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Re: A LAB TO SHARE



BW!

You mean a cap. with two dielectrics? Is this necessary?

bc

Brian Whatcott wrote:

Faraday claimed that this 'recovery' mode could be demonstrated with =
any=20
two layer dielectric capacitor - where the dielectric propertiesof th=
e=20
layers differ.

Brian W

At 12:47 AM 2/12/02, you wrote:
Maybe he wants to compete with Jacques P=E9pin?

bc

P.s. Also don't wait long after shortening the caps. (dielectric
relaxation) Another wow demo, I first saw demo'd by the NCNAAPT Pre=
z. (heard
about long ago). Rmove the dielectric (after charging), short the p=
lates,
replace the dielectric, and see if any one will volunteer to short i=
t with
their hand.


Jack Uretsky wrote:

I doubt that 2A actually means what it appears to say!
Regards,
Jack
On Mon, 11 Feb 2002, Ludwik Kowalski wrote:

In reading my own message I see that one thing was
missing. It has been inserted as point 2A below.
--------------------------------------------------------------

1) Suppose you want to verify a prediction about the
equivalent capacitance of two or more capacitors
connected in one way or another. This is easy when
a method for measuring a capacitance Cx is available.
Here is one simple method.

2A) For a moment shorten the leads of the Cx capacitor
with a wire to make sure it is discharged. If Cx is a set of
several capacitors then make sure that all component are
discharged before proceeding. Without doing this you may
get a wrong result. (The most reliable way of preparing a
setup for a measurement is to shorten all components at
the same time and then to disconnect the shortening wires.
Shortening only the external leads may not be sufficient
for some setups.)

2B) You must have a reference capacitor whose C is
known. Connect it to a dc source of known voltage, such
as U1=3D12 V, to fully charge it. Then disconnect it from
the source.

3) Connect the terminals of your fully charged C to the
terminals of the Cx box and measure the new voltage, U2.
The voltmeter must have very high impedance, so high
that U2 remains practically constant for at least 5 seconds.
Pasco electrostatic voltmeters are ideal for the purpose.

4) Convince yourself that the conservation of Qleads to
Cx=3DC*(U1-U2)/U2. Use this formula to calculate Cx.
Be aware that C should be of the same order of magnitude
as Cx. It is a good idea to have several reverence
capacitors. Use a larger C when U2 is too small, use a
smaller C when (U1-U2) is too small.

5) Do not wait too long before measuring U2, even with
a nearly ideal voltmeter, when Cx consists of capacitors
connected in series. Why is this important? Because the
implied assumption of "no effect from the leakage of
charge" may no longer be valid. This topic was discussed
here in 1994 (?) In most cases several seconds is not too
long but several hours is too long. Several minutes may
be too long for some electrolytic capacitors.
Ludwik Kowalski


--
"But as much as I love and respect you, I will beat you and I wil=
l kill
you, because that is what I must do. Tonight it is only you and =
me, fish.
It is your strength against my intelligence. It is a veritable p=
otpourri
of metaphor, every nuance of which is fraught with meaning."
Greg Nagan from "The Old Man and the Sea" in
<The 5-MINUTE ILIAD and Other Classics>

Brian Whatcott
Altus OK Eureka!