Re: RC circuits lab

[Roger Haar <haar@PHYSICS.ARIZONA.EDU>]

Justin,

        We use 1000 micro Farad (50 volt or more
operating volts) capacitors and 10,000-100,000 Ohm
resistors.  A nine volt battery can be used to
charge the capacitor.  One interesting thing to do
is to start the timing when the voltage across the
capacitor is 8 volts, record the time at 4 volts,
2, 1, 0.5, 0.25...  The halving time being
constant.

We have also had luck measuring voltage in  a 1000
micro Farad and a 100,000 Ohm circuit every 10 or
20 seconds all the way from 9 volts to 0.009
Volts.  Great for showing why a semi log plot is
useful:  1.  The later data vanishes to zero on a
linear plot.  2.  One gets a straight line.  ( One
can make a semi log plot without introducing semi
log paper, just by having the student calculate
and plot the log's of the voltage.

        A couple of things to be careful with.  First the
capacitors are probably electrolytic.  If so the
polarity MUST be observed.  During the
experiments. we connect discharging wires across
our cap's when not in use,  And once a semester,
(for a weekend) we connect them to a power supply
in the correct polarity, just to reset the
polariation of the electrolyte.

        Other report major leakage problem with cap's,
that is the cap's act like there is an internal
shorting resistor.  Our choice of R and C is for a
reasonable discharge rate that is measurable by
hand.  The choice of R is so that it is much
smaller than the internal resistor of the
capacitor or the meter.

Thanks
Roger Haar


Justin Parke wrote:
> Can someone refer me to a good lab I can do with Advanced Placement (C) physics students for RC circuits?  I want them to measure current and voltage several times as a capacitor charges/discharges and obtain the exponential curves which we can then fit and derive the proper equations.
>
> My main problem is my lack of experience with these type of circuits so that I do not know which combinations of resistance, capacitance, and applied voltage provide the proper charging time, etc.  I blew up a capacitor the other day while I was playing in the lab (and supplies are limited!)
>
> Perhaps some of you have written something of your own you could share with me, or refer me to some website.
>
> Thanks
>
> Justin Parke
> Oakland Mills High School