Surface current in conductors....

[Rohit Chatterjee]

The E field inside a conductor is zero....but there is a current sheet of infinitesimal thickness....How is that possible????
Moreover in case of a PERFECT conductor the thickness of this sheet tends to zero...in other words there is a surface current only.....now the surface of a conductor is equipotential....Doesnt the presence of current indicate the presence of a potential diference along the surface n hence an E field????

 

[Grossel]

I heard about this once, but it only applies when the frequenzy is very high. But I'm not familiar with the physics behind.

 

[devronious2009]

The E field is zero because there is no net charge. The electrons are simply chaining or moving in a long chain thru the atoms, since there is no additional or reduction in actual electrons there is no difference in the electric field. Remember electric fields require charge typically, an increase or decrease in charge to induce a potential difference. The potential difference is what drives the current, so there is some E field in a conductor that flows, but its not from the electrons, its from the driving electro-motive force.

If you were to get down to the quantum level you would see an E field between atoms as the electrons moved from one to the next, the E field would switch back and forth between forward and back as the electrons moved away from one atom and into the next. But its a very small fluctuation and the overal net at the macro level is zero.

Does that answer your question?

In short, if there is a current then there must be some E field that drives it so there is never a true zero there. Although at the distance from source to the position in the wire your messuring it it might be infitesimal since the falloff of an E field is Inverse Square.

The E field for a current is driven by things like batteries, so the battery produces the E filed and would then be the source, but its created and then satisfied over and over as it consumes and releases electrons thru the chemical reaction that drives it. So it probably fluctuates as well.

 

[ke5frf]

I'm not entirely sure of your question, but I do know at radio and microwave frequencies there can be a phase shift between the current at the core and the conductor's skin. This is skin effect. This can introduce unwanted impedances with various consequences.

In radio, this is something very important to consider with circuit and antenna system design. For antennas, as an example, power loss by way of heat can attenuate a signal, especially with longer transmission lines. With circuit design, particularly at microwave frequencies, the slightest added resistance or reactance has to be accounted for.

This is why in both cases special care has to be taken in selecting the proper cables, wires, shielding, and mimimize excessive distance between components and equipment if possible.

I'm not sure if this is what you are asking, but I hope it helps.

 

[Rohit Chatterjee]

Actually m concernd wth E.M waves n related stuff....not circuit theory....ur xplain does make sense tho...

 

[Warpspeed]

The (radial) e field within a conductor is certainly zero for steady direct current (dc).
But there will surely be a voltage drop ALONG the same conductor, due to resistance.

With alternating current, there is an induced constantly changing magnetic component that causes the current to crowd towards the outer skin of the conductor.
The higher the frequency, the greater this "skin" effect.

 

[devronious2009]

Rohit,

What warpspeed indicated as per radial e field and what I was talking about, the e field from the electromotive force are in two different directions and perpendicular. just to clarify things. The e field from electromotive force runs along the length of the wire, and what warpspeed is talking about is from the core center measured to the outside diameter.

Warpspeed, is that push to the outside created by the Lorentz Force? I think it would be but strange since its both created and used by the same electrons, which seems strange.

 

[Warpspeed]

The push to the outside is caused by the alternate building up and collapsing of a concentric magnetic field around the conductor during each half cycle of ac.

So yes, this electron displacing force is produced by the electrons themselves.

It is interesting that skin effect becomes far worse in a coil with many turns, and many layers, than in a straight length of wire carrying the identical ac current.
.
It has to do with flux density, and the rate of flux change (frequency).

It is also why winding high power, and high frequency transformers becomes such a specialised art.

 

[Rohit Chatterjee]

Sorry guys but m terribly confused right now...this "E field inside a conductor" stuff is driving me crazy right now....
may b i should get my concpts about that clear before i return to surface current n related stuff.....

 

[Rohit Chatterjee]

Btw...if we place a conductor in a static E field....in that case the E field inside the conductor would be zero wouldnt it???i hope i got at least ths part right.....

 

[Warpspeed]

Yes, that is what direct current produces, a static e field without change.
Current flows uniformly through the entire cross section of the wire.

Rohit,
Whenever a CHANGING magnetic field cuts through a conductor electrons are displaced.
That varying magnetic field can come from an external source, or be produced by the conductor itself.

The concept of a fixed e field within a conductor is not valid wherever the conductor is immersed in a rapidly changing magnetic flux.

 

[devronious2009]

there would be an electric field inside the conductor if you placed it inside an electrostatic device. Just like a changing magnetic field offsets the electrons, the same is true for an electric field. An electric field would have the same effect just at a right angle to the magnetic field. Remember an electric field can repel and attract electrons, even inside a conductor. In the conductor the electric field would pull or push them to one side more than the other and a charge would build up. This is in fact the very nature of a capacitor. If the electric field could not exist inside a conductor then I'm afraid a capacitor could not work.