Cathode/Anode signs

[going2breakdown]

So let me get this straight:

Concerning batteries,
Galvanic: anode is marked as - cathode is marked as +

This reflects the fact that electrons spontaneously move toward a higher potential?

Electrolytic: cathode is marked as - anode is marked as +

This reflects the fact that electrons are being forced to move from a higher potential to a lower potential?

I guess my question boils down to, "do the signs on the battery's cathode/anode just mean + is the higher potential and - is the lower potential?"

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[sodarayg]

I always thought about it like this.

In galvanic, electrons go from anode to cathode. electrons are negatively charged so they'll go towards the (+) cathode..

its reversed in electrolytic so electrons go from cathode to anode. since electrons need to move towards (+), make anode (+)

 

[circulus vitios]

I find it easier to memorize the acronyms:

AOL: anode, oxidation, loss (of electrons)
CRG: cathode, reduction, gain (of electrons)

 

[PikminOC]

this is actually why i am on sdn..to remind me how much i have been thru to get to where i am now...whew! that knowledge was buried deeeeep

 

[BerkReviewTeach]

I always thought about it like this.

In galvanic, electrons go from anode to cathode. electrons are negatively charged so they'll go towards the (+) cathode..

its reversed in electrolytic so electrons go from cathode to anode. since electrons need to move towards (+), make anode (+)

You're spot on with the galvanic cell, but you need to rethink your electrolytic cell. The direction of flow changes from galvanic to electrolytic, but the poles also reverse (which is what causes the change in flow direction). For a battery and/or cell, electrons flow from anode to cathode, because as circ vit pointed out in their post, the anode is the site of oxidation (by definition) and the cathode is the site of reduction (by definition).

 

[deleted303591]

I am an electrical engineering student. The anode and cathode are technically terminals for an electric circuit. Do not think of them with any relation to anions and cations. It has to do with current, which is the direction of the flow of electrons.

Remember it this way. My grade went up from a C- to an A+. (Because the flow of electrons goes up through the negative cathode to the positive anode).

The anode is + because electrons are being received at the terminal, while with the cathode the charge at the terminal is - because electrons are being lost, regardless of power source type (such as galvanic or electrolytic cells).

The bold statement is true because in the described configuration, the electrons are flowing from - to + and therefore current is positive.

 

[circulus vitios]

I am an electrical engineering student. The anode and cathode are technically terminals for an electric circuit. Do not think of them with any relation to anions and cations. It has to do with current, which is the direction of the flow of electrons.

Remember it this way. My grade went up from a C- to an A+. (Because the flow of electrons goes up through the negative cathode to the positive anode).

The anode is + because electrons are being received at the terminal, while with the cathode the charge at the terminal is - because electrons are being lost, regardless of power source type (such as galvanic or electrolytic cells).

The bold statement is true because in the described configuration, the electrons are flowing from - to + and therefore current is positive.

I've tried to understand it like that, but I always get it confused. I keep thinking that because the anode receives electrons, it's becoming more negative. It's easier for some people to memorize AOL / CRG.

 

[sodarayg]

I've tried to understand it like that, but I always get it confused. I keep thinking that because the anode receives electrons, it's becoming more negative. It's easier for some people to memorize AOL / CRG.

I agree that it's easier to memorize AOL / CRG and most likely that's all they need, but if they get unlucky and the question emphasizes electron movement in any way, they're screwed.

 

[BerkReviewTeach]

The problem with a thread like this, is that there is wrong information being posted, which unfortunately will confuse people even more. Here is the way it is.

For a Galvanic cell, the electron flow is caused by a chemical reaction, so the chemical reaction is what dictates the identity of the electrodes. Oxidation is the loss of electrons and it occurs at the anode, so the anode is the starting point for electron flow. Reduction is the gain of electrons and it occurs at the cathode, so the cathode is the ending point for electron flow. Electrons flow from the anode to the cathode. That is the way it is, no exceptions and no need to even consider charge, because a chemical reaction causes the flow, not stored electrons (like a capacitor).

For an Electrolytic cell, the electron flow is caused by an external voltage that has a greater emf than the chemical reaction. The outside voltage source couples with the cell and forces the reaction to run in reverse (meaning that whatever cation was reduced into a metal in the Galvanic reaction will now experience the oxidation of that metal back into a cation). This means that what naturally wants to be reduced will get oxidized and what naturally wants to get oxidized will be reduced. But the important fact here is that oxidation still occurs at the anode and reduction still occurs at the cathode. Electrons STILL flow from the anode to the cathode.

In terms of physics, we care less about the chemical reaction and focus on the resulting charge flow. A battery discharges and current flows from the cathode to the anode, so the cathode pole of a battery is labeled as (+) to indicate it is rich in positve charge and it repels current (positive charge) away. The anode pole of a battery is labeled as (-) to indicate it is rich in negative charge and it attracts current (positive charge) to it. Current flows from the cathode to the anode.

When it comes to a charging capacitor, such as the one used in gel electrophoresis, the definition of the poles can get confusing. The electrodes are defined based on the charge that is building up (being forced upon the plates). The plates would prefer to be neutral, but an outside voltage source is causing charge buildup. Current is being force to flow, so (+) charge is being lost from the cathode of the capacitor, leaving a negative charge behind on its surface. Current is being force to flow to the anode, so (+) charge is being gained at the anode of the capacitor, leaving a positive charge behind on its surface. That is the odd scenario that generally causes confusion.

I hope this helps and explains why there is some confusion on the subject.

 

[neurodoc]

Here's how I look at it. The words "anode" and "cathode" were coined by Faraday to describe the "electrodes" in electrochemical cells, to describe the presumed movement of charges (this was before anyone knew what electrons were). What was believed was that there were presumably two types of electrical charge, + and -, and these apparently moved from one place to another in an electrical "circuit," such as that in a battery or elctrolytic cell.

The words derive from the Greek word for "road" ("odos"). "ana" and "kata" are prepositions meaning, respectively, "up" or "down," Hence the "anode" is the "road upwards" and the "cathode" is the "road downwards" taken by the charges in question. "Electrode" is a term that refers in general to the pathway taken by moving charges, either + or -. The flow direction is a matter of convention. Franklin and others arbitrarily defined this flow as going from + to - ("Conventional Flow"). Another way of looking at this is "Electron Flow," which of course came later after we learned about electrons.. "Electrode" derives from two Greek words, one for "amber" and the other for "road." This has to do with the observation that rubbing amber produced static electrical charges.

If we are talking about electron flow, the cathode is the point out from which electrons flow. In a battery or electrochemical cell, or any other devive that generates electrons, the electrons flow out of the cell at the cathode.This is signified by -. In an electrolytic cell, electrons are pumped into the cell at the anode, signified by +. In terms of "road up" or "road down" these correspond to "anode" and "cathode" from the point of view of electrolytic cells. .In radio/TV circuits employing electron tubes,resistors, capacitors, and inductors, the anode is + and the cathode (source of electrons) is -. Solid state electronic devices are different, and by convention they involve the idea of the movement of positively charged "hole" currents.

 

[BerkReviewTeach]

If we are talking about electron flow, the cathode is the point out from which electrons flow. In a battery or electrochemical cell, or any other devive that generates electrons, the electrons flow out of the cell at the cathode.

No, this is not how electron flow is defined; it's how current is defined. Current flows out from the cathode and terminates at the anode (which is why physicists define the anode as having 0 voltage--i.e., no potential to flow). Electrons flow across a wire from the anode to the cathode.

What you are describing is the flow of charged species through electric fields (as seen in electrophoresis). That is the case where the electric field exists because of an outside voltage source that forces charges onto the capacitor plates, so the charges assigned to the capacitor plates are opposite of the other electrical systems (such as galavanic cells, electrolytic cells, and batteries).