Re: Internal resistance

[Ludwik Kowalski <kowalskil@MAIL.MONTCLAIR.EDU>]

To late to think (it is 10:30 p.m. I stayed in the
lab to collect data). But here are row data that
I just collected. All at the same room temp.

Circuit consists of a regulated power supply
on whose terminals there was a volt-meter in
parallel with a branch containing a resistor
R, an amp-meter and a switch.

Notation:
Vo volts measured when I is zero (switch off)
V1 when switch is closed (I as indicated below)
I current measured current (switch on)

1) Results for R1 (nominal 2.8 ohms, 12.3A)
Vo=6.0   V1=4.2    I=1.33 A
Vo=10.0   V1=2.2    I=2.39 A
Vo=16.0   V1=11.7    I=4.00 A
Vo=20.0   V1=14.7    I=5.06 A

2) Results for R2 (nominal 21.5 ohms, 4.4A)
Vo=6.0   V1=5.5    I=0.24 A
Vo=10.0   V1=9.3    I=0.42 A
Vo=16.0   V1=15.0    I=0.67 A
Vo=24.0   V1=22.7    I=1.03 A
Vo=30.0   V1=28.3    I=1.28 A


On Thursday, April 1, 2004, at 10:04 PM, Bernard Cleyet wrote:

> Ludwik Kowalski wrote:
>
> > The value of r was expected to be the same for
> > all currents, in the range for which the power
> > supply was designed.
>
> Why should we expect that?  Ohm's law is not a
> God-given law of nature.
>
> > But it turned out to be
>
> > different for different currents.
>
> I expect it.
>
> Maybe it's not regulated and is simply a transformer driven by a
> "variac" followed by a full wave rectifier and cap. filter.  (not even
> a
> choke input LC filter)
>
> A 50% change is a bit extreme for a regulated supply.
>
> I expect < 1%   Agilent claims mine 0.025%
>
> Their new low cost series is 0.01% * +/- 2 mV.
>
> L's cheapie is cheap, as in shoddy, but then may be it didn't cost
> $1080 (Mfg. list for E3632A - 15 V. 7 A.; 30 V. 4 A.)
>
> I couldn't find a site for Lab-Volt P/s's
>
> * Minimum for the repeller in a reflex klystron.
>
> bc, who thinks L's P/S is perfect for a lab. exercise.  He could
> perhaps wire up a regulator (see Horowitz and Hill) to follow his P/S
> with it for more exercise.
>
>
>
> John Denker wrote:
>
> > Terminology point:  On the side of the tracks
> > where I grew up, "resistance" means the small-signal
> > resistance,
> >
> >      R := dV/dI
> >
> > as discussed at
> >   http://www.av8n.com/physics/resistance.htm
> >
> > I am beginning to suspect that Ludwik might have
> > been using some other definition of R.
> >
> > Ludwik Kowalski wrote:
> >
> >
> > > The value of r was expected to be the same for
> > > all currents, in the range for which the power
> > > supply was designed.
> >
> > Why should we expect that?  Ohm's law is not a
> > God-given law of nature.
> >
> > > But it turned out to be
> >
> >
> > > different for different currents.
> >
> > I'm not surprised.
> >
> > > Nowhere in our
> >
> >
> > > textbooks was this possibility mentioned.
> >
> > The textbook assumes that everything in the world
> > is ohmic?  You need a better textbook, stat.
> >
> > > Many numerical problems would make no
> >
> >
> > > sense if r were current-dependent.
> >
> > The real world would not make sense if everything
> > were ohmic.
> >
> > > What is
> >
> >
> > > wrong with testing this experimentally? What
> > > is wrong with asking how r depends on I?
> >
> > Nothing wrong.  Excellent idea, actually.
> >
> >
> >
> > > How to explain the observed dependance?
> >
> > Draw the (I,V) curve.