What really is AC and DC?

[Pulse Cloud]

Hello, can you please tell me which of the graphs are AC and DC? I never really understood it fully.

Template so we can understand each other:
    Picture 1:
        Red: <classification>
        Green: <classification>
        Blue: <classification>
    etc.

Picture number is from the order of the attachments.
I used mostly sine waves, but I think it'd be the same if I used triangular, square, etc. Right?


Thanks.

EDIT: The vertical axis is Voltage and the horizontal axis is Time. Also, the Voltage axis should not have an arrow pointing down (damn copypasting to save time).

[amspire]

Ac and Dc are simply two different ways of looking at exactly the same thing.

Some things it make sense in working with the DC value which is the same as the instantaneous voltage level at any point in time. In reality, all DC voltages actually vary - equipment gets turned on and off, batteries discharge and recharge. Batteries, that is an excellent example of something that is most convenient to analyze as a varying DC voltage. Even the best and most stable DC voltage actually has time varying noise on it. Perhaps at absolute zero, a perfect DC voltage could exist, but we cannot do a lot at absolute zero.

AC is particularly good at analyzing repetitive cyclic waveforms.  Working with 1KHz AC sinewave of 1V RMS is much easier then trying to do the calculations for a DC voltage that is varying according to the sine function of time. It would be totally possible to treat all waveforms as varying DC, but then you would find yourself spending all day staring at complex differential and integral equations, and saying "there has to be an easier way". But when the waveform does not allow the convenient shortcuts embedded in the concept of AC to be useful, you do have to returns to analyzing waveforms as varying DC voltages. The 4th image is exactly the type of waveform that may not benefit much from using the concept of AC.

In the case of a waveform that has  both a DC and AC component like your second image, it is usually easier analyzing or understanding it as a DC voltage with an AC voltage added to it. So in the first image, the green trace is pure AC. The other two traces are AC with a DC offset.

The 5th image could be a high frequency sine wave superimposed on a large low frequency waveform, but we cannot see enough to know. Based on what we can see, it would probably be easier considering that as a sine wave added to a decreasing DC voltage.

So to sum up, there is really only one kind of voltage and that is the instantaneous DC voltages at each instant of time.  AC is a concept to make it much easier to work with cyclically varying waveforms in a much simpler way then we could if we tried to think of them as varying DC voltages.  The concept of AC works wonderfully when you have capacitors, inductors and transformers. A capacitor cannot pass any unchanging DC, but it can pass AC waveforms, along with transients. Transformers can only output AC with no DC offset, and transformers cannot cope with any DC component on the input. The concept of AC is a tool available for us to use. Potential difference (volts) is a fundamental physical property of matter - it is DC volts. There is no different fundamental property of matter called AC potential. Rather AC volts is a convenient label we apply to certain types of varying DC voltages, and that really is all that it is.

So once you realize that it is your choice, you look at a waveform and you decide if it is easier to think of it as DC or AC or a mixture of both.

Richard.

[Pulse Cloud]

What an enlightening reply, Richard!

Thank you a lot.

[ee851]

AC is wiggling (unsteady).
DC is steady (not changing in time).   Only line.png shows DC.

[HLA-27b]

Here is my take

#1 - Sine.png     
Red    - Positive DC with some AC ripple riding on it. Alternately you could say AC with lots of DC offset.
Green - AC sine wave
Bloe   - Negative DC with some AC ripple riding on it

#2 - SineVert.png
Red    - AC with small DC offset

#3 - line.png
Red    - Positive DC
Green - Negative DC


#4 lolwut.png
Red    - Positive voltage with a lot of noise
Green - noise
Bloe   - Negative voltage with a lot of noise

#5 decr.png
Red    - AC with DC offset that decreases with time


#6 - freq.png
Red    - AC with decreasing frequency

[touchh]

Ha, I've been wondering this too for a long time. I finally came to the conclusion that AC signals are DC signals just varying in voltage, however there is one thing I never was able to understand, and that is polarity switching in AC.

Maybe saying polarity switching is the wrong word... But here in the US power is delivered at 110-120 volts 60 hertz, as I've always understood it, every 60 cycles the positive goes from 120 volts to zero volts (in a sine wave) and the negative goes to 120 volts (they reverse direction?). What type of AC is this? Is this how it works? How is this different than a sine wave generated by my function generator that simply goes from 0 to 5v every 60hz but never actually switches polarity.

Hope I asked this right - its a question that absolutely baffles me, and Google has thus far been no help!

[amspire]

The average voltage of an AC signal should be 0 volts - so the polarity has to switch.

Your generator is producing a sinewave with a DC offset. If you added a large capacitor in series with output of your generator, it would remove the DC component and it would become a true AC sinewave output, with the waveform centered around the zero volts axis.

[Pulse Cloud]

So, the attached image is pretty much the summary of this thread and the answer to "What really is AC and DC?". Right?

EDIT: Changed the Blue graph to a triangular wave so it doesn't look like only sine waves can be AC.

[Mechatrommer]

any wave can be represented with fourier series Acos(0)+Bcos(pi)+Ccos(2pi)+...+inf and so on something like that. the first term Acos(0)=A is the DC component, the rest are AC.

[HLA-27b]

any wave can be represented with fourier series Acos(0)+Bcos(pi)+Ccos(2pi)+...+inf and so on something like that. the first term Acos(0)=A is the DC component, the rest are AC.

In other words, any shape of wave can be represented as the sum of some number of sine waves added together.
Even DC can be represented mathematically as two sine waves exactly opposite to each other.
This is what they call Fourier Series in math.

Here is a nice explanation on how it works:


And here is the same principle with sound waves:

[Psi]

Keep in mind that when we look at AC we are doing so from a reference point, namely one of the connections that we are calling ground.

Picture in your head two objects next to each other which are both bobbing up and down opposite to each other.
(so when one is high the other is low)
This is what AC looks like.

But if you think of yourself standing on one of the objects and looking at the other one moving, it will seem to move over twice the vertical distance. It cycles between being above you by X units and then below you by X units

Which is exactly what it looks like on a scope.

[metalphreak]

Don't forget what AC and DC stand for It's all about the current.

DC = Direct Current = electron (or whatever charge carrier) flows in one direction

AC = Alternating Current = electron flows in both directions alternately


On your graphs, I would classify anything as a constant voltage (horizontal line) as DC, anything that has equal and alternating positive and negative parts as AC, and anything AC that's offset is AC+DC.

For anything that's not DC, but isn't equally alternating (not AC), is a signal (really they are all signals)

Really, I'd only classify or distinguish stuff as AC or DC when talking about power systems. For everything else it's all about signals (DC is boring, nothing happens in a DC world )

[DaMaDo]

Is the only reason they make AC from the mains gradually (sine wave) move between positive and negative to enable transformers to work through electromagnetic induction?

Are there other reasons it can't be a square wave? What's the purpose for the difference between the two?

When I started reading about electricity, AC was described as a necessary evil to get current to flow across far distances: Making the electrons wiggle back and forth it's a lot easier than moving them completely across the state. As long as they move, it's a current; doesn't matter which direction. Then once at the destination, it can be transformed, rectified, and dampened into a nice DC current.

Also, how is common decided in AC? From what I understand, in the US, the longer left hand socket is that common which sets the standard by which the right socket is considered positive or negative.

How does that common for AC differ from circuit common in a DC circuit? In building my power supply, I have an earth ground, a "floating" circuit common, and a negative. What's the difference between the circuit common (direct connection to one of the transformer secondaries) and the negative (coming out of one side of the bridge rectifier)? If this doesn't have to do with the difference between AC and DC, then I'm extra lost

Sorry for so many questions, reading this thread made me realize there are lots of holes I need to plug in my knowledge of electricity

[IanB]

@DaMaDo: You have a few misunderstandings there.

The reason the mains is a sine wave is fundamentally because generators are circular and go round and round. In geometry sine waves come from rotational motion. You couldn't make a generator produce a square wave if you tried. It is also a happy coincidence that a sine wave is the best shape of wave for electricity transmission, so you wouldn't want a square wave anyway.

Remember that "common" is a relative term. "Common to what?" you should ask. In AC power circuits there are live and neutral conductors. Neutral is neutral because at some point it has been connected to earth, thus neutralizing its potential. If you measure between neutral and earth you should find very little voltage, and thus very little hazard.

[DaMaDo]

Ah ok, I guess I thought some motors were run by a type of PWM speed control or something and figured it worked backwards too. But I guess if those exist they wouldn't be generators.

[IanB]

Ah ok, I guess I thought some motors were run by a type of PWM speed control or something and figured it worked backwards too. But I guess if those exist they wouldn't be generators.

Stepper motors operate on switched pulses using an active driver, but I don't believe that kind of motor operates as a generator when run in reverse.

Even though you might run a normal AC motor on square wave AC, this is not the most efficient power source. The motor internally tries to "round off" the sharp corners to approximate a sine wave.

To get a little more technical, a sine wave is the only shape where the current and voltage waves will have exactly the same shape when applied to a coil or capacitor* (though the current wave may lead or lag the voltage wave, it will still be a sine wave). All other shapes of wave like square or sawtooth will become distorted to some degree when you pass them through such a device.

* This is true of ideal inductors and capacitors. Real inductors and capacitors are imperfect and may not match the ideal behaviour.

[metalphreak]

We use AC for power because it can be easily stepped up to high voltage using transformers, sent across transmission lines, and then stepped down at the other end.

For power transmission, high voltage, low current, means lower losses across transmission lines.

[Mechatrommer]

We use AC for power because it can be easily stepped up to high voltage using transformers, sent across transmission lines, and then stepped down at the other end.

with today's modern technology isnt there any large scale DC-DC boost/buck converter or giant AC-DC rectifier available yet?

[IanB]

with today's modern technology isnt there any large scale DC-DC boost/buck converter or giant AC-DC rectifier available yet?

Actually, yes, there are. For some applications, such as undersea power cables or ultra-high voltage long distance transmission lines, DC is preferred to AC. So there are AC-DC and DC-AC converters in the power transmission domain.