is software available that can do 'exact' calculations, but I
don't think the model in question falls into this category. Mathematica,
a software package from Wolfram research, can compute the sum of 1 and
1/3 exactly. It does not give an answer of 1.333333333333 as a
calculator would, but gives the answer as 1 + 1/3. It does not convert
the 1/3 to 0.33333333 first. You can easily multiply two 100 integers
numbers with Matematica and get an exact answer.
I feel as a scientist it is a my duty when talking to the general public
to use terms the terms 'exact', 'very accurate', 'approximate' etc or
similar in appropriate cases.
If you look at the abstract for my PhD thesis
http://www.medphys.ucl.ac.uk/~davek/phd/chapter0.pdf
you will see the final sentence mentions the word 'absolute accuracy'.
Someone one commented to me that the term was incorrect (not my PhD
examiners I would add)! I fully accept that person was correct and in
the context of the work I did (which involved several numerical models
and measurements with test equipment), the term 'absolute accuracy'
should not have been used. I have admitted when I'm wrong on a public
mailing list. Perhaps Eric can do likewise, rather than try to defend
the undependable.
Dr. David Kirkby PhD,
Senior Research Fellow,
Department of Medical Physics,
University College London,
11-20 Capper St, London, WC1E 6JA.
Tel: 020 7679 6408 Fax: 020 7679 6269
Internal telephone: ext 46408
e-mail davek@medphys.ucl.ac.uk
Web page: http://www.medphys.ucl.ac.uk/~davek
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