Electrical Conduction: Exploring Drift Speed and Force Requirements

[ZapperZ]

It’s probably me but I don’t understand this sentence.

You have stated (#28) that on average the mercury ions are neutral. That still leaves a non neutral net charge per atom because of the –ve electron. Are you saying that the electrons are on average neutral as well?

It's my turn to say that I don't understand what you are saying.

The Hg liquid is ELECTRICALLY NEUTRAL. You could FREEZE the Hg and turn it into a solid and it will behave the same way. A liquid metal is STILL a metal (by definition!) and the electrons are still as mobile. There's always the same number of electrons in the Hg liquid as necessary to keep it electrically neutral... just like a solid metal!

Now is that any clearer?

Zz.

 

[erickalle]

However, under normal conditions, the electrons are MORE mobile than the ions. It means that if you apply any reasonable potential across mercury, the electrons are zipping in and out of the mercury so fast, the mercury ions, on average, NEVER become electrically charged! So these so-called "ions" are never ions in the first place!
Zz.

According to all electrical conduction theories:
Say a conduction electron under the influence of an external electrical field leaves an atom. This atom now becomes an electrically charged ion. There’s absolutely no doubt about that! There are electrically charged +ve ions for as long as the current flows.

 

[ZapperZ]

According to all electrical conduction theories:
Say a conduction electron under the influence of an external electrical field leaves an atom. This atom now becomes an electrically charged ion. There’s absolutely no doubt about that! There are electrically charged +ve ions for as long as the current flows.

Think again. The conduction electrons are in BANDS already - they no longer belong to ANY ions, meaning they are non-localized! Their energy level no where near resemble that of an atom!

What does it mean? It means that even without an applied field, the conduction electrons are not a part of any atom. Yet, do you see the ions here reacting to any external field? Nope! Why? Look up "skin depth" or a metal, or why the conduction electrons can be such an effective shielding of any external field!

The metal remains neutral. The ions do not see appreciable field. It is why when you do a Gauss's law on a metal for a static field, there's zero E-field inside a metal!

Zz.

 

[erickalle]

Think again. The conduction electrons are in BANDS already - they no longer belong to ANY ions, meaning they are non-localized! Their energy level no where near resemble that of an atom!

What does it mean? It means that even without an applied field, the conduction electrons are not a part of any atom. Yet, do you see the ions here reacting to any external field? Nope! Why? Look up "skin depth" or a metal, or why the conduction electrons can be such an effective shielding of any external field!

The metal remains neutral. The ions do not see appreciable field. It is why when you do a Gauss's law on a metal for a static field, there's zero E-field inside a metal!

Zz.

Now put a pd across a conductor. What will be the net result of forces by the field exerted on the conduction electrons and forces exerted on “on average” neutral ions?

 

[ZapperZ]

Now put a pd across a conductor. What will be the net result of forces by the field exerted on the conduction electrons and forces exerted on “on average” neutral ions?

What forces? Under STATIC equilibrium, there are not NET fields in the conductor itself! This is what I've been trying to get across to you, that ON AVERAGE, due to the high mobility of the electrons, they zip into the anode and come out of cathode so fast as to shield the inner ions from any net electric field! Go do Gauss's Law on a conductor in a static field! What's the E-field inside the conductor? You continue to ignore my question on this.

At NO POINT in a simple static equilibrium electrical transport is there a net charge on the conductor. The conduction electrons make sure of that.

Besides, what IF there is a net force on those anyway? Are you saying that they SHOULD move due to your calculated "large" force? Or are you still arguing that you have calculated enough energy that a copper wire SHOULD melt?

Zz.

 

[erickalle]

What forces? Under STATIC equilibrium, there are not NET fields in the conductor itself! This is what I've been trying to get across to you, that ON AVERAGE, due to the high mobility of the electrons, they zip into the anode and come out of cathode so fast as to shield the inner ions from any net electric field! Go do Gauss's Law on a conductor in a static field! What's the E-field inside the conductor? You continue to ignore my question on this.
Zz.

Quote of Ashcroft / Mermin solid state physics near the top of page 7:

The resistivity ρ is defined to be the proportionality constant between the electric field E at a point in the metal and the current density j that induces: E=ρj.

It quite clearly means to say there’s an E field in the metal.

 

[Dr Transport]

If there is an E-field in a metal, please produce the experimental evidence and I'll be more than happy to email the people in Stockholm to nominate you for the Nobel because you would be putting the entire physics community on it ear...

 

[ZapperZ]

Quote of Ashcroft / Mermin solid state physics near the top of page 7:

It quite clearly means to say there’s an E field in the metal.

Again, what was the MODEL being used there? It is the free-electron gas based on a CLASSICAL IDEAL GAS!

Jeeeze! It's like talking to a WALL!

Zz.

 

[erickalle]

If there is an E-field in a metal, please produce the experimental evidence and I'll be more than happy to email the people in Stockholm to nominate you for the Nobel because you would be putting the entire physics community on it ear...

Hi Dr.Transport. Thanks for joining in. If you had joined earlier, it would have saved just about 25 or so replies in this thread.
So there’s no E-field in a metal and therefore the relation F=E*q in a metal is not valid anymore. This is new to me but not entirely unexpected. My edition of the book I mentioned is from 1976, it has a whole chapter called “Failures of the free electron model”. In there is no mention of the above fact, also throughout the book formula’s are used using an E-field existing in the metal.
I don’t think I’m the only one caught out here.
Is there an E-field just outside a conductor? Can you recommend a textbook?
Can you put that email on hold?

 

[Dr Transport]

If you can find an edition of Ashcroft and Mermin that isn't from 1976, please pass it along, they never wrote a second edition.

As for the electric field just outside a conductor, sure there is one. Look at Jackson's E&M book for the expression of a conductor in an electric field. If there isn't an electric field applied, there won't be one unless there is excess charge.

I'll be happy to hold off on that email...

 

[Farsight]

Perhaps "wire explosion" is relevant to what you're talking about, Eric. Search Google.

http://flux.aps.org/meetings/YR00/DPP00/abs/S1360079.html

"The Joule energy deposition in the initial stages of exploding wires has been investigated. A Maxwell 40151-B trigger generator, producing a maximum current of ~3kA and voltage of ~120kV, with current rise times of ~170A/ns in a fast regime and 22A/ns in a slow regime, was used in the explosion of different fine wires. The wires were 2 cm in length and 4-40 microns in diameter. Current, voltage, current and voltage derivatives, wire boundary evolution, interferometry, shadowgraphy, absorbography, time-resolved self-luminosity imaging, emitted light intensity were monitored in the experiment. The evolution of wire diameter was determined using a diode laser back lighter and streak camera. Two significantly different modes of wire explosion have been found: fast and slow. It has been found that electronic emission from wire surface plays important role in the Joule energy deposition inside the wire..."

Does anybody know why my cheap little DC welder exhibits a strong attraction between the rod and the subject?