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Re: RC circuits lab



You will need a large capacitance, so low voltage electolytic
capacitors are best. You will also need a fairly high resistance
voltmeter otherwise the capacitor will discharge through the voltmeter
input rather than the resistor you put in the circuit. Digital
voltmeters have such high input resistances, but are a problem because
they have a built in time delay since they usually integrate the signal
over time. Thus they are not very good for time dependent phenomena
like this unless the times are quite long, or the integration time is
small.
In addition, I have found that electrolytic capacitors are not very well
behaved if you apply a voltage much larger than about 10 percent of
their max voltage rating...I suspect that is because of internal
leakage. Finally, the value of the capacitance stamped on the capacitor
is not very reliable...perhaps 20 percent.

I hope this is some help to you.

cheers,

joe

On Mon, 5 Mar
2001, 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