Talk:Cathode

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Intro was incorrect, Cathode is Always positive in elecrolytic/galvanic cells, unless definition of cathode is incorrect

Deleted statements at intro as there is a counterexample: If electrons flow in one direction ---> during a chemical reaction in a battery (galvanic cell). Then the elecrons flow opposite <--- when the battery is recharged (elecrolytic cell)

The change in current direction requires that either the definition of cathode and anode be incorrect. Or that the entire argument about the cathode being either positive or negative is incorrect (or contains a large exception).

Why? We reversed the direction of the current therefore the definition requires that the cathode is redefined as the anode (the copper cathode in a galvanic cell, must be the anode in the elecrolytic cell). This allows allows the cathode to maintain a (+) charge. Therefore in THIS instance, the statement that electricity consuming devices have a negative (-) cathode must either be false, or the definition of cathode must be.

To be completely clear I will use the example of the daniel's cell: At the anode, zinc is oxidized per the following half reaction:

   Zn(s) → Zn2+(aq) + 2e- .

At the cathode, copper is reduced per the following reaction:

   Cu2+(aq) + 2e- → Cu(s) .

The total reaction being:

   Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s).

Electrons flowing from zinc to copper in the forward reaction. If it were reversed (making it an elecrolytic cell), then we would reverse the half-reactions, meaning that electrons flow from copper to zinc. Reversing the current direction and forcing us to redefine the zinc as cathode, and copper as anode. Else we are inconsistent with the definition of a "cathode".

I apologize in advance if this is incorrect, and my removal of the intro section was at fault.

Finniganawakens (talk) —Preceding undated comment added 02:42, 17 January 2011 (UTC).[reply]

The above argument is confused about cell potential. To first order, the cell potential does not change whether charging or discharging; the designation of the positive and negative terminals does not change. In the Daniell cell, the copper electrode is positive and the zinc electrode is negative. The direction of the current does depend whether charging or discharging, so the designation of which terminal is the anode or the cathode changes with the mode. Glrx (talk) 01:00, 17 January 2011 (UTC)[reply]

Following copied from User talk:Glrx/Archive 1#Cathode. Glrx (talk) 19:29, 17 January 2011 (UTC)[reply]

Hello, I made the revision on cathode where the section was deleted. You stated the following, and if it's correct then I agree with you the deletion was misguided:

The above argument is confused about cell potential. To first order, the cell potential does not change whether charging or discharging; the designation of the positive and negative terminals does not change. In the Daniell cell, the copper electrode is positive and the zinc electrode is negative. The direction of the current does depend whether charging or discharging, so the designation of which terminal is the anode or the cathode changes with the mode.

However, it also means that the article could use the additional information of how a terminal is defined as positive or negative (or was originally). As the difference in definitions between (+) and (-) vs cathode and anode will make it more clear as to why there's a difference.

Therefore I made the following change at the end of the intro:

The reason that the cathode can change designation (+) to (-), is that the terminal the cathode changes designation when the current changes direction while (+) and (-) do not. Take the Daniel electrochemical cell: as a galvanic cell the (+) copper is the cathode while zinc (-) is the anode, if it were elecrolytic (+) copper would be the anode and zinc (-) the cathode.

I think it would be preferable just to state the definition of (+) and (-) so it can be compared to the definition for cathode an anode, but this is the best alternative available to me (as I can't find how (+) and (-) are defined).

And thanks for correcting me! :)

Finniganawakens (talk) —Preceding undated comment added 02:42, 17 January 2011 (UTC).[reply]

Good article

I would like to express my satisfaction with the clarity of this article. The author, or authors, took the right approach to explain why a cathode can be positive or negative (respect to the other electrode). 213.243.137.56 (talk) 07:27, 13 September 2009 (UTC)[reply]

Who wrote the "Scams" section?

Why is there a signature in the text? I propose that it be removed. --Ethan 10/01/08 —Preceding unsigned comment added by 84.109.83.232 (talk) 20:13, 10 January 2009 (UTC)[reply]

There are scams involving cathodes. But of course not all offers are scams and copper cathode is also sold ex warehouse. Buyers will however want to inspect whether that is existing product. --41.14.147.158 (talk) 22:31, 14 January 2009 (UTC)[reply]

Basic Concepts of cathodes particularly regarding CRT (Cathode Ray Tubes)

hi, just clearing up some basic concepts

anodes and cathodes are defined by their relationship between the source of the electrical energy, and the thing which receives that electrical energy. on the source of the electrical energy, the anode negative, while the cathode is positive,and on the thing which receives electrical energy the polarities are opposite. hence a discharging cell has a negative terminal anode but a cell being recharged or undergoing electrolysis has a positive terminal anode instead is this correct?

The part of the cathode ray tube is incorrect. The electrons flow off of the negnative terminal: http://electronics.howstuffworks.com/tv3.htm —Preceding unsigned comment added by 75.187.47.144 (talk) 01:11, 18 May 2008 (UTC)[reply]

Ok. As far as I can tell the Cathode-ray tube is a historical mishap, but I can't tell why. The Hall Effect (1879) unambiguously determined that electrons were the carriers of current, and the cathode ray tube was invented in 1897 (Thompson), so it's not clear to me why they would call the terminal where the electrons are coming out as the "cathode".

But, either way, the definition on this page (cathode = positive (conventional) current out (neg in), anode = positive in (neg out)) works for everything except for CRTs. Circuit diagrams (physics), electronics, galvonic cells (chemistry), electrolytic cells (chemistry), all seem to be fine with this definition. As for the CRTs, it would be nice if a historian could help fill us in on this, but you'll just have to chalk this up to a historical accident, because every page (including the wikipedia page on cahode rays/CRTs) say that the cathode is where the electrons current comes out. —Preceding unsigned comment added by 75.187.47.144 (talk) 01:57, 18 May 2008 (UTC)[reply]

Tried to clarify the CRT item a bit. Better like this?

* In a cathode ray tube, it is the negative terminal where electrons flow in from the wiring and leave for the tube's near vacuum. So from the outside, positive current flows out of the device.

--Rubik's Cube (talk) 18:31, 8 September 2008 (UTC)[reply]

Yeah that makes sense to me though I may not have fully understood the complexities under discussion.

62.49.4.186 (talk) 18:27, 18 March 2009 (UTC)[reply]

"from the outside" should be removed, as positive current flows out from the inside too. MichelJullian (talk) 09:40, 31 March 2009 (UTC)[reply]

Current does not flow

The "substance" that flows in wires and electrolytes is not "current", it is "charge". Current is defined as a flux of charges, so, saying that "current flows" is like saying that "the flux flows", or calling "current" the substance that flows in a river, instead of "water". Devil Master (talk) 22:33, 31 January 2009 (UTC)[reply]

I agree but it's not frowned upon to say "The tide has turned and the current is now flowing into the bay." Maybe you shouldn't and you should only say the water is now flowing in but it's a minor point anyway.

62.49.4.186 (talk) 18:27, 18 March 2009 (UTC)[reply]

Devil Master, although what you say above makes sense in principle, replacing "current" by "charge" in the articles was not a good move for two reasons: 1/ Usage, cf Google: "current flows": 831000 hits, "charge flows": 19600 hits. 2/ There exists a convention for current flow direction (positive in the direction where net charge flow is positive), not for charge flow direction. You couldn't even get away with "cathode: the electrode where positive charge flows out", as this wouldn't be universally true, eg in a CRT cathode the only charge which flows in or out of the cathode is negative, and it flows in (although conventional current does flow out of course). Agreed? MichelJullian (talk) 09:33, 31 March 2009 (UTC)[reply]

I think the suggestion is not to always say "charge flows" but instead to always say "current points" in a particular direction. This is technically correct AND not confusing. -Tim — Preceding unsigned comment added by 205.175.121.100 (talk) 18:53, 22 September 2011 (UTC)[reply]

capacitor

after reading this article, I finally understood how it works, but what about the flow of positive ions "into" the device if the device is a discharging, or charging... capacitor??? it is full of emty between the two electrodes.Klinfran (talk) 16:26, 9 September 2012 (UTC)[reply]

The definition seems to contradict the one at vacuum tube

On this page, a cathode is defined as the place where electrons flow inwards, but on vacuum tube it is defined as emitting electrons that are accelerated towards the anode. These definitions contradict, but it also makes little sense in general. If the cathode is negatively charged, then it should have an electric field pointing towards it and thus electrons would accelerate away from it, not towards it. This is how vacuum diodes work in the first place; it's the cathode that is heated and which therefore emits electrons which move to the anode under the influence of an electric field pointing from anode to cathode (thus, cathode is negative), while the anode doesn't emit any electrons because it's cold, and the electric field prevents electrons from escaping. So this article is probably wrong, the cathode is where electrons flow outwards, not inwards. CodeCat (talk) 14:11, 16 August 2013 (UTC)[reply]

You are looking inside the vacuum tube; the definition is about the currents flowing into or out of the device -- draw a box around tube and look at the terminal currents into and out of the box.. Electrons flow from the outside into the cathode terminal; that's the same as a positive current departing the cathode (CCD). (Within the device, the electrons are emitted from the cathode and reach the anode; those electrons then flow out of the vacuum tube's anode terminal.) Electrons leaving the anode terminal is the same as a positive current flowing into the device (ACID). Glrx (talk) 14:41, 16 August 2013 (UTC)[reply]

I didn't think of it that way. But is the cathode ever really viewed in this way, "from the outside"? I thought its function within the device would be considered primary. In any case, the article should probably make it clear what is meant when. CodeCat (talk) 15:27, 16 August 2013 (UTC)[reply]

This page is incorrect

Hi everyone,

I have never posted to wikipedia before however as I see it as an invaluable resource and this page is inaccurate I thought I would bring up the issue. Anodes and cathodes are defined as a function of the charge carriers they supply, not as a function of an electrode's bias relative to ground or another electrode. Either an anode or a cathode may have a bias relative to ground that is either positive or negative however, what defines the electrode as an anode or a cathode is the type of charge carrier it supplies. In all cases, the anode supplies the positive charge carriers, these are holes or positive ions depending on the system in question, and the cathode supplies the negative charge carriers, usually electrons but sometimes negative ions. I am uncertain what the process is for marking an article as needing revision but this article as currently written is actively incorrect. For example, in the first paragraph it states "thus, electrons are considered to flow toward the cathode electrode while current flows away from it." This is wholly inaccurate. Electrons may flow from the cathode while the device discharges and flow into the cathode while the device charges but either way the cathode is the repository of negative charge carriers and in supply mode operation supplies them to the circuit.

Kind regards,

Space Systems Engineer