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According to Wikipedia's definition, a cathode is "an electrode through which electric current flows out of a polarized electrical device". However, the direction of current flow is purely an arbitrary convention, and is in fact the opposite direction to which electrons flow in a metal conductor. Hence, this definition seems to be a bit lacking.

If the definition is referring to the current carriers, then the definition might be rephrased as "cathode is an electrode through which the charged current carriers flow out" (or "in"?).

How would you define this term clearly and unambiguously? Does the common use of the term "cathode" really reverse when the current carrier is positive, as suggested in the article?

Edit: What actually confuses me is the phrase "Cathode polarity is not always negative". If electrons always flow into the cathode (by way of definition of cathode), this statement implies that the cathode can be at a positive voltage relative to the anode. Can this happen in a "simple" conductor like an electrolyte, or does this require some special circuit?

Roman Starkov
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    electrons have negative charge, a flow of electrons represents a flow of positive current in the opposite direction. This is a subject people often bring up as a "problem" that is not one, it is just a minor point people get stuck on. – Kortuk Jul 19 '12 at 15:13
  • Re your edit: Think of the difference between a source of electric energy and a sink. A (positive) resistor is always a sink for energy; electric current *always* enters the positive terminal and leaves the negative terminal. Now, consider a capacitor. If energy flows to the capacitor, i.e., the capacitor is charging, current *enters* the positive terminal. However, when the capacitor is discharging, current *exits* the positive terminal, energy is now flowing out of the capacitor. – Alfred Centauri Jul 19 '12 at 18:19

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Electric current is an abstract current, the flow of electric charge, not a physical current like, say, electron current, the flow of electrons.

But electric charge is a property of things, not a thing, i.e., electric charge is always "carried" by a thing.

So, while an electron current is necessarily an electric current (due the negative electric charge carried by the electron), an electric current is not necessarily an electron current.

For example, in a salt solution, we have two species of electrically charged ions present, the positively charged sodium ion and the negatively charged clorine ion. Imagine that the sodium ions are moving to the right and the chlorine ions are moving to the left.

Obviously, we have two ion currents in opposite directions but there is just one electric current and it must have a direction. The direction of electric current is, by convention, the direction of the flow of positive charge.

So, in this case, both ion currents contribute to an electric current to the right. The first term is due to the positive ions to the right. The second term is due to the negative ions to the left where the negative sign numerically "flips" the contribution to the electric current.

Think about it this way, if I told you that I was travelling at -60mph west, you'd know that I was actually going 60mph east. Similarly, a negative charge current leftward is an electric current rightward.

So, the above is all to simply say that the definition isn't lacking.

Alfred Centauri
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Wiki has it right, the cathode is the terminal of a component where the charge flows out. Charge flow (current) is the "standard" definition (i.e. Franklin's one from positive to negative, the opposite of electron flow).

The cathode of a component can change depending on it's state - for instance when a battery is discharging, the cathode is its positive terminal, and when charging its negative terminal (since the charge is now flowing into the positive terminal, rather than out, and out of the negative one, so they are reversed)

clabacchio
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Oli Glaser
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  • "Wiki has it right". Not necessarily. Remember our photodiode? :-). Or a zener diode. – stevenvh Jul 19 '12 at 13:30
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    @stevenvh - Oh no - not the photodiode again! The definitions of anode & cathode were made before the invention/discovery of the zener (whoever heard of a thermionic zener? :) so the definitions have become somewhat loose over time. Current most definitely flows *into* the cathode of a zener as you say (usually!). – MikeJ-UK Jul 19 '12 at 14:06
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    Let's define cathode where the colored band is located on the device ;o) – jippie Jul 19 '12 at 14:07
  • @jippie - except on the Southern hemisphere, then I agree :-) – stevenvh Jul 19 '12 at 14:08
  • @Steven - yep, very good point, it should have been a functional description for them too, not a structural one. I agree with the link Alfred posted that referring to "P doped side" and "N doped side" would be clearer. – Oli Glaser Jul 19 '12 at 15:51
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Of course, when you switch between talking about electrons and talking about electric current the direction flips.

You can define cathode as "the terminal where the electrons flow into the device", but as we are accustomed in everyday electronics life to talk about current flowing from + to -, the equivalent definition in terms of current is as your Wiki link states: "the terminal where the current flows out of the device".

IMO both definitions are totally unambigous. Their usefulness depends on the context.

Wouter van Ooijen
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  • Zener diode, photodiode, ... – stevenvh Jul 19 '12 at 13:42
  • @stevenvh, this speaks to that issue: http://www.av8n.com/physics/anode-cathode.htm#i-zener. *The terms “anode” and “cathode” properly apply to function, not structure.* – Alfred Centauri Jul 19 '12 at 13:57
  • @Alfred - Not sure I agree here. That LED could also be used as photodiode, so we shouldn't talk about anode/cathode for LEDs either to avoid confusion? Thanks for the link, though. – stevenvh Jul 19 '12 at 14:03
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    @stevenvh, you're welcome. It's always about context. For example, "anode" and "cathode" when referring to diodes is in the context of *structure*. The definition in question is in the context of *function*. Confusion results when the context isn't clear or forgotten. – Alfred Centauri Jul 19 '12 at 14:09
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I think that the definitions of 'anode' and 'cathode' have become rather loose over time. As others have mentioned, in the case of photodiodes and zeners, current can flow into the cathode.

In the days when the only kind of diode was the thermionic variety, the distinction between the terminals was clear. Electrons 'boil' off the cathode and are attracted to the anode, so convention current has to flow into the anode and out of the cathode.

When the semiconductor diode came along, the terms anode and cathode were preserved but I think they are misnomers in the original sense. But there is rarely (if ever) any confusion so I suppose it is a moot point.

MikeJ-UK
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I'd like to summarise the points raised by everyone in my own answer, because I had to pick them up from several answers (all of which I upvoted). Thank you everyone; this answer is only possible because you guys explained it to me :)

A cathode is the electrode that the current flows out of at a particular instant. This refers to the arbitrary direction of current which is a ubiquitous convention in modern electronics. If the current flows out of the electrode by this convention, the electrode is acting as a cathode. If the current flow at a terminal reverses direction, the terminal changes from a cathode to an anode.

Where the current is carried by electrons, the electrons flow into the cathode. In electrolytes, the negatively charged ions travel towards the cathode, and the positively charged ions travel away from it (though they don't physically flow into or out of the cathode, not very deep into it anyway).

Some electronic parts have one of their terminals designated as the positive terminal, and the other as the negative terminal. This does not mean that one of these is always the cathode. It depends entirely on whether the current is currently flowing into or out of a specific terminal. So, in a polarized capacitor that is being charged, the current flows into the positive terminal, thus the negative terminal is the cathode. While the same capacitor is discharging, current flows out of the positive terminal, thus the positive terminal becomes the cathode.

The potential difference between the cathode and anode can be both positive and negative, and does not determine which terminal acts as a cathode.

Roman Starkov
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