1  How to Define Anode and Cathode

• Definition: The anode of a device is the terminal where current flows in from outside. The cathode of a device is the terminal where current flows out. This is illustrated in figure 1.
  
  Figure 1: Anode and Cathode Currents

  As always, electrons flowing in is the same as positive current flowing out, and vice versa.

  That’s essentially all there is to it.

  There is one execrable exception, as discussed below.

2  Discussion

• Our definition applies easily and correctly to every situation I can think of (with one execrable exception, as discussed below). Good examples you might have heard of include cathode-ray tubes, the cathode/grid/plate in an amplifier tube, the rotating anode in an X-ray tube, common-anode LED arrays, and the sacrificial anode on a boat.
• Ours is the original, time-honored definition. It is consistent with the Greek roots ανα− and κατα−. There is no other sensible definition. I’ve seen several attempts at definitions, but unless they were equivalent to our definition, they were grotesquely overcomplicated, wrong, or both.
• By well-established convention (going back to Ben Franklin), when we speak of current we mean the conventional positive current. In wires, the current is carried by electrons moving in the direction opposite to the current. This complicates the notion of current, but is necessary because the electron is negatively charged.
• For the vast majority of people, there is no point in memorizing the meaning of anode and cathode. The terms just aren’t very useful, unless you get a job in an electrochemistry laboratory or some comparably narrow specialty. If some day you do need to know the meanings, you can look them up that morning and forget them again that evening.
• Note that when we say current-in, we mean current flowing into the device from the external circuit. Similarly when we say current-out, we mean current flowing out of the device toward the external circuit. We are treating the device as a black box, and we are not talking about whatever currents flow within the device. This black-box terminology is standard in all branches of engineering and science, unless the context clearly requires otherwise.
• To avoid misconceptions, remember that the anode/cathode distinction is based on current, not voltage. Anode/cathode is not the same as positive/negative or vice versa. Illustrative example: for a battery being discharged, the positive terminal is the cathode, while for the same battery being recharged, the positive terminal is the anode.
• As a trustworthy rule, keep in mind that anode and cathode refer to function, not structure. There are lots of devices where it would be madness to permanently label the structures as anode or cathode, because their function changes from time to time. Rechargeable batteries are a common, very important example.
• Although anode and cathode are fundamentally defined in terms of function not structure, there are some exceptional devices where it is arguably permissible to label the structures as anode and cathode, because only one direction of current-flow makes sense. Examples include CRTs, non-rechargeable batteries, rectifier diodes, and light-emitting diodes. However, this must be considered a risky exception, not the rule.
• Even in cases where it is arguably possible to identify a definite anode and cathode, there are usually simpler and better ways to designate the terminals. Specifically, for a battery (rechargeable or not), it is conventional and sensible to speak of the positive terminal and negative terminal. For a diode, it is conventional and sensible to speak of the P-doped side and the N-doped side. In particular, for a LED display module, common-anode means the same thing as common-P-side.
• In all cases, you can use the descriptive terms current-sink and current-source as synonyms for anode and cathode. Description is generally preferable to jargon.
• It is possible to buy an array of Zener diodes. Alas, according to an established but illogical convention, the so-called common-anode configuration is structurally analogous to a common-anode array of LEDs, in the sense that the P-doped sides are tied together. This is an abomination, because in normal Zener usage, the P-doped side is where the current exits, and should logically be called the cathode. Apparently somebody was under the misimpression that anode/cathode referred to structure instead of function.

  You should never apply the terms anode or cathode to a Zener diode, because the potential for confusion is too great. Instead you should refer to the P-doped side and the N-doped side, and you should insist that others do the same.

  Note that reversing the labeling convention for Zener diode arrays would not solve the problem in any fundamental sense, because there are perfectly reasonable circuits in which – part of the time – a Zener diode is forward biased, so that it conducts just like any other diode. This is the same situation we encounter in connection with rechargeable batteries: if you attach permanent anode/cathode labels to the structure, you will be wrong at least part of the time.

  The terms “anode” and “cathode” properly apply to function, not structure.

• Electrochemical corollary: In any electrochemical cell, oxidation reactions take place at the anode, and reduction reactions take place at the cathode. (If you don’t know what this means, don’t worry about it.) This includes batteries being charged (anode=positive) as well as batteries being discharged (anode=negative). This is a corollary flowing from our definition, and from the conventional viewpoint that the cell is the black box, and everything external to the cell is the external circuit.

• Let us make a brief exception to the black-box viewpoint, and consider what happens inside an electrochemical cell. Inside the cell, cations (positively charged species) moving toward the cathode make a positive contribution to the conventional current inside the cell, as shown in figure 2. Similarly, anions (negatively charged species) moving toward the anode make a positive contribution to the conventional current inside the cell. No anions are shown in the figure. The rule anions-to-anode, cations-to-cathode applies only inside the cell. This rule is required by the fact that current obeys a conservation law; current that flows into the cell at the anode must flow through the cell and then out the cathode. Outside the cell, current flows toward the anode; inside the cell, current flows away from the anode. (By the way, it is usually assumed that outside the cell, there are no mobile anions or cations, just electrons moving via the external circuit.)

  
  Figure 2: Anode and Cathode : Inside the Black Box

  When talking about ions, you need to remember that cations are positively charged. The mnemonic for cations is to view the ‘t’ as a plus sign: ca+ion. Meanwhile, the mnemonic for anions is something of an acronym: A Negative ION = ANION.

  When remembering the cations-to-cathode rule, you need to remember that cations go to (not from) the cathode: ca+ions +o ca+hode. The corresponding anions-to-anode rule is equally valid, but you have to work harder to remember that the anions go to (not from) the anode.

• Similarly there is some slight potential for confusion when thinking about cathode ray tubes, because of the temptation to deviate from the black-box viewpoint. Remember, from outside the device we see positive conventional current coming out of the cathode, in accordance with our definition. If we peek inside the device, we see electrons streaming out of the cathode.
• There is even more potential for confusion if you try to explain or define anode/cathode in terms of electrochemical cells. In particular, in a battery being discharged, inside the cell we have positively-charged ions flowing toward the positive-voltage electrode. Obviously this cannot be explained in terms of the electrical potential alone. You need to understand the electrochemical potential. Even among chemists and physicists, it is rare to find someone who has a fluent, detailed understanding of where that comes from. See reference 1 for the next level of detail on the subject. As the saying goes, learning proceeds from the known to the unknown. Our definition of anode/cathode is simple. Batteries are not simple. It makes no sense to “explain” the former in terms of the latter.
• Boats and other structures in contact with salt water give rise to a situation with some potential for confusion about anode versus cathode. At first glance it might not be obvious what’s considered the “black box” and what’s considered the “external circuit”. The conventional viewpoint is that the water, and the metals touching the water, are to be thought of as a giant electrochemical cell, and treated as a black box, while the structure of the boat (or whatever) is considered the external circuit. That is, the boat is external to the water, not vice versa. This may seem arbitrary, but at least it is consistent with the aforementioned electrochemical corollary, so that oxidation reactions take place at the anode, and reduction reactions take place at the cathode. This leads us to the useful concept of a sacrificial anode, which is just a cheap, easily-replaceable electrode that is placed in the water and arranged to have a large positive voltage with respect to the rest of the boat. That makes everything else on the boat a cathode, greatly reducing corrosion, because most forms of corrosion involve oxidation reactions. To say the same thing in other words, inside the water, highly corrosive anions such as OH^– and Cl^– are flowing toward the anode and away from everything else, in accordance with the anions-to-anode rule. The anode, of course, corrodes rapidly, and needs to be replaced on occasion.

3  Summary

I am astonished that some people take a concept that is simple and unimportant, make it needlessly complex, and pretend it is important.

When dealing with batteries, don’t think in terms of anode and cathode; think in terms of positive terminal and negative terminal.

When dealing with semiconductor diodes, don’t worry about anode and cathode; think in terms of P-doped side and N-doped side.

The general rule is: Anode means current into the black box and cathode means current out from the black box. Zener diodes give rise to an execrable exception that should be avoided like the plague.

There is abundant evidence that even people who call themselves experts cannot keep the anode/cathode terminology straight. In any practical situation, there is always a way to figure out how to hook things up without a deep understanding of anode versus cathode.

In almost all situations, it is better to avoid the terms anode and cathode. There are better ways to say what needs to be said. Constructive suggestion: it is better to talk about the current (rather than the electrode). It is better to talk about what the current is doing (rather than what the electrode “is”).

4  References

1.

John Denker,“How a Battery Works”
http://www.av8n.com/physics/batt.htm