# Re: [SI-LIST] : Question on propogation of high freq. signal in conductors

From: Michael Nudelman ([email protected])
Date: Thu Sep 21 2000 - 06:47:05 PDT

Prasanna:

>>>>>
And why is it that more flux is linked with inner part
of a conductor so that by Lenz's law the opposing
force for current is high and hence no current flows
in the inner parts of the conductor?
>>>>

OK. Let me use another model, that is used by a "Electricity" course for
universities for physicists (I am one, actually).

Let the altern. current flow through the conductor. Say, the direction is from
you to the horizon.
Then, the magnetic induction ( or field, in some systems they are the same,
though in SI they are not, but....) B, made by the current, will look like
clockwise circle aroud the current (perpendicular to the currrent and the
conductor).

Now. Let the current increase. The increasing B (remember, the derivative is
used in Maxwell's eqations, so it is positive for increase and negative for
decrease) will produce the electric field E, having the direction opposite to
the current's direction inside the conductor, and the direction, same as the
current's on the sirface of the conductor.
Thus increasing the current on the surface and compensating the current inside.
The current flows on the surface.

Now, let the current decrease. The decreasing B (of the same direction) will
produce the E of the opposite to the former direction, that is inside the
conductor it will oppose to the current decrease (coinside with it) and outside
it will support the current decrease, thus augmenting the effect of the current
change on the surface.

prasanna kumar wrote:

> Hi all,
> Thanks for the answers.But i still dont have my doubts
> cleared.
> I have learnt about skin effect but had the idea that
> it is only concerned with EM waves travelling from one
> medium(say air) to another(a conductor).I am unable to
> digest the concept of using skin effect here.Please
> clarify.
> And why is it that more flux is linked with inner part
> of a conductor so that by Lenz's law the opposing
> force for current is high and hence no current flows
> in the inner parts of the conductor?
> As quoted by Mr.Clewell,
> "'First slice a wire lengthwise to make concentric
> tubes, like growth rings on a tree trunk. Now if you
> consider the inductance of each ring separately
> the inner rings would be long and skinny and have more
> inductance than the outer rings which are fatter.'
> and what here is meant by sayin that the inner rings
> are long and skinny?Why should the inductance be more
> for such rings?"
> Thanks,
> Prasanna.
>
> --- "Jian X. Zheng" <[email protected]> wrote:
> > Hi,Prasanna:
> >
> > In short, the skin effect causes the current to be
> > flowing on the surface of
> > a conductor. In fact, you can demonstrate such a
> > phenomenon on a plane wave
> > incident onto a big sheet of metal:
> >
> > \
> > \ plane wave
> > \
> > ---------------------------------------- top surface
> > of the metal sheet
> > |
> > | plane wave in the metal
> > |
> > ---------------------------------------- bottom
> > surface of the metal sheet
> > \
> > \ transmitted plane wave
> > \
> >
> > When a plane wave is incident onto the surface, you
> > will see the electric
> > field is exponentially decaying in the -z direction.
> > J = Sigma * E, where E
> > is the electric field, J is the current and Sigma is
> > the conductivity. In
> > the other word, the J is exponentially decaying and
> > the decaying factor is
> > very big. It is just like the current only exists on
> > the surface of the
> > metal sheet. Thanks!
> >
> > Best regards,
> >
> >
> -----------------------------------------------------------------------
> > Jian-X. Zheng, Ph.D
> > Zeland Software, Inc., 48890 Milmont Drive, 105D,
> > Fremont, CA 94538, U.S.A.
> > Tel: 510-623-7162, Fax: 510-623-7135, Web:
> > http://www.zeland.com
> >
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> ----------------------------------------------------------------------------
> > --
> >
> > > -----Original Message-----
> > > From: [email protected]
> > > [mailto:[email protected]]On Behalf
> > Of prasanna kumar
> > > Sent: Tuesday, September 19, 2000 6:26 AM
> > > To: [email protected]
> > > Subject: [SI-LIST] : Question on propogation of
> > high freq. signal in
> > > conductors
> > >
> > >
> > > Hi,
> > > i have a small doubt.
> > > in high frequencies why does the signal is
> > confined
> > > only to the surface of the conductor?
> > > ie,(if i am putting it right)why does the current
> > > flows only at the surface of the conductor?
> > > thanks.
> > > Prasanna.
> > >
> > > __________________________________________________
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