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Wikipedia says "an electrolytic capacitor is a capacitor whose anode or positive plate is made of a metal that forms an insulating oxide layer" (1, link). Elsewhere, wikipedia seems to define the anode as the terminal where conventional current enters a device (2, link). These statements are contradictory when the electrolytic capacitor is discharging. What is the solution for this contradiction? Should the positive plate be called cathode when the electrolytic capacitor is discharging?

--EDIT--
I like the answer given in Electronics Stack Exchange (link), that the well known definition of anode in chemistry is invalid for electronics. In chemistry, the anode is the terminal where the conventional current enters the device (and it is the plate where oxidation occurs).

In electronics, the anode is defined differently. For example, in a diode, the anode is the terminal that accepts current most readily. It remains the anode if the voltage is reverse biased. In an electrolytic capacitor, which is viewed to be an electronic device, the anode is the terminal that should be connected to the positive voltage. It remains the anode if the current reverses (when the capacitor discharges).

A battery is the only device in electronics where the anode conforms to the chemical definition.

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  • $\begingroup$ Please reopen. My question about the anode/cathode terminology was closed, and the stated reason for its closure was that it "appears to be about engineering". However, my question is definitely not about engineering; it is about established terminology that is widely used in high school physics and chemistry and beyond. Engineering would be: how to build or design an electrolytic capacitor. Faraday defined the anode/cathode terminology in 1834 in a way that has been preserved in chemistry, but as we know, its established use in electrolytic capacitors is inconsistent with Faraday's choice... $\endgroup$
    – jkien
    Commented Apr 7 at 7:37
  • $\begingroup$ ... The best answer up till now is that specifically for components in electrical circuit (capacitors and diodes) this terminology has changed. In addition, one of the close-votes claimed that the anode/cathode terminology is a matter of opinion. That is incorrect, it is established terminology. $\endgroup$
    – jkien
    Commented Apr 7 at 7:40
  • $\begingroup$ Voting to reopen. Obviously not an engineering question. $\endgroup$
    – gandalf61
    Commented Apr 7 at 8:30

2 Answers 2

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In chemistry, you should not think about signs while assigning Anodes and Cathodes. Just apply the rule, "Reduction will always occur at cathode and oxidation will always occur at anode." Its then easier to work out the signs.

Hence while generation of current in galvanic cell, let's say in Danial Cell, cathode is the one where reduction occurs, i.e. free electrons from the rod are absorbed by ions in reduction half-cell and rod becomes positively charged.

On the other hand, in an electrolytic cell where electricity from external source is used for electrolysis, let's say in the electrolytic decomposition of NaCl, the cathode is the one where again reduction occurs but here free electrons from source make the rod negative and are absorbed actively by the positive ions in the cell.

Discharging is the classic example of Galvanic cell and charging is an example of electrolytic cell.

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Should the positive plate be called cathode when the electrolytic capacitor is discharging?

Yes. The Wikipedia article on anode says that in a galvanic cell

... the anode switches ends between charge and discharge cycles ...

i.e. the positive and negative terminals of a galvanic cell swap roles between cathode and anode depending on whether the battery is discharging or being charged. For consistency the same should apply to a capacitor.

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