Why is AC current represented with a sine wave?

[Vovk_Z]

By the way, "AC current" is redundant.

I'm not talking about that post, but rather in general:
I may be wrong but I guess we use this a bit redundant form of "AC current" because we use "AC voltage" too, so we add "current" to "AC" when we want to say that we are talking about a current but not about a voltage.   

[Siwastaja]

By the way, "AC current" is redundant.

No, it's not. "AC" has developed a set of completely new meanings which have nothing to do with current, and just become a term of its own. "AC voltage", "AC current", "AC component" are all widely used and understood, not only colloquially but rigorously scientifically, and they can't be written open like "alternating current voltage", "alternating current current" and "alternating current component", obviously. Why? Because "AC" in such contexts does not mean "alternating current".

[tooki]

By the way, "AC current" is redundant.

Except not really. As others have explained, “AC” just refers to alternating current in the abstract (it means the entire concept of electricity whose polarity changes constantly), not to “current” in the narrow engineering/physics sense. You’re not going to convince anyone that our multimeters should say DV, DC, AV, and AC on the dial (for DC V, DC I, AC V, and AC I, respectively).

English is happy using many, many redundant abbreviations/initialisms (LCD display, PIN number, etc).

Some languages, like German, expressly discourage them, resulting in awkward constructs like “LC-Display” and “ISB-Nummer” (the latter of which really irks me because there is an internal hierarchy to the 4 words in ISBN, and “ISB-Nummer” violates the hierarchy by plucking out a word from a deeper level: structurally, it’s “international+(standard+(book number))”).

[Gyro]

By the way, "AC current" is redundant.

Unless you can persuade the world to adopt AV as a standard abbreviation for Alternating Voltage, we are stuck with 'Alternating Current voltage' and 'Alternating Current current'.


Edit: ... and as tooki says, DV too.

[m k]

Many times I find myself changing AC to local expression of AE, where E is electricity.
Or more closer WE, where W is wall.

My head is also so tightly in DC side that my intuitive wall electricity peak value is wrong.
Of course the reminder is pretty instant but it's not a value, it's a picture of a scope screen with almost vertical lines.
Values become later, if ever.

[golden_labels]

To add to “it’s a convention” comment: using a sinusoid is a convention too. One does not only choose a specific point to be t=0, but also chooses to ignore what happens sufficiently far away from that point. Sine wave extends infinitely far in both directions, which does not happen with any real-world signal. So it’s all about saying “look, around that point we observe that frequency” and, if more than one sinusoid is considered, “look, they differ in phase that and that way”. Whatever choice serves that purpose best is the right one.

[TimFox]

To my many respondents:  don't worry, I find it very difficult to avoid "AC current" in technical writing.  It is still redundant, as is "Gobi Desert" (another pleonasm).

[tooki]

To my many respondents:  don't worry, I find it very difficult to avoid "AC current" in technical writing.  It is still redundant, …

No it isn’t. The “current” in “AC” doesn’t mean the amount of flow. It’s referring to the abstract concept of electrical flow, for the purposes of describing what type it it. So the phrase “AC current” generally means “flow rate of an alternating current”.

When we want to refer to the abstract current itself, as I did above, we do not abbreviate it, and we use a pronoun: an alternating current. You’d never refer to it as “an AC”.

[TimFox]

"Redundant" does not mean "incorrect", "negating", or "oxymoron"--it just means "unnecessary for meaning".
Others include "PIN number" that you mentioned:  the concept is still understood.
A common expression in English technical writing is "3.2 A rms AC", which could be written "3.2 A rms AC current".

[JohanH]

Because "AC" in such contexts does not mean "alternating current".

And yet it might. Because it isn't a standardized abbreviation or term. Edit. It is, but it isn't always used correctly.

[tooki]

Because "AC" in such contexts does not mean "alternating current".

And yet it might. Because it isn't a standardized abbreviation or term.

That’s an absolutely absurd claim.

[JohanH]

That’s an absolutely absurd claim.

I might have missed it, I haven't read all standards. Care to quote?

[tooki]

That’s an absolutely absurd claim.

I might have missed it, I haven't read all standards. Care to quote?

You sound like Bill Clinton arguing the meaning of “is”.

We can argue about what the exact meanings of “alternating current” are, but you only look like a fool trying to argue whether “AC” stands for “alternating current” in the context of electricity.

[TimFox]

I seem to have stirred up a flurry amongst the language snowflakes.

[JohanH]

We can argue about what the exact meanings of “alternating current” are, but you only look like a fool trying to argue whether “AC” stands for “alternating current” in the context of electricity.

That was not my intention. The thought behind my post is whether "AC" as a term has been used in an ambiguous ways not defined in standards. I do find it mentioned in IEC IEV 151-15-01.

Note 3 – For the marking of electric equipment, either the notation AC (see IEC 61293) or the graphical symbol ~ (see IEC 60417 item 5032) may be used. Example: AC 500 V or ~ 500 V.

Note 4 – According to ISO 31-0 and IEC 60027-1, unit names and unit symbols shall not be qualified by "AC" as an attachment. Example: U_AC = 500 V is correct, U = 500 V_AC or U = 500 V AC are incorrect.

   

So it is comparable to the "~" sign. But it is not allowed to use "AC" as a unit (attachment).

The common usage VAC and IAC (and variations) is thus wrong.

So I cannot see that there is a fully satisfying situation with regards to standards.

[CatalinaWOW]

For those uncomfortable with the use of AC as a descriptor what is your preferred way to describe and differentiate in situations where there is both a zero frequency component and also current at a single dominant frequency?  The use of AC and DC is usually very clear in context and the situation is not uncommon..

[xrunner]

A lot of instruments use symbols. Such as this example from a Fluke 117. Again the sine wave for alternating volts or amps, and the stable line above a ground for non-alternating voltage or current.

[CatalinaWOW]

That is fine for an instrument, and probably could work in written communication (with some typography issues) but how do you pronounce those symbols in spoken conversation or in a lecture?

[Siwastaja]

DC and AC are used in contexts such as DCT transforms in JPEG images to differentiate between the zero-frequency component (put simply, same color across the macroblock) and higher frequency components (varying color across the macroblock). If you think about it in power line terms, it sounds ridiculous, but this is what it is. Language lawyerism is futile; reality wins over theory, especially if it's so widely accepted both in colloquial and scientific use as AC/DC terms are.

[Siwastaja]

That is fine for an instrument, and probably could work in written communication (with some typography issues) but how do you pronounce those symbols in spoken conversation or in a lecture?

"AC voltage range", for example. This is unimportant; the lecturer needs to carefully explain what the meter actually measures, anyway, and this is far beyond just one simple term: does it measure RMS? Average? Peak? Peak from which RMS is calculated assuming sine waveform? Something else? In presence of DC offset, how does it behave? Is there a maximum frequency limitation? And so on, and so on.

After that, the choice of exact term is unimportant, it's just an identifier.

The lecturer would also make clear that a multimeter AC functionality is a very limited instrument and oscilloscope should be used to see all the details of the actual waveform.

[SandyCox]

sine is cosine with a phase lag of 90 degrees.

Must textbooks use cosine as their phase reference.

[Vovk_Z]

I seem to have stirred up a flurry amongst the language snowflakes.

That's because we don't really know about a topic question, but we always know when somebody is wrong on the internet.  ( )

[Vovk_Z]

sine is cosine with a phase lag of 90 degrees.

Sine is just more beautiful. That's it. 

[Cerebus]

sine is cosine with a phase lag of 90 degrees.

Sine is just more beautiful. That's it.

Not so. In human perception more beautiful is generally associated with symmetry, with symmetrical being seen as more beautiful than asymmetrical. Vis:

If you start at the origin a sine wave is perceptually asymmetric, a cosine wave symmetric:



corresponding to asymmetric and symmetric perceptions of natural beauty:

[newbrain]

I've known loads of people who carry around a head full of trig identifies that they have learned by rote and have no clue why those identities are the way they are.

This is a terrible way of learning, and it can happen due to faulty teaching but sometimes also due to some student's attitude.

Quite a bit OT, but relevant:
During the "Sequential and Combinatory Systems" course at my uni, we were studying among many things (the usual: Karnaugh maps, Mealy and Moore FSMs etc. etc.) Z80 assembler programming.

A friend and colleague, smart and with very good marks, came to visit.
He was flabbergasted when I explained that the board in front of him was a tangible, real Z80 with RAM, EPROM etc.
"Oh, I'd never have thought that this thing existed!"

For him the instruction set, ROM, RAM and ports were all just a conceptual exercise - bearing no relation whatsoever with the physical world - he had categorized it more or less as an overly complicated, abstract, way of representing state machines.