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

From: Vinu Arumugham ([email protected])
Date: Tue Sep 19 2000 - 13:11:47 PDT

If i_surface is the surface current and i_center is the current at the center
of the conductor and i_eddy is the eddy current, the net current at the center
is i_center - i_eddy and the net current at the surface is i_surface + i_eddy.

I was referring to this as the strengthening of the current at the surface.

Vinu

[email protected] wrote:

> 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:[email protected]]
> > Sent: Tuesday, September 19, 2000 11:12 AM
> > To: Kim Helliwell
> > Cc: prasanna kumar; [email protected]
> > 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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> > > Kim Helliwell
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