Electrical code Compliance - Isolated Cylinder Capacitance

Author

Isolated Cylinder Capacitance

Hayato

2005-06-17, 11:30 pm

Hello,
I'm sorry to bother.
But I've been searching for a long time, and I cannot find a formula
to calculate the capacitance of an isolated cylinder.
Can you help me?

Thanks

Paul E. Bennett

2005-06-17, 11:30 pm

Hayato wrote:

quote:

> Hello,
> I'm sorry to bother.
> But I've been searching for a long time, and I cannot find a formula
> to calculate the capacitance of an isolated cylinder.
> Can you help me?
>
> Thanks

One quick Google later and I found:-

<http://home.san.rr.com/nessengr/tec...itance/cap.html>

Looks like just the formulae you need.

--
********************************************************************
Paul E. Bennett ....................<email://peb@amleth.demon.co.uk>
Forth based HIDECS Consultancy .....<http://www.amleth.demon.co.uk/>
Mob: +44 (0)7811-639972
Tel: +44 (0)1235-811095
Going Forth Safely ....EBA. http://www.electric-boat-association.org.uk/
********************************************************************

Hayato

2005-06-17, 11:30 pm

Thanks,
But the formulae shows the capacitance of two coaxial cylinders.

"Paul E. Bennett" <peb@amleth.demon.co.uk> wrote in message news:<d4giog$opa$1$8300dec7@news.demon.co.uk>...

quote:

> Hayato wrote:
>
>
> One quick Google later and I found:-
>
>
> <http://home.san.rr.com/nessengr/tec...itance/cap.html>
>
> Looks like just the formulae you need.

Ned Forrester

2005-06-17, 11:30 pm

Hayato wrote:

quote:

> "Paul E. Bennett" <peb@amleth.demon.co.uk> wrote in message news:<d4giog$opa$1$8300dec7@news.demon.co.uk>..
>
> Thanks,
> But the formulae shows the capacitance of two coaxial cylinders.

What, exactly, are you looking for? For an infinite length cylinder at
infinite distance from ground, the capacitance is infinite. Even the
capacitance per unit length is infinite, because the field strength
falls as 1/r, not 1/(r^2), so the capacitance per unit length goes as
log(r).

To have a defined capacitance, the cylinder must be of finite length,
making the calculation of capacitance to infinity much more complicated.
I bet that there is not an analytic solution for all cases, but rather
some good approximations for cylinders that are very long or short
compared to their radius. The last time I needed this, I used a
numerical solution, though there may be better ways.

--
NOTE: to reply, remove all punctuation from email name field

Ned Forrester n_f_orrester@whoi.edu 508-289-2226
Applied Ocean Physics and Engineering Dept.
Oceanographic Systems Lab http://adcp.whoi.edu/
Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA

Paul E. Bennett

2005-06-17, 11:30 pm

Hayato wrote:

quote:

> Thanks,
> But the formulae shows the capacitance of two coaxial cylinders.
>

There were two formulae, one for a flat plate solution and one for a
coaxial solution. So, if neither of those did it for you then you are
probably looking for the formula for a cylinder above a flat plane.

http://www.eece.ksu.edu/~gjohnson/tcchap2.pdf

should answer your question.

Incidently, I used just two words in the google search and feel that you
could have easily come by the two links I provided by learning how better
to use the Google Search Engine.

The two words I used; "Capacitance" and "Cylinder".

--
********************************************************************
Paul E. Bennett ....................<email://peb@amleth.demon.co.uk>
Forth based HIDECS Consultancy .....<http://www.amleth.demon.co.uk/>
Mob: +44 (0)7811-639972
Tel: +44 (0)1235-811095
Going Forth Safely ....EBA. http://www.electric-boat-association.org.uk/
********************************************************************

Andrew Thomas

2005-06-17, 11:30 pm

Hayato wrote:

quote:

> Hello,
> I'm sorry to bother.
> But I've been searching for a long time, and I cannot find a formula
> to calculate the capacitance of an isolated cylinder.
> Can you help me?
>
> Thanks

Interesting question. I'm tempted to say that there is no such thing as
an isolated cylinder: and if there were, it may well have 0 capacitance.
On the other hand, an "isolated" object such as the oblate of a Van
deGraf (sp?) generator can hold a charge, and in proportion to voltage:
but is it truly isolated? Absolutely not. I think the "capacitance" of
such objects can vary greatly depending on subtle details of geometry,
including for example the sharpness of the edges.

I think this is a question for the physics folks, not the electricians:
not even the electrical engineers.

-Andrew

Paul E. Bennett

2005-06-17, 11:30 pm

Andrew Thomas wrote:

quote:

> Hayato wrote:
>
>
> Interesting question. I'm tempted to say that there is no such thing as
> an isolated cylinder: and if there were, it may well have 0 capacitance.
> On the other hand, an "isolated" object such as the oblate of a Van
> deGraf (sp?) generator can hold a charge, and in proportion to voltage:
> but is it truly isolated? Absolutely not. I think the "capacitance" of
> such objects can vary greatly depending on subtle details of geometry,
> including for example the sharpness of the edges.
>
> I think this is a question for the physics folks, not the electricians:
> not even the electrical engineers.

If by isolated the OP meant at infinite distance from anything else then
the capacitance is infinite according to the notes on one of the links I
posted. However, it does make more practical sense to consider the cylinder
at a specific finite distance from another object.

--
********************************************************************
Paul E. Bennett ....................<email://peb@amleth.demon.co.uk>
Forth based HIDECS Consultancy .....<http://www.amleth.demon.co.uk/>
Mob: +44 (0)7811-639972
Tel: +44 (0)1235-811095
Going Forth Safely ....EBA. http://www.electric-boat-association.org.uk/
********************************************************************

Hayato

2005-06-17, 11:31 pm

Hello and thanks everybody.

Paul,
Thanks for that PDF link!
I did an extensive search in Google search engine, before I post here,
and if I missed that PDF document it was due a lack of attention! ;)

Well, I was thinking of a formulae to calculete an isolated cylinder
capacitance, "similar" to the isolated sphere capacitance, which
formulae is:

C = 4*Pi*Epsilon0*R,
Where Epsilon0 is 8.85E-12;
R = the sphere radius;

So, like Andrew said,
I'm thinking in an isolated cylinder, like a Van Der Graaf's oblate /
dischage terminal.

Thanks everybody!

"Paul E. Bennett" <peb@amleth.demon.co.uk> wrote in message news:<d5281r$sub$1$8302bc10@news.demon.co.uk>...

quote:

> Andrew Thomas wrote:
>
>
> If by isolated the OP meant at infinite distance from anything else then
> the capacitance is infinite according to the notes on one of the links I
> posted. However, it does make more practical sense to consider the cylinder
> at a specific finite distance from another object.

Roy Q.T.

2005-06-17, 11:31 pm

sheesh CrashCup !

Still trying to Foil the Master Cylinder ? good calcs dude..
=AEoy

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