# AC Voltage source terminal polarity and grounding

Ill try to make it as clear as possible. This may seem like a simple question, but it is something that I havent quite figured out. It has to do with polarity on the leads of an AC source. Every book I read, pictures an AC source with two leads such as this:

Now, on the books, I see that the + symbol is used on upper lead, and the - at the bottom when they are picturing the positive half of the waveform, and they use a - symbol on the top and + on the bottom when they are picturing the negative half of the waveform. So far so good.

Here are the 3 scenarios and the thing that really confuses me:

Is only one lead changing voltage while the other stays stationary at 0V, meaning that the upper lead will go from + to - and from - to + and so on while the lower lead stays at 0v?

Or

For the positive part of the waveform the upper terminal will be + and the lower 0V, and for the negative part of the waveform the lower terminal will be - and the upper 0v?

Or

The upper terminal will have an alternating current and the lower terminal will have a mirror signal with opposite polarity? in which case, where is my 0v reference?

Comparing it to the AC wiring on any house, theres 1 cable which is "Hot" and the other which is the "neutral", I know the neutral doesnt carry any voltage and its the 0v reference, so the HOT cable is the one which changes its polarity.

Now, what exactly happens when I ground something? for example what is the difference of having that 0v floating or connected to ground, and what happens when I ground a circuit at something else rather than 0v? For instance say I have the following circuit: An ac source feeding a transformer, there are a couple of resistors across its secondaries and the mid point between the resistors is connected to ground.

So let say that the transformer has a 1:1 ratio and both resistors have the same resistance, 100ohms for this example, that means they will form a voltage divider of half the total voltage.

I know all that because thats how they thaught it to me, what I dont know is this, if ground potential is 0V, and my transformer's (bottom lead) is at 0v, why will I get a voltage at point B with respect to ground?

I get very confused between the diference of 0v and ground reference, and when I see circuits like the previous one which are grounded at some place different from 0V, specially on AC circuits. On DC it makes sense, if I ground the negative side or 0V side, I will have a positive voltage, if I ground the positive, I will get a negative voltage. But on AC it confuses me the fact that I dont know if the only alternating current is on one side of the terminals or if the 2 terminals are switching between each other.

It seems like on some cases its only one side (like in a house electrical installation) and on others like in a balanced push pull circuit its both sides.

It seems I dont have a clue of how exactly AC works.

• I think im getting how AC works thanks to the comments below, what still remains to be cleared out is the fact of adding a ground. I will put more thought into it using the "bird on hanging cable" analogy given below, let me ask something, when I ground one side, is there any current flowing to ground? or its only used to tell the circuit "this is going to be the point at which everything will be compared to?" – S.s. Nov 19 '12 at 17:25
• Follow-up: So if I ground one terminal, ground being the planet itself, it basically means that that terminal and the planet will be at the same potential, so when the other terminal has more electrons in contrast to the planet it has a positive voltage (conventional), and when it has a lack of electrons it has a negative potential in contrast to the planet. Now the question, the terminal connected to the planet is also alternating? meaning its also injecting electrons to the planet (adding up to the total) when its + and sucking electrons (substracting electrons from the planet) when its -? – S.s. Nov 19 '12 at 18:12

The key is to realize that there is no such thing as 0 volts in an absolute sense. Voltage is a measurement of difference in potential between two points. You can say that something is 0V with respect to something else, but you can't say that a conductor is at 0V without including a reference.

Consider a bird sitting on a high voltage power line. The power line and bird are both at 13,800V relative to the ground, but the bird is at 0V relative to the wire.

To answer your question about AC, the AC source provides a potential difference (voltage) between the two conductors. It alternates in the sense that sometimes the first conductor is at the higher potential, and sometimes the second conductor is at the higher potential. "Higher potential" is an absolute concept; current will always flow from the higher potential to the lower potential if you were to connect them with a wire.

For a floating AC source like you've depicted (or a transformer), there's simply no concept of 0V until you define one. And you can define that any way you want: call the bottom terminal 0V, then you're in your first scenario. Or call whichever terminal has the lower potential at any instant 0V, and you're in your second scenario. Or define 0V as the "midpoint" when the top and bottom are at the same potential, and now you're in your third scenario.

When you use words like "stationary" you need to define what they're in reference to. Certainly the hot line does not stay "stationary" with respect to the neutral line, nor vice versa. In practice, it's common to use the earth ground as the reference point. In a house, it's common to physically connect neutral to the earth ground. Then, it's generally the case (modulo effects like resistance in wires) that the neutral will be at a potential of 0V relative to earth ground.

It sounds like you understand the concept of a floating DC supply, where there is no intrinsic reference to ground and you can choose to connect ground to either terminal. Extending this understanding to AC should be straightforward: consider that an AC supply is just like a DC supply where someone's constantly adjusting the knob, except that you can also bring the voltage negative (e.g. bring the black terminal to a higher potential than the red terminal).

• Thank you for your comments. Let me ask even further, is the top conductor the one changing its potential relative to the second, meaning at a given moment it is positive relative to the second conductor thus at higher potential, then it goes negative relative to the second conductor, hence making the second conductor be at higher potential, yet the second conductor is not "moving"? Sometimes I think of AC as if it were a circling DC voltage source like a spinning hourglass, is that the wrong way to see it? – S.s. Nov 19 '12 at 8:04
• Furthermore, heres a better analogy for your consideration, imagine im standing still, and a car is ahead of me, lets assume that means that its at higher potential than me, then the car moves towards my back, that means the car is behind me or at lower potential, hence making me at higher potential than the car, yet I never moved. So, back to AC, which conductor is the car, and which conductor is me? Or both the car and I are moving at the same time? – S.s. Nov 19 '12 at 8:12
• In your analogy: if you declare yourself to be ground, then the other terminal goes ahead and behind you. However, imagine the same situation on the deck of an aircraft carrier. Now you are moving relative to the sea (floating reference). Within the circuit, you can't tell and it doesn't matter. – pjc50 Nov 19 '12 at 9:47
• That helps up clear things up a bit. The way Ive always thought of a voltage source, is that one side has an excess of electrons and the other side has a lack of electrons, thus electrons flow to the other side. I have a problem visualazing that in AC. So in the positive half cycle side X will have an excess of electrons, side Y a lack of electrons, but on the negative it means Y will have an excess of electrons and X a lack, or it means Y still has a lack an electrons but X has an even bigger lack, thus making Y at higher potential? – S.s. Nov 19 '12 at 16:15

The + sign does not denote polarity as in DC, it is simply a convention to denote phase. This way you know that if you have two of them in series, they will either add or subtract depending on how they are wired. Assuming sinusoidal sources:

$$sin(t) + sin(t) = 2sin(t)$$

vs.

$$sin(t)-sin(t) = 0$$

So the + side merely means that that side will become positive at t=0, and half a period later it will become negative. In relative terms of course, since you can define voltage and time offsets, and different waveforms, but they are all adjustments to this reference.

Additionally, It has nothing to do with which side is constant while the other one goes positive or negative. In fact, it does not matter. You will not be able to come up with any circuit at all where this makes any difference, because all that matters is the difference in potential.

• I see what you are saying, still doesnt answer my question of what could be bottled down to: "which conductor is carrying the electrons and which one has the holes" – S.s. Nov 19 '12 at 8:09

As to which conductor is carrying the electrons and which one has the holes, both conductors alternately carry electrons. If you look at an AC waveform, you'll see the wave crosses a zero point, after which the electrons start flowing the other direction.