# Why don't we say alternating voltage and direct voltage instead of AC and DC?

Considering that, usually, current is a consequence of a voltage over a load, why do we focus on current when talking about alternating or direct electrical quantities (i.e alternating current and direct current)?

In the control of static converters, for instance, normally we have a voltage reference that is either alternating or constant. In electric generators, it is an induced alternating voltage that is produced from the resulting spin of the machine's rotor. Transformers relate voltages, even without any current flowing.

Wouldn't it be more accurate to say AV/DV instead? Maybe this is something that we say just because "everybody does"? Maybe it's historical, but it would be interesting to know where all this started.

There is a related question (Why is DC Voltage called Direct Current ?) where some answers mention the fact but doesn't really explain why is it.[

Thanks!

• Because what we care about is the charge carriers moving, which is called current. Especially in those cases where the current creates a voltage, like in a generator Commented Mar 14, 2017 at 14:11
• And by the way, why is the current STILL backwards compared to the direction to which the electrons move ? Plenty of things makes no sense but it's that way just because changing them is almost impossible. Commented Mar 14, 2017 at 14:13
• @Bregalad We need something to discriminate between physicists and engineers. Commented Mar 14, 2017 at 14:44
• If you apply Alternating Voltage to a higher Direct Voltage you still get a direct current, Hence DC Commented Mar 14, 2017 at 15:11
• Current is physical, quantifiable, and has mass. You can count current's electrons moving as in coulombs. Voltage is a measure of potential energy but it does not have any physical characteristics. Volts is to electricity like what a pint is to beer. Commented Mar 14, 2017 at 15:39

I don't know for sure and I'm not sure if there's a book that discusses the etymology, or not. But I can take a shot at it which may help find an answer.

First off, we're discussing English and, as it turns out, Benjamin Franklin was the first person to construct a workable mental model to explain Leyden jars, the earlier German friction generators, and the concept of conduction. He was also pretty famous for other reasons and had the ears of the general public. He created the concept of an electric fluid to do that.

Volta developed/refined the battery pile, which at the time wasn't known for its voltage but instead for its current. It didn't produce sparks, but was used for its steady current in laboratories of the time. In fact, within weeks of his announcement of his piles, two English scientists (William Nicholson and Anthony Carlisle) were already using its currents in order to separate oxygen and hydrogen from water! Davy also immediately stopped his earlier efforts (he was 21 years old then) and shifted over to try and find out "what these amazing currents can do." [See note at bottom.]

It also got into "medical" use (muscle twitching, for example), which I think is again where popular use of terms occurs.

(Who, outside of engineering for example, immediately knows the origin of "push the envelope?" But yet the term is still widely used by the population, at large, too.)

Ørsted, not long after, found that magnetic effects could cause a current in wires and Faraday the very next year was able to show how to apply these two ideas to create a motor.

I suspect, but not having directly lived through that time myself I remain unsure of course, that Benjamin Franklin's fluid ideas were easiest for the lay public to put in mind when thinking about the developments taking place around them. And I suspect that the idea stuck. Marketing folks would have later simply leveraged the public's concepts when the "war of currents" began, and the ideas become set in verbal concrete.

A note about Davy: In checking on my facts above, I uncovered that Davy was an amazing popularizer of science and thought I'd share a few interesting bits I uncovered today. After starting to explore with Volta's battery piles, he soon transferred to The Royal Institution of Great Britain (in 1801, at the age of 22!!) One of the stated missions of the institution was to provide public demonstrations of science meant to stimulate an interest in science by the elite of the day (for monetary support, I suppose.) To achieve that, they created a theater in their building on Albemarle Street in London. Davy started out as an assistant lecturer, but the lectures at that moment were also dwindling in popularity and weren't serving the institution's hoped support from the elite. Davy was very soon "promoted" to this top job of public lecturing. To make the most of it, he chose to make his lectures seem "spontaneous" and "shocking." It worked! And almost the moment that Davy took over the lectures, the audiences were packed in. They quickly became so popular, in fact, that Albemarle street became the first one-way street in London because of all the carriages that were bringing people to hear his lectures. (Within 5 years or so, he was able to tell the Institution that he had filled their coffers well enough doing "their work" that he now wanted their support for him to go off and do his research.)

I found it very interesting to learn that the popularity of science demonstrations led to the first one-way street in London!!

We call it alternating current and direct current because it is in fact a current that results in the voltage that we measure. Without this initial current the voltage would never exist.

Imagine a battery being built from scratch. The positive and negative terminal and plate are first inserted into the housing. Is there a voltage? No. Both plates are at the same potential. Now we add some electrolyte. What happens? The electrolyte causes free electrons that are on the positive plate to be deposited on the negative plate. At this point you have a voltage. But what caused that voltage to exist? The movement of free electrons from the positive plate to the negative plate....... which is electric current. So we call it AC and DC because it's the current that creates potential (voltage)

• Without an initial voltage there would be no initial current. You may shift single electrons and create an electric field. There may be a potential difference without a current.
– Uwe
Commented Dec 28, 2021 at 1:42
• I think AC/DC is just a historical misnomer. No need to overthink it. The battery example you have included could not possibly answer the AC case anyway. A voltage can exist without a current. Current cannot flow without a voltage difference.
– Syed
Commented Dec 28, 2021 at 7:33
• @Syed In a superconductor a current may flow without a voltage difference.
– Uwe
Commented Jan 1, 2022 at 1:27
• I was referring to everyday usage. I am not aware of the theory of superconductors. Thanks for your useful comment. @Uwe
– Syed
Commented Jan 1, 2022 at 5:02

In German we use Gleichpspannung (direct voltage), Gleichstrom (direct current), Wechselstrom (alternating current) and Wechselspannung (alternating voltage).

• Which terms are most commonly used (both in EE and with the general public)? Commented Dec 28, 2021 at 8:02
• @AlexHajnal We often use Strom or Wechselstrom when we are talking about electrical energy. A Starkstromelektriker (strong current eletrician) is specialised to work at Hochspannungsleitungen (high voltage power lines). Starkstrom is used in the meaning of high power as well as high voltage.
– Uwe
Commented Dec 28, 2021 at 10:02

I think your link pretty much explains it, and this post may be off topic, but I love analogies.

Could it just be as simple as: The potential for X volts is always X volts in both cases, but the current either flows in one direction, or it alternates direction, hence direct vs. alternating current?

If you have a 2 ton car (volts) moving forward (current), vs a car alternating between forward and reverse, the car is still a 2 ton car.

• This just isn’t true (except in unusual circumstances). For an ordinary AC power source powering an ordinary load, both the voltage and the current alternate. I don’t think that the comparison between the mass of a car and voltage makes sense. Commented Dec 29, 2021 at 18:37