AC confuses me.

[Hextejas]

Please excuse a complete noob question.

I am doing an exercise on a breadboard that has Vin as 9v AC.
My wall wart takes the 120 down to 9v and I have a plug that lets me plug it into the breadboard.
The breadboard of course has the + and - rails running down the side.
So, since this is AC, does it have a + and - or does it make any difference ?

Thanks and please excuse if a stupid question.

[james_s]

The + and - are simply labels for your convenience, when working with AC they are irrelevant. There is no difference internally in the breadboard, it's just like the color of wire insulation.

[paulca]

Yes, AC is confusing.  It all gets into trigonometry to explain simple things like current, voltage, power.

If you have two wires coming from the power supply I expect one of them is a "reference" voltage and the other will alternate around it.  My maths brain doesn't work these days, I expect it doesn't really matter which is which.  They will go +9V potential between them at some point and -9V potential at another.  So if you reverse them there is no difference.  They will alternate between each other at the frequency, probably 50 or 60Hz in your case.

I hate AC.

[james_s]

I don't understand what is confusing about AC, you don't need any fancy math to understand it on a basic fundamental level. Just look at how a simple AC generator works, a magnet rotating on an axis with the poles passing by a coil, the voltage will ramp up in one direction, ramp down to zero and then ramp up in the other direction as the other pole passes the coil then repeat.

[retrolefty]

  I can get why AC voltage understanding can be confusing for beginners to get their head around for some. Even just stating 9 vac, one has to understand the different ways and AC source voltage can be stated, RMS value, peak value, peak to peak value, average value.

 When stating a source is 9 vac one has to understand that it's assume to be the AC RMS value unless specified specifically. And after all that one learns that these value relationships depend on the AC wave shape and if other then a pure sine wave source the voltage values relationships differ.

 There is a reason that electronic fundamentals courses start off with DC fundamentals before AC fundamentals. 

 

[TimNJ]

Here's one way to look at it.

AC is alternating current. And while you need a voltage to drive that current, let's first look at it as its name implies. In AC, the direction of current changes with respect to time. That means electrons might flow from left to right in a wire for a second, and then from right to left for a second, and then left to right, and on and on. How quickly the current changes direction is called frequency.

Lets say we attach a load (a resistor for instance) up to an AC source. (Look at my attached drawings.) If the direction of current is downwards through the resistor, then the voltage on the white wire must be positive with respect to the black wire. This is because current flows from high potential (voltage) to low potential. Similarly, if the direction of current is upwards through the resistor, then the voltage on the black wire must be positive with respect to the white wire. This is exactly the same as saying the voltage of the white wire is negative with respect to the black wire.

That kind of introduces the ideas of a reference node, or "ground", or "0V" or "-". If we make all of our measurements with respect to the black wire, then the voltage on the white wire will alternate between positive and negative values. This is because we lock down the black wire as the 0V reference point of the circuit. All other measurements in the circuit must be referenced to it (i.e. put the black lead of your multimeter on the reference node and make all voltage measurements with respect to it.)

Maybe that's all very confusing, still..

So to answer your question, no, AC does not have a + or -. You can get really philosophical when you start talking about the existence of "ground" or "-" (or whatever). But for your purposes, if you hooked up an AC voltage to the + and - rails on a breadboard, then 50% of the time the markings on the breadboard are going to be correct (when current flows out of the + terminal and back into the - terminal). But the other 50% of the time, current will flow from - to +, which is the opposite of what the markings suggest.

[Zero999]

Please excuse a complete noob question.

I am doing an exercise on a breadboard that has Vin as 9v AC.
My wall wart takes the 120 down to 9v and I have a plug that lets me plug it into the breadboard.
The breadboard of course has the + and - rails running down the side.
So, since this is AC, does it have a + and - or does it make any difference ?

Thanks and please excuse if a stupid question.

With single phase AC (two wires) it doesn't matter which way it's connected. To make it easier to distinguish between the two wires, call one side 0V or neutral and the other side phase.

If this were the mains or a control panel, one side will be connected to earth (neutral) and the other side phase, but the output from your transformer will be floating, so it's arbitrary.

[Hextejas]

AC is alternating current. And while you need a voltage to drive that current, let's first look at it as its name implies. In AC, the direction of current changes with respect to time. That means electrons might flow from left to right in a wire for a second, and then from right to left for a second, and then left to right, and on and on. How quickly the current changes direction is called frequency.
/quote]
Maybe I shouldn't say this but that statement doesn't compute. What happens, or how can it be so if your length of wire is infinitely long. It seems that you would need to have a current source on both ends and i dont think that that is the answer ?
Probably it's beyond my level of comprehension.

[TimNJ]

AC is alternating current. And while you need a voltage to drive that current, let's first look at it as its name implies. In AC, the direction of current changes with respect to time. That means electrons might flow from left to right in a wire for a second, and then from right to left for a second, and then left to right, and on and on. How quickly the current changes direction is called frequency.
/quote]

Maybe I shouldn't say this but that statement doesn't compute. What happens, or how can it be so if your length of wire is infinitely long. It seems that you would need to have a current source on both ends and i dont think that that is the answer ?
Probably it's beyond my level of comprehension.

I know no one likes getting this answer, but I suggest taking some time to digest the fundamentals of DC (first) and then (I think) AC will make sense based off your understanding of DC.

Here's a video I made a few years ago:



To help understand your question, can you explain why you think you might need a current source on "both ends"? What do you mean by "both ends"? Remember, a circuit is a complete loop/path (just like a auto race, bicycle race, etc.) An infinitely long real wire (if one existed) would have an infinite resistance so it wouldn't do you very good. An infinitely long ideal wire has no resistance, and therefore a current source or voltage source theoretically would have no problem driving some sort of load very far away.

[IanMacdonald]

Look, you humans think it's confusing? You should try being IN one of these wires..