Talk:Cathode

"Drive" is ambiguous in this context

"In an electrolytic cell, the cathode is where the negative polarity is applied to drive the cell. Common results of reduction at the cathode are hydrogen gas or pure metal from metal ions. When discussing the relative reducing power of two redox agents, the couple for generating the more reducing species is said to be more "cathodic" with respect to the more easily reduced reagent."

What does the word "drive" mean here? It only makes sense if one already understands electrolytic cells. Otherwise, what does electrolytic cells have to do with golf, cars, and Sunday afternoons. Please delete this after this is fixed. — Preceding unsigned comment added by 24.19.71.186 (talk) 14:49, 11 September 2022 (UTC)

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).

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)

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

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).

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)

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)

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)

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)

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)

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)

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)

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

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)

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)

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)

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)

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)

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)

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)

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)

I agree with Glrx's explanation, but I also emphasize the confusing descriptions that concern CodeCat. The current version of the In Electronics section says that "the cathode is an electrode that emits electrons into the device"! So what is the device in this case? Unlike what Glrx suggests that a box be placed around the entire electronic component which would be called "the device", in context of electronic tubes, the "device" implies the space within the tube between the elements. It is also worth noting that some cathodes in electronic devices have two leads, so there is technically current flowing both into and out of the cathode. However, keeping with what Glrx described, there is a net flow of electrons entering the cathode--electrons flowing in through one cathode lead is much greater than the electrons flowing out through the other lead since some electrons are "lost" from the wire, emitted into the space inside the tube. --cperk

Further problems arise between the varying interpretations because in context of many electronic devices/tubes, there is actually no definite flow of electrons outside the device as stated in the Flow of Electrons section: "from anode to cathode outside of the cell or device". This is especially true in vacuum tubes/chambers where traditional electronic flow rules (e.g. Kirchoff's law) do not apply due to emitted free electron currents. In vacuum chambers, some stray electrons charge up insulating material, some are absorbed by the chamber walls, etc. Even if one adopts the definition of device to include the entire contraption that wires are poking out of, the flow of charge out of the anode may likely go to a completely different external device where the only common "circuit" is through the building ground. It's questionable whether one could trace a complete circuit in this case going strictly from cathode to anode (or vice versa). In this case, the definition should simply be about the flow of current into or out of the individual elements and not the current between them. --cperk

Lastly, although this page tries to unify the definition of cathode, I suggest creating a disambiguation page for the different contexts... likewise for anode. If that's not acceptable, what about providing a clear disambiguation in the introduction paragraphs? There is so much confusion and contradiction (all over the web and in classrooms) about the definitions of cathodes and anodes that we should address it directly instead of trying to craft an all-encompassing definition then warring over the minutia. --cperk (talk) 02:43, 11 July 2018 (UTC)

cperk: I don't think the definition is as complicated as you are trying to make it. As Glrx said, an electric circuit consists of electronic components each of which connects to the external circuit with terminals (which may include the building ground). It is the direction of the current with respect to the external circuit, not the direction inside the component, that defines a terminal as a cathode or anode. A cathode is a terminal in which electrons flow from the external circuit into the component. It seems to me the definition gives unambiguous answers to all the questions you posed:

• If DC current flows out of a component into the building ground, that is a cathode.
• As you mention, some vacuum tubes have directly heated cathodes in which the cathode is a filament powered by two terminals emerging from the tube.
  • If the filament is powered by DC, regarding the filament as a separate component you could say the filament terminal where electrons enter is the "cathode terminal of the filament" and the terminal from which electrons leave is the "anode terminal of the filament".
  • However, regarding the tube itself as the component, the "cathode of the tube" is both terminals together, as the cathode current is the algebraic sum of the current in both filament wires (the number of electrons entering minus the number of electrons leaving).
• In the rare case of electrons flowing out of a component not through a metal terminal but through the case or wall of the tube into the air, you could say either "this component doesn't have a defined anode" or "the wall of the tube serves as the anode".
• When you say: "It's questionable whether one could trace a complete circuit in this case going strictly from cathode to anode..." remember that the terms anode and cathode apply to DC circuits. In a DC circuit there must be a complete circuit to conduct current.

--Chetvorno^TALK 23:59, 11 July 2018 (UTC)

I oppose cperk's suggestion of creating a disambiguation page for Cathode and Anode. The definition is a little complicated, what with the exception for diodes, but for that reason the last thing we need is to split the explanation between different pages. --Chetvorno^TALK 00:13, 12 July 2018 (UTC)

Chetvorno: Point taken, a separate disambiguation page is not necessary. Your bulleted items would be a great addition to various sections of the article and would make it much clearer.

I agree that the definition does not need to be complicated, but I'm not necessarily the one complicating it. My understanding was primed by reading various textbooks and other online resources, many of which are contradictory. My goal was to reconcile seeming contradictions with the terms in physics and chemistry. The main article did not help without re-reading and analysis. The learning curve is unnecessarily steep. Rereading the article now, it does not seem as confusing, but that's only because I have been mulling over this for a couple days. But even if descriptions are technically correct, different parts of the article choose different terms and references to use, so that the overall picture when going between the different contexts is still confusing: One snippet refers to conventional current, then another uses electron flow; one area talks about the circuit outside the device, but another talks only about activity inside the device--hardly providing a clear comparison.

Agreed that the ground can be part of a DC circuit, but it's still a bit simplistic in the cases I pointed out. In a vacuum chamber with electron guns and open mass spectrometers and pressure gauges (all with cathodes), the entire chamber and internal elements can become multiple "anodes" as stray free-electron currents charge up insulating materials and are deflected by high-potential sources, etc. The exiting electron flow can be split among the grounded chamber and various terminals of multiple external equipment. Indeed, there must eventually be an equilibrium reached and so there is an overall "circuit", but even if the electron flow through the cathode might be well defined, the flow through an undefined anode can be impractical to trace. Does that destroy the definition of a cathode or anode? Perhaps not, but it **is** worth considering just as much as the details of an electrolytic cell can be important for a chemist working in that context. Is it too complicated to talk of ion movement in the electrolyte, redox reactions, salt bridges, half cells, etc.? Certainly not, so why are the details of physics research equipment any less in need of explanation? If someone does not point out these relationships and how the overarching definition of terms applies, it can be truly confusing for someone going from one context to another. ---cperk (talk) 06:39, 12 July 2018 (UTC)

• Oppose DAB page for cathode and anode. I'll agree the definitions require care and have conventions that further cloud them (electrons have negative charge; choosing the KCL direction into/out-of the node; that current flow determines the label and is independent of the sign of the voltage/potential). That does not mean the terms are inconsistent. If you believe in ideal semiconductor diodes (0 reverse current), the only odd ball is the Zener/avalanche diode. Glrx (talk) 01:33, 12 July 2018 (UTC)

cperk: I think as you note the terms "anode" and "cathode" are used rather loosely in electronics literature and often not in accordance with the official definition, as in your example of electrodes in a tube that have no external connection. I think all we can do is follow the definition given in reliable sources.

I absolutely agree with your point that the detailed application of the definition is complicated and is inadequately explained in the article. It seems to me all the above details are too much to explain in the introduction, and anyway the introduction is supposed to be a summary of information in the article (WP:LEAD). I suggest a new overview section be written that explains -- in a consistent way that covers both electrochemical cells and electronic components -- the rules of how terminals of devices are named cathodes and anodes. I think the introduction could also be rewritten to be a better summary and less electrochemical-centric. --Chetvorno^TALK 11:05, 12 July 2018 (UTC)

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 — Preceding unsigned comment added by Bret Bronner (talk • contribs) 23:45, 26 May 2014 (UTC)

You need to think about this more.

CCD means cathode current departs: a positive current always departs the device; electrons carry a negative charge, so a positive conventional current is the same as electrons entering the device. That's the point of statement in the first paragraph.

Your last sentence is confused; electrons will always flow into the cathode; if electrons are flowing out of a device's electrode, then that electrode is an anode.

Glrx (talk) 23:40, 28 May 2014 (UTC)

Flow of electrons

Preface: My opinion in the following paragraphs regarding anode and cathode applies only insofar as diode terminals. I understand the terms are also used in other applications of chemistry and physics, I pass no judgement on those usages.

Update: See also my follow-up at the end.

The "Flow of electrons" section in this article, "Cathode", and that of the "Anode" article state different things. Both are incorrect.

In both articles the section starts with the same description of electron flow:

"The flow of electrons is always from anode to cathode outside of the cell or device, regardless of the cell or device type and operating mode..."

However in the latter, diode behaviour is excepted wholly from said description:

"...with the exception of diodes, where electrode naming..."

While in the former, diode behaviour is excepted only partly:

"An exception is when a diode reverse-conducts..."

Notice "exception of diodes" vs. "when a diode reverse-conducts". (Also, "the exception" vs. "an exception".)

For a forward bias p-n diode in an electric circuit (of solid metal conductors), electrons flow from cathode (n-side) to anode (p-side). Conversely, from anode to cathode when in reverse bias. (Ideally, when a diode is in reverse bias there should be no flow at all; in practice, however, there's always some leakage and in some cases this is intended —Zener diodes are an example.)

Note that, in this usage, the name assigned to the terminals (electrodes) does not change with the orientation of the device. The electrode attached to the p-side of a diode is always called "anode", and the one attached to the n-side always called "cathode". This is supported by the "Related antonym" section in the "Anode" article and by the "Diodes" subsection in the "Cathode" article.

According to this, of the two texts, the one in the "Anode" article is almost correct, the other is blatantly incorrect.

The section in "Anode" is not entirely correct because it continues (emphasis added by me):

"...where electrode naming always assumes current in the forward direction (that of the arrow symbol), i.e., electrons flow in the opposite direction, even when the diode reverse-conducts..."

The emphasised passage is incorrect. (Note that "opposite direction" refers to "the forward direction" not to "current", here used as shorthand for "conventional current", and that these two, conventional current direction and a diode's forward direction, do not coincide when the diode is in reverse bias!)

In sum, I propose to use the section in "Anode" for both articles after removing (or correcting) the aforementioned passage.

Czukrae (talk) 06:34, 1 October 2014 (UTC)

It's been brought to my attention that I may have overlooked the significance of "outside of the cell or device": Since the diode is inscribed in a circuit, saying electrons flow from anode to cathode "outside of the device" (as in, they leave the anode and arrive at the cathode traveling round the circuit and not directly across the device —the diode's p-n junction) would be the same as saying electrons flow from cathode to anode through the device —directly across the diode's p-n junction.

If this is the case then though the two sections are still inconsistent, my conclusion and proposal would have to be flipped over: keep the section in "Cathode" (and clarify the wording).

Czukrae (talk) 05:32, 3 October 2014 (UTC)

I don't think you have a clear understanding of what is intended. For example,

"...where electrode naming always assumes current in the forward direction (that of the arrow symbol), i.e., electrons flow in the opposite direction, even when the diode reverse-conducts..."

I don't think the statement is clear because the parenthetical is messed up, but the overall statement is correct. For the semiconductor diode, the electrode naming (anode/cathode) uses the forward direction names even when the diode reverse-conducts. The parenthetical is electron flow is opposite to conventional flow.

Glrx (talk) 18:02, 4 October 2014 (UTC)

The battery anode is always negative and the cathode is positive

"The battery anode is always negative and the cathode is positive." Quote from batteryuniversity.com dot com. Which is on Wikipedia's blacklist (which is amazing since it is more credible than Wikipedia) so I cant add a proper link.

The text here is VERY confusing. It actually says anode can be either negative or positive... — Preceding unsigned comment added by 212.181.101.51 (talk) 12:38, 25 January 2017 (UTC)

Confusing? The battery anode is not always the negative electrode! Don't start with the battery polarity. Anode and cathode are defined from the external current direction, pls see the original, the M Faradays paper of 1834, paragraph 11, "section" 663.[1] If the battery type is a primary cell(s) (non rechargeable) the current direction leads to that the anode is the negative electrode. But if the battery type is a secondary cell (rechargeable) the current direction is into the positive electrode during charging (which means anode) and from the positive electrode during discharge (meaning cathode). In the case of rechargeable batteries it is wise to be carefully about the use of the words anode and cathode. However some scientists of lithium ion cells research confusingly call the negative electrode material "anodematerial" although one of its feature is its rechargebility that is to function as a cathode (as opposed to metallic lithium in a primary lithium cell not suitable for charging). The scientists should find a new name - why not negatrode as suggested by professor George Z Chen,[2] or maybe a still shorter name - negode.

But diodes seam to have their own definition so my comment doesn't involve diodes. -Stenallan (talk) 00:24, 2 February 2017 (UTC)-Stenallan (talk) 17:46, 3 February 2017 (UTC)

CCD Mnemonic

Hi guys CCD is not a mnemonic. It is an abbreviation for cathode current departs. A mnemonic would be "cathode current departs".

Tanis8472 (talk) —Preceding undated comment added 09:01, 29 November 2017 (UTC)

Contribution of Rontgen in cathode research:

Wilhelm Conrad Röntgen and the Early History of the Roentgen Rays https://www.google.com/books/edition/Wilhelm_Conrad_R%C3%B6ntgen_and_the_Early_Hi/5GJs4tyb7wEC?hl=en&gbpv=1&bsq=cathode

Wilhelm Conrad Röntgen was the first scientist to observe that cathode rays can pass through opaque substances which later leads to discovery of Xrays, page 14 Drisha herjee (talk) 00:14, 23 August 2022 (UTC)