Re: Elecraft Digest, Vol 168, Issue 30: current flow on copper strips

[Elecraft mailing list]

My thinking is the current density is uniform in the outer surface of a round tube falling according to skin depth at that frequency. At higher frequencies there is no current on the inner surface so whether they touch when squashed is immaterial, no current there anyway.
What is relevant is the shape of the magnetic field as the tube is progressively squashed. As it gets flatter the current distribution moves closer and closer to that of a strip. There are no sudden effects.
73 Alan
G0HIQ

> Message: 3
> Date: Fri, 27 Apr 2018 01:35:36 +0000
> From: "Dauer, Edward" <[hidden email]>
> To: "[hidden email]" <[hidden email]>
> Subject: [Elecraft] Current Flow on Copper Strips - a Question
> Message-ID: <[hidden email]>
> Content-Type: text/plain; charset="utf-8"
>
>    Since I don't know what precedes the word "thus" in the quotation below I would ask if someone could explain this phenomenon for me.
>
> I am trying to visualize this with a thought experiment.  In an earlier post someone (Skip?) mentioned that early transmission lines were sometimes hollow copper tubes, to respect the fact that AC flows only on the outside of a conductor.  So, imagine a hollow tube carrying RF (which may approximate the fast rise and fall times of a high voltage strike).  Current is flowing all over the surface, I gather.  Now squeeze the tube along its length so that a cross section becomes an ever flatter ellipse.  At the last instant squeeze it so that the sides are in contact with each other.  What happens to the current flow as that squeezing occurs?  Is it still all around the squished tube until the instant the two sides join?  And then it all flows primarily along the edges of the now flat conductor?  

______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [hidden email]

[David Woolley (E.L)]

I think that thinking in terms of skin depth, is causing a lot of the
confusion here.  Skin depth is a consequence of how the fields
representing the electromagnetic pulse propagate in the electron plasma
of the metal, so is a secondary effect, not a primary one.

This whole thing is better addressed assuming a perfect conductor, so no
wave penetration, and therefore no skin depth at all.

Far from the tube, the magnetic field is circular.  Immediately outside,
it parallel to the surface.  Transiting between the two, that means
field is compressed at the ends of the major axis.  The field density is
proportional to the current, so the current will be concentrated near
the major axes end points.

--
David Woolley
Owner K2 06123

On 27/04/18 23:47, Alan B wrote:
> My thinking is the current density is uniform in the outer surface of a round tube falling according to skin depth at that frequency. At higher frequencies there is no current on the inner surface so whether they touch when squashed is immaterial, no current there anyway.
> What is relevant is the shape of the magnetic field as the tube is progressively squashed. As it gets flatter the current distribution moves closer and closer to that of a strip. There are no sudden effects.

______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [hidden email]