Anode and Cathode naming confusion for batteries

[hurricanehenry]

Hi. If I understand it correctly, the anode is where electrons come out of to enter the circuit, and they go around until they reach the cathode right? That is, the anode is the bottom of an AA battery and the cathode is the top of it.

But inside a battery, the naming is reversed, because the electrode that loses electrons into the electrolyte is the one that is the anode, although outside the battery this same electrode is actually the one receiving electrons and is named the cathode.

Is my understanding correct?

Thanks.

[fivefish]

There is "Conventional Current" flow, and there is "Electron Flow"

CC assumes current from from (+) to (-).... back in the early days of electricity discovery.
But EF is what really happens at the electron level... electrons flow out of (-) terminal back to (+) source.

When you're analyzing circuits, just make sure to pick one or the other and use it consistently.

The end result is the same.

ADDENDUM:
You can think of "Conventional Current" flow as "Hole Flow"... where Hole is defined as absence of an electron, i.e. a positive charge. But protons do not really flow, instead it's electrons that flow.  So you can think of it as when an electron flows, it leaves behind a "hole", and let's define absence of the negative electron is presence of a positive hole. So as electrons move their normal way, the positive holes move in the opposite way.

[TheElectricChicken]

After this one has been settled, I shall not pipe up whether the battery is charging and discharging. I'll zip my lip.

I think the best idea is to avoid labeling battery terminals with those words altogether.

this might help

You are completely correct in your statement that oxidation occurs at the anode and reduction at the cathode.  It can be confusing assigning positive and negative signs to the electrodes, because some electrochemical processes are spontaneous (voltaic) and others are not spontaneous (electrolytic).

In a voltaic process (spontaneous) like your common dry cell battery, the anode is giving off electrons (oxidation), so has a negative charge.  This will be the zinc on the bottom of the battery.  The cathode is taking in and consuming electrons (reduction).  This means there is a deficiency of electrons here, so it is positive.  In the battery, this occurs in a manganese-based paste.  To get the electrons to the manganese, the battery uses a graphite rod.  The tip of this rod is the knob on top of the battery, or the positive terminal.

In an electrolytic cell (not spontaneous), you need to initiate the reaction with an outside power source, like a battery.  If you wanted to silver-plate a fork, for example, the fork would be the cathode, so that silver ions in solution will reduce on the surface of the fork.  You would have to connect the negative terminal of your battery to the cathode, so that electrons can be added to the fork and reduction can occur.  In other words, the anode of you voltaic cell connects to the cathode of your electrolytic cell.

-Chris: CT, USA

[hurricanehenry]

I'm not sure I know what you guys meant.

I do know about electron flow. I understand holes and such. I know about the depletion region in semiconductors, and how the charge carriers move around.

Is what I wrote about the anode and cathode correct? The terminology is still being used in many places so I need to know.

[helius]

It's a common newcomer mistake to expect that anode and cathode denote a polarity in voltage. The words have nothing to do with voltage and everything to do with current.
Cathode is derived from late Latin cathodus meaning "way out". It is the terminal of the device from which conventional current goes out into the external circuit. Conventional current is opposed to the direction of electron flow, so it is also the terminal where electrons enter the device from the circuit (the cathode of a vacuum tube emits electrons into the vacuum).
In a battery, the direction of current is different during charge and discharge. During charging, the current is forced through the battery by the circuit, so it flows through the battery from the + terminal to the - terminal. This makes the - terminal the cathode during charging.
During discharging, the battery forces current through the external circuit, so current flows through the battery from the - to the + terminal. This makes the + terminal the cathode during discharging.

You asked about what happens "inside the battery". From the point of view of electrical theory it's simply a voltage source in series with a resistance, there is nothing else in there. The electrochemical reaction between the cell plates and the electrolyte is a separate topic.

From the point of view of chemistry you are talking a different language, with things like redox potentials, half-cell voltages, and reaction formulas.  Unlike wires in an electrical circuit, electrons really do not flow at all in the electrolyte, only ions do. The electrolyte contains + charged cations and - charged anions. The cations flow towards the cathode, and the anions flow toward the anode. Electrons do not flow.

I would also disagree with those who say that "it doesn't matter what convention you use, it's entirely arbitrary." Words are symbols, so they have meaning entirely and solely by convention. The relationships between words describe reality, which is independent of any convention. So to invert the meaning of any word destroys all knowledge we have about reality.

[TheElectricChicken]

now that is an awesome explanation !!!
even I understand it now 

[Circlotron]

You can think of "Conventional Current" flow as "Hole Flow"... where Hole is defined as absence of an electron, i.e. a positive charge. But protons do not really flow, instead it's electrons that flow.  So you can think of it as when an electron flows, it leaves behind a "hole", and let's define absence of the negative electron is presence of a positive hole. So as electrons move their normal way, the positive holes move in the opposite way.

It's like saying you didn't drive your car from home to work; rather your parking space moved from work to home.
Basically, back in the bad old days they didn't really know which way electricity flowed so they took a guess and they got it wrong. Legions of people are still trying to save face over this by spouting this "conventional current" nonsense.

[Wim_L]

Be careful when overgeneralising, the conventional current direction isn't always 'wrong'. There are plenty of cases where the charge carriers are indeed positive, and they move in the same direction as the conventional current. There even are cases where you have both positive and negative charge carriers in the same region, so the 'real' current (charge carriers) move in two opposite directions. Such bidirectional current isn't handy when you want to use it for calculations, so you have to define a direction for practical reasons.

The conventional current through some surface (e.g. a section of wire) in a circuit describes the amount of charge that flows through that area, with the direction determined by the direction a positive charge would move at that point, but without saying anything about the nature of the real charge carriers present.

[retrolefty]

Be careful when overgeneralising, the conventional current direction isn't always 'wrong'. There are plenty of cases where the charge carriers are indeed positive, and they move in the same direction as the conventional current.

 Can you give an example case? Otherwise I tend to stay in the reply #7 camp. 

[hurricanehenry]

It's a common newcomer mistake to expect that anode and cathode denote a polarity in voltage. The words have nothing to do with voltage and everything to do with current.
Cathode is derived from late Latin cathodus meaning "way out". It is the terminal of the device from which conventional current goes out into the external circuit. Conventional current is opposed to the direction of electron flow, so it is also the terminal where electrons enter the device from the circuit (the cathode of a vacuum tube emits electrons into the vacuum).
In a battery, the direction of current is different during charge and discharge. During charging, the current is forced through the battery by the circuit, so it flows through the battery from the + terminal to the - terminal. This makes the - terminal the cathode during charging.
During discharging, the battery forces current through the external circuit, so current flows through the battery from the - to the + terminal. This makes the + terminal the cathode during discharging.

You asked about what happens "inside the battery". From the point of view of electrical theory it's simply a voltage source in series with a resistance, there is nothing else in there. The electrochemical reaction between the cell plates and the electrolyte is a separate topic.

From the point of view of chemistry you are talking a different language, with things like redox potentials, half-cell voltages, and reaction formulas.  Unlike wires in an electrical circuit, electrons really do not flow at all in the electrolyte, only ions do. The electrolyte contains + charged cations and - charged anions. The cations flow towards the cathode, and the anions flow toward the anode. Electrons do not flow.

I would also disagree with those who say that "it doesn't matter what convention you use, it's entirely arbitrary." Words are symbols, so they have meaning entirely and solely by convention. The relationships between words describe reality, which is independent of any convention. So to invert the meaning of any word destroys all knowledge we have about reality.

Many thanks.

[rs20]

Be careful when overgeneralising, the conventional current direction isn't always 'wrong'. There are plenty of cases where the charge carriers are indeed positive, and they move in the same direction as the conventional current.

Can you give an example case? Otherwise I tend to stay in the reply #7 camp. 

-- P-type semiconductors
-- Cu++ ions in copper plating solutions (actually, majority current flow in most electrolyte I believe, including batteries?)
-- Er, proton beams at particle accelerators (OK, yes, I'm overreaching now)

It's true that electron flow dominates in vacuums and metals. But conventional current works just fine, the reality of electron flow is of minimal concern to electronic engineers. It's just a nice way to weed out physicists who aren't up to the task 

[rs20]

It's like saying you didn't drive your car from home to work; rather your parking space moved from work to home.

And if you stand a million miles above the Earth, that may as well be exactly what's happened. What's your point? What are you proposing? Switching conventional current around on January 1st? I just sent off for some PCBs to get made this morning, and I designed them without remembering once that the electrons are actually going "up" my circuit diagram. I think it'll work just fine.

[retrolefty]

It's like saying you didn't drive your car from home to work; rather your parking space moved from work to home.

And if you stand a million miles above the Earth, that may as well be exactly what's happened. What's your point? What are you proposing? Switching conventional current around on January 1st? I just sent off for some PCBs to get made this morning, and I designed them without remembering once that the electrons are actually going "up" my circuit diagram. I think it'll work just fine.

 I agree it's not that big a deal once one decides on what 'reality' one wants to use. My main complaint is that all the arrows used in schematics for semiconductors point backwards to the real electrons flowing, so sure to frequently confuse people starting out in learning electronics theory. The military electronics training made it very simple, they just ordered us to ignore conventional current flow theory and just run our pencil tracing against the arrows.   

[rs20]

I agree it's not that big a deal once one decides on what 'reality' one wants to use. My main complaint is that all the arrows used in schematics for semiconductors point backwards to the real electrons flowing, so sure to frequently confuse people starting out in learning electronics theory. The military electronics training made it very simple, they just ordered us to ignore conventional current flow theory and just run our pencil tracing against the arrows.   

I claim that's an odd complaint because you don't care how the semiconductor works. Why are you thinking about the electrons at all? How a transistor works is not electronics theory, it's physics theory, and only when you cross into physics theory do you announce the simple sentence "electrons are negatively charged, so when conventional flow goes one way, the electrons are actually going the other way." And I'm sorry, but if someone can't remember that electrons are negatively charged, they might not be cut out for electronics. I think "frequently confuse" is really pushing it.

[CatalinaWOW]

So you are saying that the military picked one, stuck with it, and made sure you did too.

That is certainly a valid position, even though some might find it a little confusing to always go against the arrows. 

Since you will find quite a few books written with both of the assumptions I think the best practical answer is to be familiar with both, and use whichever works best with your neurons.  When talking to others make sure you know how they think and conform to the majority in your location.  If there is any chance that your audience doesn't share your personal preference, it always pays to state the world view you are using.

And remember, none of us really knows what is going on.  We just have some math models and theories that seem consistent with our observations.  From one point of view, electrons never move around, there are just a large number of coordinated phase shifts in the wave functions.  How would you tie that into direction of current flow?

[SL4P]

It's like saying you didn't drive your car from home to work; rather your parking space moved from work to home.

Do you do that too ?
Only on Mondays though... other days it seems to work <normally>.