Anodes and cathodes: Why are these terms mixed up so much? - Page 1
Forum section: electronics / beginners

author: yada
author_rank: Frequent Contributor
timestamp: May 27, 2017, 12:18:52 am
content: Looking into the chemistry of a lead acid battery I was trying to figure out which terminal released the hydrogen gas. I found this but it doesn't make sense:QuoteNegative anode? My understanding is this:~Cations are positively charged atoms.~Anions are negatively charged atoms.~Cathodes are negatively charged conductors: The cathode in a cathode ray tube is at the back of the tube and shoots off electrons (cathode rays).~Anode is positively charged: That's the thick red high voltage wire toward the front of the CRT, you don't want to touch that.So in a battery surplus electrons/neg. charged atoms or anions migrate to the (-) terminal MAKING it the cathode (cause and effect here is important here). They then flow out through the car where they o to the (+) terminal where hydrogen ions (protons or H3O) are waiting for electrons to become neutral.Hydrogen gas is made when the Pb H2SO4 chemistry is done then electrolysis starts occurring. The negative terminal starts to turn H2O into H+ then neutral H2.Correct? I saw them mix this up on PBS youtube scishow, and they people in the comments like to pretend they know what they are talking about.

author: retrolefty
author_rank: Super Contributor
timestamp: May 27, 2017, 12:29:04 am
content: I've never seen or heard of battery terminals named cathode and anode. Vacuum tubes, diodes, yes but not batteries.

author: helius
author_rank: Super Contributor
timestamp: May 27, 2017, 12:36:33 am
content: Conductors in a circuit are notcharged. That is what happens to capacitors.In the CRT example, the cathode is never charged. It emits electrons thermionically in a continuous process. The first anode (the plate of the electron gun) is also not charged. The second EHT anode is part of a capacitor formed by conductive coating on both sides of the glass bell, and its positive charge is balanced by a negative charge on the outside.The terms anode and cathode have nothing to do with charge, they are about current direction. The terminal where current flows out of the device into the circuit is called the cathode, and the terminal where current flows into the device is called the anode. When batteries are recharged, the anode and cathode swap places because current is flowing in the reverse direction. It is a common cause of confusion because our culture overemphasizes objects (in this case, charges) and underemphasizes processes (in this case, current).In electrochemistry, cations are ions attracted to the cathode, anions vice-versa. During recharging, the electric field is imposed by the external circuit, so the cathode (where current flows out) has a lower voltage than the anode. During discharging, the electric field is produced by the battery, so the cathode has a higher voltage than the anode.

author: sleemanj
author_rank: Super Contributor
timestamp: May 27, 2017, 12:40:10 am
content: Any confusion you are having may be down to conventional vs electron flow.Inconventionalflow (how people almost always think of electricity working from the "+" to the "-" of a DC circuit), the Cathode is where electrons leave [a thing] and the Anode is where electrons enter [a thing], so in a battery supplying current we consider the positive terminal the Cathode, and the negative terminal the Anode, in diodes we consider the (forward) "positive" side the Anode and the "negative" side the Cathode.So a battery connected with a (forward) diode, we consider the electricity leaving out of the battery Cathode, entering into the diode Anode, leaving out of the diode Cathode and entering into the battery Anode.Inelectronflow (how it actually physically works), the electron flow is opposite, electrons enter at the Cathode and exit at the Anode.It is easier, unless you really really need to, to just go with it and use conventional flow, forget about electron flow, it really doesn't matter that we think of it backwards.It is a little bit unusual to refer to the Cathode and Anode of a battery (or supply), probably because of this confusion especially since most people would most commonly use Cathode and Anode in reference to (forward) diodes.

author: yada
author_rank: Frequent Contributor
timestamp: May 27, 2017, 01:01:32 am
content: Quote from: sleemanj on May 27, 2017, 12:40:10 amSo how do you label the cathode and anode of a rechargeable battery? If its being charged they are labeled one way and if its supplying current they are labeled the other. What do you call them when the battery is not hooked up to anything? Wouldn't that also apply to capacitors?

author: sleemanj
author_rank: Super Contributor
timestamp: May 27, 2017, 01:21:42 am
content: Quote from: yada on May 27, 2017, 01:01:32 amIn short, yes.QuoteConvention is to use the discharging state, the reality is that's what it is doing if not being charged anyway, if nothing else through it's self-discharge.QuoteYes at least in conventional sense (I am not an electrochemist/physicist), but most people in common terms would simply call the negative of a polarised capacitor the cathode and the positive the anode in all cases, if they used those terms at all, which as I say isn't that common outside of diodes really.

author: helius
author_rank: Super Contributor
timestamp: May 27, 2017, 01:26:17 am
content: When the battery is disconnected, there is no external circuit, and so no place for current to flow out or in. The ions in the cell are also not attracted to either end. The same applies to an isolated capacitor even if it is charged.Diodes however always have the cathode on the same side, and it is where the diode is marked. It makes sense to call that end (the N side of the PN junction) as the cathode even if it is disconnected. Similarly for vacuum tubes. But consider a straight fluorescent tube: somewhat worryingly, it has two cathodes and no anode. In this case both terminals are homologous with cathodes in a vacuum tube, and they both emit thermionically at opposite parts of the cycle, so to save trouble they are both called cathodes.

author: jpanhalt
author_rank: Super Contributor
timestamp: May 27, 2017, 05:31:57 am
content: "Anode" goes back to Michael Faraday, 1839. Here is a link to an online version:http://www.gutenberg.org/files/14986/14986-h/14986-h.htmFaraday defined the anode of an electrochemical cell as the electrode at which oxidation took place ("decomposition"). Thus in a typical battery, that ends up the negative electrode. That is, in a common battery with a zinc electrode, the zinc is oxidized, is the battery's negative terminal (electrons flow out), and from the battery's point of view is the anode.One way to keep it straight is to consider a battery is in series, rather than parallel, with a circuit it powers. If one makes that more generalized, the same convention would apply to any power supply. Thus, cathode to anode and visa versa.For batteries, I personally prefer calling the terminals the positive and negative terminals. However, when you do electrolytic rust removal, it does help to follow Faraday's definition to know which terminal is connected to the terminals from the power supply.Several years ago I read the Wikipedia discussion, and apparently in an attempt to satisfy every one, it said that by convention in the US and other places, the anode of a battery was the positive terminal. I was probably the one person in the world not satisfied with that description. Faraday is credited with inventing the terms and had the right to name them, which he did.John

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 27, 2017, 10:12:02 am
content: Anode and cathode are named for their actions: one attracts positive ions and the other attracts negative ions. What electric potential is associated with these actions, depends on the chemistry.Here's a good resource:http://mysite.du.edu/~jcalvert/phys/elechem.htmTim