Copyright © 2004 jsd

How to Define Anode and Cathode
John Denker

*   Contents

1  Definition

2  Some Examples

Our definition applies easily and correctly to every situation I can think of (with one execrable exception, as discussed item 11 below).

  1. Voltaic cells and batteries.
  2. The hot cathode in a cathode-ray tube, as found in an old-style television or oscilloscope.
  3. The hot cathode in an electronic amplifier tube (“Fleming valve”).
  4. The hot cathode in an X-ray tube, as in figure~2.
  5. The rotating anode in an X-ray tube, as in figure~2.
    Figure~2: X-ray Tube
  6. A common-anode LED array, such as a 7-segment digit array, although this is not optimal terminology, for reasons discussed in item 8.
  7. The sacrificial anode in a boat; see item 16.
  8. The anode plate and cathode plate (as well as anode mud) in an electrolytic refining cell; see item 9.

Crucially, our definition applies just fine to things like a rechargeable battery, where you cannot identify the anode and cathode until you see how the device is being operated, as discussed in item 6.

Our definition also applies just fine in cases where it is relatively easy to distinguish anode from cathode “just by looking” as discussed in item 7.

There is one execrable exception, as discussed item 11 below.

3  Discussion

Ours is the original, time-honored definition. It is consistent with the etymology, as discussed in item 17. There is no other sensible definition. I’ve seen several attempts at definitions, but unless they were equivalent to our definition (as given in section~1), 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 metal 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 emphatically 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.

If you insist on peeking inside the black box, the story gets more complicated, as you can see in figure~2. However, this does not change the letter or the spirit of the definition, which is based on the behavior of the black box, as seen from the outside.

It is crucial to 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, as mentioned in item 5.

Although anode and cathode are fundamentally defined in terms of function not structure, there are some exceptional devices where the function is essentially locked to the structure. In such a case, it is arguably permissible to label the structures as anode and cathode, because only one direction of current-flow makes sense. On the list in section~2, all the examples except for the rechargeable battery are in this category.

In any case, keep in mind that this category must be considered the risky exception, not the general rule. The trusty general rule is explained in item 6.

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, the so-called common-anode configuration would more properly be called the common-P-side configuration.

Here’s an interesting and important example: Consider the electrolytic refining of metals such as copper.

During normal operation, a large current flows through the cell, imposed from the outside. Current is pushed into the cell at the anode, and taken out at the cathode. The terminals are labeled according to their normal function, in accordance with the definition given in section~1.

At the beginning of the operation, the anode is impure copper. At the end of the operation, the cathode is much higher purity copper. Try googling for anode mud.

If some wise guy temporarily reversed the direction of the current, the normal anode would become the temporary cathode and vice versa. However, this possibility is so kooky that it is usually not even considered. The terminals are labeled according to their normal function.

Note the contrast:

Electrolytic refining cell.   Ordinary battery

In the refining cell, the open-circuit cell voltage, if any, is very small and completely irrelevant.   In the battery, there is a definite positive terminal and a definite negative terminal.

The voltage drop across the cell is roughly proportional to the current. During operation, the anode will be at a positive voltage relative to the cathode.   The voltage drop across the cell is qualitatively the same, no matter whether the current is positive, negative, or zero. The positive terminal is the cathode during discharge, but it is the anode during recharge.

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 use the terms anode or cathode to describe the structural parts of a Zener diode, for the same reason you should not use such terms for the structure of a rechargeable battery. Anode and cathode refer to function, not structure. 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.

The situation is summarized in the following table:

− plate:~~~~~cathode
being reduced
being oxidized
+ plate:~~~~~anode
being oxidized
being reduced

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~3. 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 carried by metal wires in the external circuit.)
Figure~3: 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 inside the cell, 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.

Please remember that the cations-to-cathode rule is subject to multiple caveats. It is at best a chemical corollary to the real definition. It cannot possibly serve as the definition of cathode, because the cathode is well-defined for all sorts of devices that don’t have any mobile cations, e.g. semiconductor diodes, cathode-ray tubes, et cetera. Another caveat is that this rule applies to what’s going on inside the cell, whereas for most purposes (including the anode/cathode definition) it is conventional and appropriate to focus on the properties of the black box, as seen from the outside. (Similar issues arise in item 14 and item 16.)

There is some slight potential for confusion when thinking about cathode ray tubes and X-ray tubes, because of the temptation to deviate from the black-box viewpoint. (Similar issues arise in item 13 and item 16.) In an X-ray tube, the interesting physics is happening inside the device, whereas the definition of anode is expressed in terms of conventional current flowing into the external terminal, flowing into the black box from outside. Remember, from outside the device we see positive conventional current coming out of the cathode and going into the anode, in accordance with our definition, as seen in figure~1 in section~1. The rule is ACID: Anode Current Into Device. (If we peek inside the device, we see electrons streaming out of the cathode, but that’s only a corollary of the definition, not the definition per se.)

There is even more potential for confusion if you try to explain or define anode/cathode in terms of electrochemical cells, if only because very very few people understand how such things work. See reference~1 and references therein. As the saying goes, learning proceeds from the known to the unknown. Our definition of anode/cathode, as given in section~1, is simple and useful. The internal mechanism of a battery is not simple. It makes no sense to “explain” the former in terms of the latter.

Battery terminals are labeled positive and negative. They are labeled according to voltage, not charge or current. This is conventional and entirely appropriate, because the positive terminal remains at a positive voltage (relative to the other terminal) during all normal conditions, including when the battery is discharging, recharging, or just sitting there in equilibrium with no current flowing.

In contrast, as mentioned in item 5, it would be wildly inappropriate to label the battery terminals as anode and cathode. That’s because the terminal that is the cathode during discharge becomes the anode during recharge ... and is neither anode nor cathode in the equilibrium (no current) situation.

Furthermore, it makes no sense to define anode and cathode in terms of electrochemisty, because the terms are used in all sorts of situations where there is no electrochemistry involved, including semiconductor diodes, X-ray tubes, et cetera.

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 this:

That is to say, the convention is to consider the boat as external to the water ... even though it might seem more logical to think of the water as external to the boat. This may seem arbitrary, but at least it is consistent with the aforementioned electrochemical corollary (item 12), 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.

Etymology: The words anode and cathode were introduced in 1834 by Michael Faraday on the advice of William Whewell, the polymathic scientist and prolific wordsmith. Whewell understood quite a bit of Greek, and put it to good use:

One should never place too much emphasis on etymology, because meanings can drift over time. Indeed ἀνά and κατά have drifted from their ancient roots. However, ὀδός has not, and that’s the key. The English words, when coined, were clearly intended to describe flow, not voltage. The same roots are used in other Greek and pseudo-Greek terms in English, e.g. anabolic, cataract, odometer, et cetera.

4  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.

The terms anode and cathode are sometimes convenient, in situations where only one direction of current makes sense.

In other situations, it is usually 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”).

5  References

John Denker,“How a Battery Works”
Copyright © 2004 jsd