# Why do we use the terms AC and DC if they have nothing to do with current?

It seems archaic, maybe that we use the term AC (alternating current) and DC (direct current) to talk about power systems. After all, it is the voltage waveform we're usually referring to. In my first power electronics class, we learned that the current of a power system often seems to mimic the envelope of the voltage input waveform, but that's hardly the case with active loads.

Why do we use alternating current and direct current as opposed to alternating voltage and direct voltage?

• Natural language is like that. Some word is chosen for a concept, and it sticks even after its literal meaning is no longer strictly fitting, like "dialing a phone", or "rolling up" the car window. Mar 30, 2021 at 4:01
• I think that term was coined before many loads existed that would have current waveforms strongly different from their voltage waveforms, and similar to water, it is the current that is actually being pumped around to serve the customer. Looking at the source component providing power to the device it is still arguably accurate, as a source is offering either current pumped in a single direction, or current pumped in both directions. Only utilizing a portion of the wave doesn't change what the source makes available.
– K H
Mar 30, 2021 at 4:01
• Incidentally I think you may be on the wrong stack. Although this would fall loosely into history of Electrical Engineering, the English Stack takes questions on etymology (the origin of words) and they would likely have more interest in finding the original reason, where as this stack is full of people like me who can simply see many plausible arguments or just reasons something like this would happen.
– K H
Mar 30, 2021 at 4:07
• Voltage doesn't really do much of interest to the consumer without current. The difference is like the difference between the lightning bug and lightning. Mar 30, 2021 at 4:13
• Interesting enough, it varies by language. E.g. in Swedish, it is referred to as "växelspänning" ("switching voltage") Mar 30, 2021 at 5:13

If the voltage connected to a resistor is alternating, the current is alternating too. The same is true for a circuit of resistors, capacitors and inductors without diodes or transistors.

If a constant (not alternating) voltage is connected to a resistor, the current is constant and not alternating too.

A hypothesis about the linguistic origins of AC & DC:

When electricity became a utility, we already had water conduits, gas conduits and sewer conduits, all carrying a stream of some sort. So it would be natural to have electricity be a stream supplied through some sort of conduit. Clearly, these conduits are adapted for electricity, and wires are generally preferred over hollow conductors.

One readily appreciates that a utility is delivered as a stream: water, gas and sewer are direct streams, but electricity has two modes, the direct stream and the alternating stream.

Compare this to other languages: when referring to wire, in German and Dutch we speak of "Stromleitung" and "Stroomleiding" respectively, literally meaning stream conduit. The conduits for water are "waterleiding" and "Wasserleitung" (... must remember to capitalize German nouns). Gas and sewer are similar. Equally, AC and DC refer to currents in these languages too, "wisselstroom" and "Wechselstrom".

And current is related to the French "courant", meaning "running", like a runner or a stream that runs. And the French "conduit" means to guide in this context, which is also related to our general term for the thing that "carries" electricity, the "conductor".

So it stands to reason that the thing of value that alternates along its utility conduit is the stream. Similar to other utilities, value is delivered when water or gas flows, not just when water or gas pressure is present. Also, billing is based on the amount of stream used (admittedly a very loose interpretation), not the pressure.

So perhaps it is the perspective of electricity as a utility similar to other utilities at the time, that lifts current over voltage as the primary source of value. And consequently the thing that alternates is the current.

Other languages do have a word for "alternating voltage" (e.g. the German "Wechselspannung"), and it is used when linguistic precision or a diversity of labeling is required to study and tackle electricity from a physics or engineering angle.

This is similar across Indo-European languages, but I'd be curious to know if other language families (Afro-Asiatic, Asia-Pacific families etc..) have a similar or different view.

1. Ah, let me see. During The Current War, it was the direction of electrical current: "Direct" or "Alternate" that two big guys were fighting for.

2. Since "AC" voltage, with the help of the use of transformers, current can transmit longer distance, then DC. So the AC guy won.

3. The names "AC", "DC" got stuck after the War, and people have been saying for two hundred years the funny names "AC current", "DC current".

References

Appendices

Appendix A - War of the currents

/ to continue, ...

• The question is more about the use of "current" in AC/DC instead of "voltage" as in if you said "AV/DV" instead. The argument is that because the source is usually a voltage source or voltage converter in distribution and can tolerate current deviations from the AC wave, we should be saying "Alternating Voltage" and "Direct Voltage" instead. Similarly we could also replace "Direct" with "Unidirectional" to be more accurate, as arguably both are equally direct in some contexts. I think they haven't been changed because the current terminology is well understood.
– K H
Mar 30, 2021 at 4:15
• Ha, I very much agree with your saying that "the current terminology is well understood.". In my country, we often say something like "We people have been using the wrong word for a long time, but it has now become "customary", or "of historical reasons", "Let the mistake be used as mistake". Of course it is all nonsense. BTW, all these years I have had a hard time explaining to the laymen that OK, OK, the direction of electrical current is actually wrong, because electrons flow the opposite direction. In short, don't take what the EE pros say too seriously, because they are often wrong. Mar 30, 2021 at 4:38
• That phrase also makes me happy because of the double entendre =). The current terminology is current terminology lol. The common phrase that comes to me is "Don't fix it if it ain't broke." which in real life can turn out to be a variety of things. Sometimes it's good advice, sometimes it gets misused to encourage laziness. By my standards, OP is right, but the terminology used isn't wrong enough to justify correction unless it's easy. At this point if we did switch, it would just mean there were two pieces of information to know instead of one because of the volume of existing work.
– K H
Mar 30, 2021 at 4:41
• There is also the question of whether it would be right to do that to AC/DC the band at this point in time.
– K H
Mar 30, 2021 at 4:45
• And if you get French AC/DC is used to say something else - aka voile ou vapeur... Mar 30, 2021 at 5:07

It's not true that you usually talk about voltage, in transistor application current is usually way more important. In fact galvanometers actually measure current (magnetic field is generated by current), and that was until some years ago the preferred way to measure voltage, too.

The trick is that they are related by resistance (Ohm said so) and since usually resistance is constant (at least for the loads of the time) a continuous current produces a continuous voltage.

If you really want to go linguistic, the real issue is saying "DC voltage" or "DC current": the first has not really sense (direct current voltage? meh), the second one is like saying "LED diode" (current is already in acronym)

By the way, languages vary: in italian we use "corrente continua", "corrente alternata", "tensione continua", "tensione alternata": no redundant acronyms and "direct" is replaced by "continuous" which is arguably a better choice.

PS: and don't forget about the "conventional direction of flow" thing!