Re: Internal resistance

[Bernard Cleyet <anngeorg@PACBELL.NET>]

L!

I don't think that was JD's objection, and I suspect (I may be missing
something here.) you haven't given a complete description of your test,
since you write monotonically decreasing, etc.

One can only (practically) approximately test the internal R (Z if AC)
at a particular current as the only way I know of finding R is to vary
the current (by varying the load).  You picked a huge delta I.

bc, maybe confused, and can't read

p.s. that's why in the low freq. Z lab (UCSC intermediate lab.) the
student measures the internal R of: a dead cell (batt.), a new one, a
P/S regulated (well), a poorly regulated one (single transistor), and
the output of an HP analog VTVM [TVM is more appropriate] (meant to feed
a strip chart or X-Y recorder).  [Maybe more, but I'm not going to check
the man.]  Not BTW.  they do this over the practical range and plot it,
etc.   BTW they also measure a heavy wire and a deposited metal on glass
resistor.  These are in black (literally) boxes and they must discover
how to do it.  I instructed the TA's to "not give the game away" and
only when the student was "really" stumped to give hints.  We also had
several non-linear R's  in addition to the usual L's C's and LCR's, etc.
all in black boxes.


Ludwik Kowalski wrote:

> 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