RE: [SI-LIST] : Re: Skin effect (Was: Nil)

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From: sam.kvk@amd.com
Date: Tue Sep 19 2000 - 12:13:50 PDT


My 2 cents (along the lines of Vinu's explanation in circuit theory terms):

Take a round conductor and assume that it is made up of many hypothetical
wires with smaller diameters, all bunched together. Alternating Current
flowing in one such smaller wire will induce currents in the adjacent wires
flowing in the opposite direction, through mutual inductance (transformer
action). (Magnetic field is present even for d.c. currents. But, transformer
action takes place only for a.c. The amount of induction increases with
increasing frequency.) Thus the original currents in the adjacent wires
experience an increased inductive reactance and such phenomenon is called
"internal inductance" for the conductor.

The hypothetical wire that is exactly at the center of the conductor cross
section will have the maximum of such "internal inductance" among all the
wires, as it is surrounded by the maximum number of adjacent wires. And this
inductance value keeps reducing as the location moves towards the perimeter.
Those hypothetical wires at the perimeter (or in other words "at the skin")
of the conductor have the minimum internal inductance, as they do not have
any adjacent wires at all on one side. As a.c. currents always take the path
with minimum impedance, high frequency currents tend to flow through these
wires at the perimeter and thus the crowding of current at the skin of the
conductor.

Vinu...I don't understand your statement that there will be strengthening of
current at the surface of the conductor. Can you pls clarify?

-Sam KVK

> -----Original Message-----
> From: Vinu Arumugham [SMTP:vinu@cisco.com]
> Sent: Tuesday, September 19, 2000 11:12 AM
> To: Kim Helliwell
> Cc: prasanna kumar; si-list@silab.Eng.Sun.COM
> Subject: Re: [SI-LIST] : Re: Skin effect (Was: Nil)
>
> Another way of looking at it:
>
> When an alternating current flows through a conductor, a magnetic field is
> created around it. If the conductor has finite dimensions, some of the
> magnetic field is contained within the conductor. Such a changing magnetic
> field inside a conductor, creates eddy currents that tend to oppose the
> main current at the center of the conductor and strengthen the current at
> the surface of the conductor, resulting in the skin effect.
>
> Vinu
>
> Kim Helliwell wrote:
>
> > I don't know how in-depth (pun not intended!) an answer you're seeking.
> > The name of this effect is the skin effect. It comes
> > directly out of Maxwell's equations when considering
> > electric fields in conductors. What happens is that
> > an electromagnetic wave gets damped out inside the
> > conductor, and the penetration distance, or skin depth,
> > is inversely proportional to the square root (to a first
> > approximation) of the frequency of the wave. Since charges
> > in a conductor are propelled by electric fields, current
> > can only flow where the electric fields have not been damped
> > to (essentially) zero. Which is near the surface.
> >
> > Nearly any good graduate level E&M text will discuss this
> > effect with all the mathematical machinery. My reference is
> > _Classical_Electrodynamics_, by J.D. Jackson.
> >
> > Jackson says that the skin depth for copper at 60 Hertz is
> > 8.5 mm, and for 100 MHz, it's about 7 microns.
> >
> > Kim Helliwell
> >
> > prasanna kumar wrote:
> > >
> > > 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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