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Re: RC circuits lab



Electrolytic capacitors from Radio Shack are very inexpensive.
Be careful to connect them properly; a capacitor can blow up if
if its + and - leads are not properly connected with respect to
the power supply. If the capacitor is rated as 10 V, for example,
do not exceed the limit.

Suppose C=500 microF. Connect it to a DC source, for example,
9 V, through a resistor of R=100 kohms. In that case the RC is
500*10^-6 * 10^5 = 50 seconds. Thus expect about 3*RC
(or 150 seconds) for the capacitor to be charged or discharged.

With R=100 Kohms the current from the 9 V source will never
exceed 9/10^5 = 10^-4 A or 100 microA. Thus you can use a
typical 100 microA galvanometer in series with R and C and
the battery. Ask kids to measure the current every 15 seconds
after charging or discharging begins. Then ask them to plot
the I=f(t) curve. Repeat the same with a larger R, etc.

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