But “frequency” doesn’t work in my Jeopardy clue. Is there no other word that does?
Inverters do quite a nice job of applying a frequency to DC, thank you very much.
Alternating vs. fixed polarity is the fundamental difference. I don’t know that we have a single word that carries that meaning.
Any single micro-second long timeslice of AC or DC looks identical. What differs is only the fact that in AC the voltage is rhythmically changing significantly over timescales of milliseconds (or shorter) and in DC it isn’t significantly changing over timescales of seconds (or longer).
At least the above is true about mains power supplies, both current and historical.
We had a fascinating thread a couple years ago about whether the essential AC/DC difference is rhythmic change in voltage, or voltage crossing zero 2x/cycle. IOW, if you have a standard US 120VAC supply and bias it with 200VDC, is the result AC or DC?
The answer seems obvious. Until the experts weighted in on the weird corner cases and the sometimes goofy implications of the naïve interpretation.
I’ll see if I can locate the thread. It’s gonna be a bear to search for.
I will say this.
A good friend of mine, who sadly is no longer with us, was an engineer who, many years ago, worked with a doctor who was doing research on pacemakers. In order to test pacemaker designs, they needed to reliably throw heartbeats out of whack (animal hearts, not human, but if you’re an animal lover that might not be much better).
Anyway, what they found was that the best frequencies for screwing up your heartbeat were right around 50 or 60 Hz or so.
So from a safety point of view, we really couldn’t have picked worse frequencies to base our power systems on.
To LSLGuy, I personally would call that AC with a DC bias.
That is how you bias a transistor.
Or alternating DC, although there’s not any alternating going on.
Well, yes, but the goal there is to simulate AC.
Simulated AC is still DC, though, so in the context of what distinguishes AC from DC, “frequency” isn’t valid DC can have a frequency.
Simulating AC isn’t always the goal; in many cases energy control is the goal. With AC you only have the ability to switch on a thyristor or SCR every half cycle; with an inverter you’re only limited to the speed of your technology.
Found it. It turns out the “what is biased AC called?” was a digression, albeit an interesting one.
For your reading pleasure from 8 years ago: Is an analogue audio signal DC or AC? - Factual Questions - Straight Dope Message Board
It was already revived once by an uninformed question post; perhaps best to let it sleep.