Chemistry-Electronics naming discrepency? - Page 1
Forum section: electronics / general

author: FenderBender
author_rank: Super Contributor
timestamp: March 22, 2013, 02:46:26 am
content: Hi everyone,I'm a senior taking Chem II in high school right now and we are doing electrochemistry, mainly focusing on voltaic cells. Interesting to see our more abstracted electronics world meet a more realistic chemistry world, though much of chemistry is still an abstraction.So I was in class and my teacher was saying that electrons flowed from anode to cathode, which immediately set off an alarm in my head which said "electrons flow from - (cathode) to + (anode)". Why is it that they are flipped, (if they actually are)?According to all chem literature I've read, at the anode the oxidation reaction occurs and electrons are lost, and at the cathode reduction occurs and electrons are gained...which indicates that electron current moves from anode to cathode.No?P.S. Not to pat myself on the back, but I've gotten accepted to the 6 colleges I've applied to for EE! Thanks for all of the motivation and encouragement this forum has directly and indirectly given me.

author: c4757p
author_rank: Super Contributor
timestamp: March 22, 2013, 02:51:25 am
content: "Positive and negative" is not the same as "anode and cathode". The anode is the point at which electrons exit something - positive on something that absorbs power, negative on something that supplies power.

author: FenderBender
author_rank: Super Contributor
timestamp: March 22, 2013, 02:58:24 am
content: Interesting, I was almost confident a battery's + was associated with the term anode and - with cathode, but perhaps I am just mistaken.

author: c4757p
author_rank: Super Contributor
timestamp: March 22, 2013, 03:09:35 am
content: Probably because most people forget the correct definition of anode and cathode. People learn to associate anode=positive because the only time (other than electrochem) the terms are still in common use is when dealing with diodes.Wikipedia, the world's most credible source, agrees with me:QuoteOf course, EE isn't the only place stuffed with misconceptions. My high school chem teacher taught us the exact opposite misconception: anode is always negative! She even had a little mnemonic: "anode =anegative electrode"... I'm not sure why people think "electrons exit at the anode" is too confusing to teach.

author: BravoV
author_rank: Super Contributor
timestamp: March 22, 2013, 03:15:42 am
content: I was having similar problem in the past, then I use this basic as the base to make my mind holds firmly about it.Think how diode works connected to voltage source like battery, the current flows from (+) positive source from the left sideenteringthe anode, andexitsto the right side (cathode) to the (-) voltage source.Since this is a "series" connection, hence the voltage source at positive (+) side is cathode as it sources current flow (exit), and negative of the battery (-) is anode where the current flow enters (sink).

author: notsob
author_rank: Frequent Contributor
timestamp: March 22, 2013, 04:21:40 am
content: Try this, an ANode is where the ANIONs arehttp://simple.wikipedia.org/wiki/AnionA CAThode is where the CATIONs arehttp://en.wikipedia.org/wiki/Ion#Anions_and_cations

author: amyk
author_rank: Super Contributor
timestamp: March 22, 2013, 05:56:05 am
content: I think it's also an issue of perspective: with a battery in a circuit, the electrons flow out of the battery via its - terminal, and enter via the + so you say the flow is from - to +, but if you lookinsidethe battery the electrons flow from + to -!

author: IanB
author_rank: Super Contributor
timestamp: March 22, 2013, 12:05:24 pm
content: Quote from: FenderBender on March 22, 2013, 02:46:26 amYou have just discovered why it may be better to use the terms "positive electrode" and "negative electrode" rather than anode and cathode. Depending on the circumstances the anode and the cathode can switch places!The anode is the electrode from which electrons leave the device and enter the external circuit. Therefore the identity of the anode and the cathode depends on the direction of current flow.In a primary cell (e.g. an alkaline battery) electrons flow from the negative terminal through the load to the positive terminal, making the negative terminal the anode. In such a cell oxidation is occurring at the negative electrode, providing the source of electrons to flow out into the external circuit.The same is true of a secondary (rechargeable cell) when it is discharging.However, consider a secondary cell when it is being recharged (for example, charging a NiMH cell). It is now necessary to force current to flow through the cell in the opposite direction in order to reverse the chemical reaction. In this case electrons are being made to flow into the negative electrode and out of the positive electrode due to the imposed external charging voltage. The positive terminal is now the anode and the negative terminal the cathode. They have switched places!The oxidation reaction that was occurring at the negative electrode during discharge has now become a reduction reaction and oxidation is occurring at the positive electrode instead. This case applies equally to electrolysis, e.g. in an electro-plating bath.We see therefore that in a silicon diode the electrons are flowing into the negative terminal and out of the positive terminal and that makes the positive terminal the anode.For the above reasons I try to avoid using the terms anode and cathode. If we use the terms positive terminal and negative terminal then everyone is clear and there is no ambiguity.

author: IanB
author_rank: Super Contributor
timestamp: March 22, 2013, 12:16:46 pm
content: Quote from: FenderBender on March 22, 2013, 02:46:26 amNote that further to my post above there are two kinds of electro-chemical cell: galvanic cells and electrolytic cells. In a galvanic cell the cell is producing electricity and the negative electrode is the anode. In an electrolytic cell the cell is consuming electricity and the positive electrode is the anode. In both cases the electrons are flowing out of the anode and into the external circuit. Electrons always flow from anode to cathode outside the device and hence from cathode to anode inside the device.

author: ftransform
author_rank: Frequent Contributor
timestamp: March 22, 2013, 06:14:57 pm
content: i WISH THE terms anode and cathode were abolished. I cannot for the life of me remember which one is witch and I find myself constantly having to look it up.I think I might put up a sign.

author: IanB
author_rank: Super Contributor
timestamp: March 22, 2013, 06:37:44 pm
content: Quote from: ftransform on March 22, 2013, 06:14:57 pmJoin me in making sure that you never use those terms, ever, and perhaps if enough people follow the trend we can see those confusing words as a forgotten footnote in history.

author: c4757p
author_rank: Super Contributor
timestamp: March 22, 2013, 06:43:18 pm
content: Hmm. I actually rather like them. They come to me easily because they are nouns - it seems simpler to refer to the "anode" of a diode than the "positive terminal", and the concept of "anode = electrons exit" fits in well with my usual "visual" understanding of a circuit. I never realized they were such a big source of confusion, though - thought they were just more of a "trap for young players" than something almost everybody agreed to hate. Maybe I will try to stop using them as well.

author: SeanB
author_rank: Super Contributor
timestamp: March 22, 2013, 07:16:51 pm
content: Chemists will never give up the convention. It is too useful to them, and they are the ones who design and ensure consistency of most of the packaging and components used to fabricate electronic components. To somebody not in the industry a battery is a battery, but in the chemistry there are so many compromises and trade offs. You can make a cell that is usable for thousands of cycles, can handle massive charge and discharge rates and will survive extreme temperatures. Just that the only customers are NASA and Hughes Telecoms, as the cells are both wounded buffalo expensive and made from ultra high quality components, as they are used in a lot of the satellites in high reliabilty applications.

author: IanB
author_rank: Super Contributor
timestamp: March 22, 2013, 08:30:14 pm
content: Quote from: c4757p on March 22, 2013, 06:43:18 pmWell the situation is not brilliant for electrochemists, if you consider that for a rechargeable battery like an NiMH cell the negative electrode is either an anode or a cathode depending on whether the cell is discharging or charging at the time. If the cell is open circuit the negative electrode is presumably both an anode and a cathode all at once?

author: c4757p
author_rank: Super Contributor
timestamp: March 22, 2013, 08:52:36 pm
content: Quote from: IanB on March 22, 2013, 08:30:14 pmThat's easy, it's either "undefined", or more commonly, defined based on standard usage (discharge, in the case of a battery). Hence why the terminal that is taking current on a reverse-biased Zener diode is still called the cathode, even though it's actually operating as an anode.(Yeah, I guess I see the point about confusion.)

author: FenderBender
author_rank: Super Contributor
timestamp: March 23, 2013, 10:02:14 pm
content: Very interesting indeed, Ian, especially. Thanks for the insight.