Re: capacitor

[forinas@indiana.edu (kyle forinash)]

Ludwik and Joe (and those doing a CBL capacitor experiment);

You are right, the CBL must have an input impedance near that of my 1Meg
resistor. With a 780k resistance and the 10 microF cap I get a maximum 5.5V
on the cap, with a 390k resistor I get 6.7V. I have switched to a 330
microF cap and get a nice curve with a 10k resistor that plateaus at around
8.8V. Reducing to a 3.3k gives an ok curve (sampling at 2 per sec) which
plateaus at 8.9V. Lower resistances come out closer to 9V but begin to look
more like a step than an exp.

So I guess the moral is to use a larger cap (so that the time constant is
in the range of the capability of the CBL) but a smaller resistor so that
the CBL does not become part of a voltage divider.

One other small point is the voltage leads to the CBL will slowly charge
the cap if connected directly accross it (0.6V in 50s). A way to avoid that
is to set up the experiment with one lead of the CBL on the switch side
(the other on the cap) so that the voltage and CBL are applied at the same
time.

Thanks for the help.

kyle

> Kyle Forinash wrote:
> > I had students charge up a capacitor (10 microF) in series with about a
> > 1 megOhm R with a 9V transistor battery. They measured the voltage on the
> > cap with a CBL at about 2 samples per sec and get a very nice 1-exp curve
> > which levels off nicely after a few seconds. But it levels of at a voltage
> > much lower than 9V (around 5V)! Am I missing something? Shouldn't the final
> > voltage be 9V? Is there something about the CBL which would prohibit doing
> > this? It seems to be about the same for each setup.
>
> I do not know what the input imperdance of a new CBL probe should be but your
> result may be explained by assuming it is nearly as low as 1 mega-ohm. You
> have a resistive voltage divider with C connected to one of its components.
> Test this by lowering your R=1 megaOhm to 0.5; this should move you closer to
> the expected 9 volts. You can measure the probe impedance by discharging a
> known capacitor through it (from the rate of discharing). Please share your
> conclusion; we can all learn from it.
>                                                 Ludwik Kowalski


!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!  kyle forinash   812-941-2390                 !
!  forinas@indiana.edu                          !
!  Natural Science Division                     !
!  Indiana University Southeast                 !
!  New Albany, IN    47150                      !
!  http://Physics.ius.indiana.edu/Physics.html  !
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!