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When I was younger I learned in Chemistry class that Anions were negatively charged and Cations were positively charged (Fun fact: I memorized this because anion sounds like onion which makes you cry and is therefore negative; while cation has cat in it and cats are cute, so it's positive :P).

Now that I've started getting into electronics, I've learned that the Anode is the positive lead while the Cathode is the negative one. It seems odd to me that they're reversed; can someone explain why this is?

HellaMad
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    Another possible mnemonic: A "cation" is positive because it has the letter "t" in it and it looks like the positive symbol. :-) – In silico Apr 15 '12 at 22:35
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    also see http://en.wikipedia.org/wiki/Cathode#Etymology and http://en.wikipedia.org/wiki/Anode#Etymology – Stefan Paul Noack Apr 16 '12 at 11:39
  • For diodes the mnemonic I use is AAA - Anode Acceptor Arrow: The anode is comprised of acceptors (i.e. holes are the conductors) and on the symbol it is the arrow. The complementary mnemonic isn't as nice. CDB - Cathode Donor Bar, The cathode is comprised of Donors (i.e. electron conduction) and the symbol has a bar on that end. – placeholder Mar 05 '13 at 17:05

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From Wiki:

Faraday also introduced the words anion for a negatively charged ion, and cation for a positively charged one. In Faraday's nomenclature, cations were named because they were attracted to the cathode in a galvanic device and anions were named due to their attraction to the anode.

In the diode, and specifically in the so-called depletion region, there is diffusion of carriers (electrons and holes) from one region to the other. Since the Anode is positively doped, it will attract electrons from the cathode, and this will cause the formation of Anions in its side of the depletion region.

clabacchio
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    The names *cathode* and *anode* were around before semiconductor diodes. The name *diode* was extended to semiconductor devices that had the same one-way current property of the existing diodes of the day, which were mostly vacuum tubes. The "di" in diode refers to 2. A "diode" was a 2-element tube, as apposed to a triode, pentode, and others. – Olin Lathrop Mar 05 '13 at 16:55
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The anode is not always the positive lead.

It is the electrode through which electric current flows into a polarized electrical device. For a discharging battery, this is the negative lead.

This is how I connected anode/anion and cathode/cation during chemistry class when we were discussing batteries. I was confused when I learned that for other components, such as an LED, the kathode is negative, and the anode is positive. Until I learned that it's not about polarity but about the direction of current flow (anode = in, cathode = out), and the naming of ions is based on how they move in a discharging battery, probably because that's what chemists often deal with.

Stefan Paul Noack
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IIRC anions are the ions that are attracted to the anode in an electrolytic cell. Opposite charges attract, hence the charge of an anion is opposite of that of an anode.

An electrolytic cell (not to be confused with a galvanic cell!) decomposes chemical compounds by means of electrical energy, in a process called electrolysis [ http://en.wikipedia.org/wiki/Electrolytic_cell ]

Wouter van Ooijen
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  • +1. That's what I recall from my chemistry classes, too. (Also, I recall my favorite chemistry teacher saying: "If you know what you're doing, you may do anything." True for electronics engineering as well!) – zebonaut Apr 15 '12 at 20:44
  • -1 an anode does not have a specifically signed charge. This will get you confused when looking at batteries. – Stefan Paul Noack Apr 16 '12 at 12:10
  • added 'in an electrolytic cell' – Wouter van Ooijen Apr 16 '12 at 13:45
  • still, in a battery the charge of an anion has the same sign as the anode. your explanation refers to anodes always being positively charged, which is not correct. – Stefan Paul Noack Apr 16 '12 at 13:58
  • @noah1989 it's true, but in this case electrons are attracted by the anode because of carrier diffusion, so the principle somehow holds – clabacchio Apr 16 '12 at 14:10
  • of course, anions go to the anode, that's how it is defined. but this can be for other reasons than "opposite charges attracting" in which case the anode might be negative as well. this answer suggests that anions move to the anode because they're negative and the anode is always positively charged. that's not true. – Stefan Paul Noack Apr 16 '12 at 18:23
  • @noah1989: In an electrolytic cell anions alway move to the anode because they are attracted by its charge. A battery is not an electrolytic cell. – Wouter van Ooijen Apr 16 '12 at 20:33
  • I know the difference between an electrolytic cell and a galvanic cell. I think I now understand what you mean. When you're saying "of an anode" you mean an anode of an electrolytic cell or any component that electrically looks like one from the outside, not an anode in general. – Stefan Paul Noack Apr 17 '12 at 07:09
  • Yes. I edited the text 17 hourse ago to state "to the anode in an electrolytic cell". . That should remove all ambiguity, I think? – Wouter van Ooijen Apr 17 '12 at 07:13
  • @WoutervanOoijen: Seems like I am easily confused by linguistic details. It says "an anode" while an electrolytic cell only has one, which made me think you were referring to all anodes of all devices. – Stefan Paul Noack Apr 17 '12 at 07:21
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The anode have negative chare because it don't do workers without the cathode , so cathode have positive charge due to its efficiency of giving - es

Saqib
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As the anode have negative charge because it takes the m - es from cathode i-e positive charge because influence of the nucleus to the around revolving es is more as well as less in rare cases so it hold the es permanently in this way the consumer is cation and saver is anion sion work done is taken by cation therefore it is positive and opposite to this is for cation because it alone do nothing

Saqib
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