What is the physical significance of skin effect?

[koushikr_in]

what is the physical significance of skin effect? how it applies to a waveguide?

the em wave tries to penetrate into the conductor? will it not lead to powe dissipation if the wave travels within the conductor?

what is the gen requirement---should the sigal flow along the cond surface or should it be confined within the conductor?

 

[sergio mariotti]

skin effect

The phisical sognificance of the skin effect is that the em signal don't use all the conductor thikness available to flow, but a little part only. It depend on freq., resistivity and permittivity.
The penetration factor is the thikness at wich the current density is 1/e (0.3678...).
Of course less metal is used for the electrical conduction, higher is the series resistance.
Tipical values for copper are microns at GHz.
Inside the waveguide, dependig on the exited mode, the attenuation is dominated by the metal losses. Only few, exotic, and difficult to exite, modes are insensitive to the metal resistivity.

 

[dowjones]

skin effect

Hello sergio,

a couple of things: what is the meaning of "less metal"?, do you mean metal thickness?. To my knowledge series resistance of the conductor is dependent on the conductivity and the skin depth (at less in a first approximation), and it does not depend on the metal thickness.

second: you say "depending on the mode, attenuation is dominated by metal losses". To me, in a hollow waveguide, the unique loss responsible is the metal loss, and therefore attenuation is always dictated by metal loss, independently of the mode, although the value of the loss is dependent on the excited mode.

regards

 

[rajesh13]

Re: skin effect

With in the wave guides the signal is not confined to the surface. Actually it is reflected by the internal surface of the wave guides.

 

[djalli]

Re: skin effect

With in the wave guides the signal is not confined to the surface. Actually it is reflected by the internal surface of the wave guides.

Can you elaborate this a bit more?

Also as frequency goes up less we are eager to coat waveguides with GOLD. This saves money.

You can also fill the internal transmission line structure with something cheap because due to skin effect you only need few micron metal coating for signals traveling in these lines with GHz frequencies.

 

[Maya]

Re: skin effect

The reason that a person is safe inside a metal car during a thunderstorm (or in the operators cage in our theater) is not, as many assume, because the metal container forms a Faraday cage but because of something known as the skin effect.


A Faraday cage only works with static electricity (the storage spheres on top of our Van De Graaff are Faraday cages.) What happens there is that the negative charges on a Faraday cage repel each other and try to get as far away from each other as possible. The best way to do that is to occupy only the outside of the cage. Hence there is no electricity on the inside.


When a lightning bolt hits, however, the electricity is not static--it is moving very rapidly! Some other effect must be at work.


Lightning is an example of a very high frequency alternating current. This may seem illogical as the electricity in a lightning bolt normally only moves down, but looking at the current/time diagrams below will show that it is more like a short piece of alternating current than direct current.


All electric currents generate magnetic fields that in turn can affect the current (this is the principle behind electric guitar pickups). In a direct current case everything is constant and so nothing seems to happen. With an alternating current, however, there is a delay in the magnetic field's response to the change in current and the 'old' magnetic field tends to push the current towards the outside of the conductor. As the frequency increases, so does the effect until at very high frequencies the entire current flows in a very narrow skin on the conductor--hence the name.


The earliest work on explaining the skin effect was done by Lord Kelvin (of temperature fame) in 1887. Tesla also investigated the effect.

 

[koushikr_in]

Re: skin effect

nice reply maya.

can u explain how a delay is introduced in the magnetic field's response to the change in current and how the 'old' magnetic field pushes the current towards the outside of the conductor. can u clearly explai the freq dependence?

if possible give me some web links?

 

[sergio mariotti]

Re: skin effect

Hello sergio,

Sorry for the late, only now i've seen you asked to me...

>a couple of things: what is the meaning of "less metal"?, do you mean metal thickness?.
Yes, i mean less metal thickness in the case that thickness is comparable to penetration factor.


>second: you say "depending on the mode, attenuation is dominated by >metal losses". To me, in a hollow waveguide, the unique loss >responsible is the metal loss, and therefore attenuation is always >dictated by metal loss, independently of the mode, although the value of >the loss is dependent on the excited mode.


??? You write the same concept i wrote.
In any case, the WG attenuation is related to: metal resistivity, magnetic permeability of the metal, metal thickness (if less or comparable to penetration factor), surface roughness, and the mode of propagation.

The tipical case is the circular waveguide (WC) where the exotic mode TE01 has much more less attenuation than the usual TE11.

 

[dowjones]

skin effect

Hello Sergio, OK , I think we are telling the same

Maya, this is the first time I read that lightning is not an electrostatic effect!! (charges on clouds and charges on ground and so on).

I thought that Benjamin Franklin explained correctly the physics of lightnings by 1750.

 

[sergio mariotti]

skin effect

dear dowjones, I agree maya; of course lightings are electrostatic fenomena, but during the discharge there is a transient, so an impulsive effect, and as you certaintly know, a pulse have it's frequency spectrum. Faster is the pulse, wider is the spectrum.
So all fenomena related to a variable current exist (in a limitated time). One of this fenomena is the inducted current that is the cause of many electronic damages during storm.

 

[dowjones]

skin effect

Hello Sergio,

OK but...

I agree that the discharge of the lightning is a pulse, and being a pulse it contains spectral components. however the main components of the spectrum are limited to a few kilohertz (from 0.1 to 10kHz more or less as the references below say). In my opinion this is far from being high frequency (although this concept is relative, I know).

The skindepth for an aluminium sheet at 10kHz is 0.8mm, and at 0.1kHz is 8mm, which is quite far from the micron units we are used to handle when we talk about skin depth effects. (honestly if I were in the middle of a lightning storm I should prefer a box with 8mm wall thickness than a 0.8mm one !).

About the effect of discharges on electronic equipments, I think that are the low-frequency high-voltages the causes of the damages to the equipments due to their effects on transistors, diodes... I understand that no high-frequency effects appear (NOTE: to me high frequency is microwave).

In summary Sergio, as you have correctly pointed out, the origin of lightning is electrostatic and the discharge of the lightning is (Low Frequency) electromagnetic due to the pulsed effect.

regards

**broken link removed**

http://www.madsci.org/posts/archives/nov98/912136278.Ph.r.html