Anode / Cathode
isofuncurves
Posts: 19
Am I losing my mind or are anodes and cathodes the reverse of what I was taught growing up? The explanation of an LED seems all backwards to me--electroplating for instance.
Comments
[noparse][[/noparse]quote]A widespread misconception is that cathode polarity is always negative. This is often incorrectly inferred from the correct fact that in all electrochemical devices positively charged cations move towards the cathode (whence their name) and/or negatively charged anions move away from it. In fact cathode polarity depends on the device type, and can even vary according to the operating mode, as per the above electric current direction-based universal definition. Consequently, as can be seen from the following examples, in a device which consumes power the cathode is negative, and in a device which provides power the cathode is positive
What may be confusing is that it once was taught that current flowed from positive to negative. We now know that the electron current flows from negative to positive. Confusing this further is the fact that there's what's called the "hole current" in semiconductors where current still flows from positive to negative, but this time it's "holes" which is a crystal lattice cell that's short an electron and that net shortage of an electron acts like it flows from anode to cathode. When a hole combines with an extra electron, a photon is emitted.
What is really true is that the terms anode and cathode have not been used consistently in electronics literature either historically or in the present.· One must be careful in stating which conventional usages one is using, or one's writings will cause confusion -- much like the case of conventional current, which some writers have going from positive to negative, and others the reverse.· Too bad old Ben guessed wrong.
Specifically, a writer on electronics ought to make clear at the outset whether he uses cathode to mean positive electrode, or electrode at which positive carriers flow out of the device, or electrode at which positive carriers flow into the device (converse for negative carriers).· I've seen all three; and of course the same is true for anode.
Perhaps the best thing is to avoid these terms altogether, but unfortunately there doesn't seem to be any universally adopted term to use as a substitute for either.
It's like that accursed tree falling in the forest -- no one is confused about what's happening physically, but they'll fight to the death to insist on using (or not using) particular words to describe it.
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· -- Carl, nn5i@arrl.net