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Re: elctronic components




On Mon, 30 Dec 1996, roger haar wrote:

Hi,

The very high capacitance, supercaps are a batttery-capacitor
hybird. They are more like batteries with a very thin electrolyte, so
the time for the chemicals to drift from terminal to terminal is short.
These supercaps are the subject of much research because of their high
energy density. Hoped for uses, are things like storage as part
of regenerative braking in electric buses.

Thanks
Roger Haar
U of AZ


I've been only sporadically reading this thread, so forgive me if I repeat
something someone else already said.

Chemical effects in capacitors have been with us ever since the invention
of the electrolytic capacity. They, too, had the character of a chemical
cell, forming a very thin layer of polarized material near the can, so
thin that it was in effect a very thin capacitor. But they didn't achieve
the rated capacitance until they were charged or discharged a few cycles,
a process called 'forming the capacitor'. Photographers who use flash guns
with such capacitors know this well. The instructions say "If the unit has
not been used for some time, form the capacitor by flashing it several
times." If this is not done, the effective capacitance is too small on the
first flash, which might not give enough current when discharged to flash
the bulb.

I understand that some of these modern high-tech capacitors take this
ion-layer principle one step farther by making the layer even thinner, and
forming it on a porous material, or a surface with microscopic roughness
to greatly increase the effective surface area on which that layer forms.
I think there was a nice article in TPT about that this year. So, by
making A larger and d smaller, you get large values of C. A nice example
as worthy of inclusion in textbooks as any of the other 'sidebar' examples
they are cluttered with these days. Why do so many textbooks fail to
mention electrolytic capacitors, which are found in so many everyday
electronic circuits?

Even non-electrolytic capacitors have some resistance, and most meters
have capacitance, and the circuit has resistance, capacitance and
inductance... and so it goes. The experimental determination of decay
curves is fraught with difficulties. I'm not sure it's suitable for
freshmen, unless one chooses the experimental components very carefully.
But is this fair? To choose things which won't cause trouble, giving the
students a false confidence that they can go and measure any capacitor
with equal ease?

-- Donald

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Dr. Donald E. Simanek Office: 717-893-2079
Prof. of Physics Internet: dsimanek@eagle.lhup.edu
Lock Haven University, Lock Haven, PA. 17745 CIS: 73147,2166
Home page: http://www.lhup.edu/~dsimanek FAX: 717-893-2047
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