More of a physics or is it electrical problem??

[sayee1]

Imagine I have a HT cable running from a power station to a distribution station. The moment I have a potential difference in the form of some volts, In this case it should be very high voltage and low current, the free electrons start flowing from the negative to the positive terminal. What happens when the free electrons at the end? If this goes on for a while, there should be a termendous build up of charge and electrons at one end and positive kernels/atoms at the other end. So then what happens , since the electrons are not moving outta the end of the wire, the build up should go on till it is not possible for any more free electrons to meve. Is this correct, anyone any ideas. Sorry for this stupid question>.........

Thanks and regards
Sayee Prasad R
Ph: 0097143968906
Mob: 00971507682668
email: sayee_prasad@yahoo.com
The black holes of nature are the most perfect macroscopic objects there are in the universe: the only elements in their construction are our concepts of space and time.

 

[Metalguy]

DC here, of course, butI think you forgot about the return path/ground.

 

[jackboot]

I would have to guess (for a DC situation):

If you have a voltage potential only (no current) the answer is yes - the electrons will build up at one end. Increasing the voltage would mean that more electrons would be stripped from their orbits and make a bigger "pile" at the opposite end.

Further - unless we are considering something other than metal conductors - you shouldn't have any positive ions moving. The increase in the positive charge is the result of moving more and more electrons out of the positive side and therefore resulting in an increased positive charge.

Relaxing/reducing the voltage would mean the electrons would flood back to the positive side and therefore balance the electric charge.

I don't know if this is helpful or if I am just rambling-

jackboot

 

[jetmaker]

I have heard about cases in the semicondutor world where there has been sufficient electron migration that the gauge of wire actually changes over time, with a buildup at one end and a stripping of thickness at the other. Eventually, enough electons migrate that the wire fractures due to insufficient gauge.

Now I may have this wrong in that it was another phenomena.. so if people know same/different, would appreciate the update.

jetmaker

 

[kenvlach]

sayeeprasadr,
You described a serious technical issue from the early days of electrical power distribution, and the distinguished Thomas Edison got it wrong. The polarization described is a very real problem in distributing DC power over long distances, which is why the AC power system of George Westinghouse prevailed.

jetmaker,
This is a phenomenon known as electromigration. An electric field or current can cause the diffusion of atoms (interstitial or substitutional) or vacancies. See Physical Metallurgy Handbook, pages 2-69 to 2-76 (2003). In &[ignore]alpha[/ignore];-iron, the highly mobile carbon interstitial moves toward the cathode. For in-depth coverage of this problem in electronic materials, see Electromigration and Electronic Device Degradation (1994).

 

[sayee1]

Kenvlach/Metalguy
So it means that power transmission is using AC....
Now consider a welding machine operating on DCEP. In this case just consider the return cable from the earting clamp to the welding machine. Since on DC, and a low voltage and high current prevails electrons in large numbers keep moving from machine to the return cable thus constituring flow of current. In this case when we say that the current flows across the junctions like say the cable end to the connector, connector to clamp, clamp to base metal, base metal to the electrode(thru the arc) and so on, I can only understand electrons moving across the arc.But does it mean that electrons freely move across the boundaries from one material to another constituting the flow of current? Please elaborate.....And these are the free electrons which are moving right, any changes in material properties because of the flow of these electrons?

Thanks and regards
Sayee Prasad R
Ph: 0097143968906
Mob: 00971507682668
email: sayee_prasad@yahoo.com
The black holes of nature are the most perfect macroscopic objects there are in the universe: the only elements in their construction are our concepts of space and time.

 

[Metalguy]

Electricity and free electrons are not something I know a lot about, but I don't *think* there's any macroscopic materials changes in your example. I've always heard that the free electrons "travel around the OD of the wire".

I suspect some of this is like the speed of light phenomenon-you always must ask "in relation to what?".

In high school someone built a Van de Graff machine which would through 6"+ long sparks-thin, red, ultra-low current, but something like 300,000+ volts. Somehow I can't envision that machine making any observable physical changes to a piece of thin wire-but it did make everyone's hair stand straight out when we all held hands and one guy (a brave one) touched the alum. globe on top.

Anyway, back to welding-here's a good one for you. *Why* do we put more heat into a workpiece with EP in SMAW, but with EN with GTAW? (hopefully I didn't get them backwards!).

Also, I thought T. Edison's plans for supplying home electricity was to use low voltage-something like 25VDC. Since he knew he couldn't send it very far, he thought of building many small gen. stations throughout a neighborhood.

 

[sayee1]

Metalguy
This has been the nemesis for me since I have been a welding engineer. When I was in school I was explained that the heat is on the +ve always. Whether GTAW or SMAW.... As a result you cannot use DCEP for GTAW/TIG. That is why we use DCEN for GTAW/TIG. Hence also in Al/Mg GTAW welding, weher we want the stirring of the base metal as well as more heat on the workpiece, so using DCEP GTAW would be more beneficial(The combined effect of ions hitting the Al oxides, the stirring action by the electrons and the heat on the bm would be best. However due to chances of W electrode getting melted, prefer AC GTAW for AL/Mg where half of the cycle DCEP and half DCEN. Just got my theories knocked out on AWS welding forum, where I was told even DCEP GTAW used for welding Al or otherwise w/o a problem. I know that we put more heat into a workpiece with EP in SMAW, but with EN with GTAW and was told so in a training program I attended, but the physics of that is beyond me. Maybe because of the arc, is it because of the shielding gas which is also ionised to form the arc in case of GTAW....Please explain in a way that I would remember this forever ;-)))

Thanks and regards
Sayee Prasad R
Ph: 0097143968906
Mob: 00971507682668
email: sayee_prasad@yahoo.com
The black holes of nature are the most perfect macroscopic objects there are in the universe: the only elements in their construction are our concepts of space and time.

 

[Metalguy]

I wish I could, but I'm not really sure myself. I looked it up once and I "think" it involved the fact that in SMAW you have all kinds of ions, some of them heavy, from the flux in the arc, and they apparently transfer quite a lot of heat into the work-maybe it's as simple as the +ions being repelled from the electrode (+) and attracted to the work (-), for DCRP, or EP.

For TIG/GTAW, perhaps the ions aren't as heavy or somehow don't transfer heat that way. Guess we'll just have to wait until someone who knows provides the answer. If they don't, I'll try to find out myself when I get a little time.

I don't see your 2nd post here-wish I had read it better than the 1st quick glance I gave it-it sounded good.