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Re: Internal resistance



On Thursday, April 1, 2004, at 04:56 PM, John Denker wrote:

Ludwik Kowalski wrote:
... r ... was found to decrease
monotonically from 1.4 ohms, at I=0.85 A,
to 1.0 ohms at 3.8A. .... (V1-V2)/I

The question is ill-posed.

In the key formula, V1 is not measured "at"
3.8A so the quotient (V1-V2)/I cannot be called
the resistance "at" (V1-V2)/I.

The value of r was expected to be the same for
all currents, in the range for which the power
supply was designed. But it turned out to be
different for different currents. Nowhere in our
textbooks was this possibility mentioned.
Many numerical problems would make no
sense if r were current-dependent. What is
wrong with testing this experimentally? What
is wrong with asking how r depends on I?
How to explain the observed dependance?
Ludwik Kowalski