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RE: Toroid capacitance reduction
Original poster: "John H. Couture" <couturejh-at-mgte-dot-com>
Paul -
Thank you for your reply. I agree with what you have said below regarding
charged objects. However, what happens when one of the objects is grounded
as with Tesla coils? You cannot charge the earth. I agree that the fact is
the toroid capacitance does reduce when placed on the Tesla coil. But Why?
Is it because of the charges on the toroid? Does the toroid capacitance
decrease when placed on the secondary if there are no charges?
I made some simple tests to see what would happen. I placed a toroid at
various distances from a grounded plate. As expected the toroid capacitance
increased as the toroid was placed closer to the plate. I also tried placing
the toroid on the top of two TC secondary tubes one with windings and one
without. The toroid capacitance was greater when the toroid was on the tube
with windings. Both of these tests indicate that the toroid does not
decrease when placed on the TC secondary coil. It is obvious that the
decrease is possible only when the toroid is charged.
My conclusion was that when the toroid when placed on the TC secondary the
capacitance did not change provided the TC was not energized. If the TC was
energized the toroid capacitance may be less depending on the amount of
charge on the toroid? I have not researched this further.
How would the 34pf for Peter Reid's TC be determined theoretically? ETesla6
gave me 69.007 for capacitance and 72.7 KHz for the frequency for Peter's
TC. These two parameters do not give the proper 34pf toroid capacity. Bart
and Antonio's programs also do not give the 34pf toroid capacity. The
problem is that all of these programs including the JHCTES program are
giving outputs based on toroids that are not charged. How do you make these
programs recognize the varying charges on the toroids and showing the proper
toroid capacitance reduction?
My main interest is to find an equation that would give me the reduced
toroid capacitance at the design stage like the 34pf of Peter's TC. At
present the 34pf can only be determined after the TC is built and then
tested for the operating resonant frequency. With this very important test
frequency (84.43 KHz) the 34pf can then be easily calculated. But that
doesn't help the designer at the design stage. The JHCTES Ver 4.42 on-line
computer program will only help you to find the reduced toroid capacity when
you have the proper test frequency. To my knowledge there are no computer
programs that will give the coiler the proper toroid capacity he needs at
the design stage.
John Couture
------------------------------
-----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: Thursday, December 25, 2003 9:48 AM
To: tesla-at-pupman-dot-com
Subject: Toroid capacitance reduction
Original poster: Paul Nicholson <paul-at-abelian.demon.co.uk>
John Couture wrote (in 'First Light for 10" Coil'):
> I do not believe anyone has ever explained this Toroid reduction
> with Tesla coils. It should be an increase because the toroid is
> closer to other objects. Maybe Paul or Antonio will look into it
John's comments about reduction of toroid capacitance when it is
in-situ over the coil are quite correct. However, there is no
mystery to the reason why - it is a well known and understood
phenomena in electrostatics.
Generally if you have two objects A and B, having self capacitances
of Ca and Cb respectively when each is measured (or calculated) in
isolation, then when they are brought together, joined or just close
to one another, the combined capacitance of the two will always be
less than Ca + Cb.
You can think of it as each object shielding the other to some
extent. The effective electrical 'surface area' of the two objects
combined is less than the sum of the two separate objects. This is
because charge which would have been distributed fairly evenly over
each of the separate objects will be displaced away from the other
object when the two are brought together.
With fixed width font and some allowance for the ascii artwork,
the charge distribution around two isolated blobs is something
like:
+ + + + + +
+ ____ + + ____ +
+ / \ + + / \ +
+ | | + + | | +
+ \____/ + + \____/ +
+ + + +
+ + + + + +
where the + indicate charges on the surface of each blob (not in
the air around it as the sketch might suggest!)
When they are brought close to each other, we get
+ + + + + +
+ ____ + ____ +
+ / \ / \ +
+ | | | | +
+ \____/ \____/ +
+ + +
+ + + + + +
If you count the charges (ie measure the capacitance) you see
there are now fewer, because charge is not inclined to occupy the
surfaces of the objects which are close to each other. Thus the
total charge needed to raise the combined objects to some given
potential is less than the sum of the amount needed for the two
separate objects.
Why doesn't charge want to sit on the adjacent surfaces? Because
each charge can find a lower potential energy by moving elsewhere.
Each charge is immersed in the E-field generated by all the other
charges, and those that are free to move (ie not bound into atoms,
etc) will 'fall' through the field until they find their lowest
level. In effect the charges are just trying to get as far away
from each other as they can. The upshot is that less total charge
is needed to reach some potential because the adjacent surfaces
don't need to be covered.
It isn't too hard to calculate this behaviour, because at low
frequencies the only significant force on each charge is just the
Coulomb force.
Programs made available by Terry, Bart, and more recently Antonio,
will all take these effects into account quite accurately, because
one way or another they compute the actual charge distribution of
the in-situ objects.
And as a corollary, not only does C_toroid fall when in-situ, so
does C_secondary too.
Apparently my hands are waving so much they risk bringing down the
xmas decorations, but hope that helps!
Greetings All,
--
Paul Nicholson