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Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
Ash Small wrote "The heat is generated by the electrons essentially losing kinetic energy when they enter the positive electrode."
Does it follow that, the energy of the electric current in a circuit is carried by the mass and velocity of the electrons so that at any point in ark the heat radiated is due to the slowing of electrons?
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Radiotech, I was repeating what I'd read regarding welding. DC straight and DC reverse, with DC straight, the electrode is at negative potential and the workpiece is heated more (by electrons hitting it and imparting kinetic energy), in DC reverse welding, the workpiece is negative and the electrode gets hotter.
This is the accepted explanation (as far as welders are concerned). The positive electrode gets hotter.
(This is one of the reasons I got confused earlier, other reasons were I'd been up all night and I got confused with something else)
The electrons ionise air molecules, assuming the arc is in air, and there is a general movement of positive ions towards the negative electrode, but these don't enter the electrode (except, possibly, in welding).
The accepted explanation is something along the lines that the electrons have to give up energy when they enter the positive electrode.
The light given off by an arc is due to electrons dropping to lower energy levels when they re-combine with positive ions, and emmitting photons.
I'm as intrigued by Grenadier's observations as he is.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
I think the references to DC arc welding polarities are good starting places for an explanation.
Must let go of the idea that "electric current is primarily carried by moving electrons". That's essentially true in metals. In semiconductors a practical model has 2 kinds of charge carriers: electrons and holes. In electrolytes you have moving positive ions and/or negative ions and/or (rarely) electrons. Same for plasmas (in which e- is not an uncommon species).
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Klugesmith, I agree that you have both positive and negative ions in plasmas, but with CCP (which is what an arc is) the electrons enter the positive electrode (and are emitted by the cathode). positive ions are generally confined to the plasma itself.
It is supposedly (according to welding literature) this phenomenon that causes the heating, as electrons 'lose' energy.
I assume this is similar to the mechanism by which electrons emit photons when they drop to a lower energy level in other scenarios.
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