hey menticol

u said AC changes polarity. but the wall outlet always gives LIVE at one wire and ground at the other... where does the polarity change happen? and how does it affect us?

 

If AC is floating you can imagine that line A and line B repeatedly have A and B go above and below each other (sometimes A>B, other times B>A).

If B, for example, is tied to a ground, then what results is that you can still describe them using the above description (which is to describe their polarities relative to each other). But now you have an additional description you can use: you can describe their polarities relative to ground. If B, is tied to ground for example, you can say A goes above and below ground (or B since they are the same in this case). The first description still holds true because you can actually say that from B's polarity goes above and below A's polarity (if you observe B's polarity using A as a reference).

It's just like relative motion and reference frames!

 

Household AC polarity changes from +120 to -120, taking the neutral wire s a reference. The AA battery was a "For dummies" example, didn't wanted to confuss you!

For dknguyen, the spanish joke: El que sabe, sabe! (Who know... knows!) Good explanation

__________________
Felipe

 

If you ran AC into a rectifier to turn the AC into DC, you could actually call either line AC line DC ground. It would be okay as long as your circuit is "isolated" and does not plug into something else that actually has the true AC neutral as it's neutral. As soon as that happens...

BTW. AC has 3 plugs. THe two "eyes" are hot and NEUTRAL (very often mistakenly called ground). ANd the "mouth" is actually ground. Neutral carries the return current, in a perfect world ground is supposed to be at the same voltage as neutral (don't stick your finger in to test it because this isn't a perfect world), and ground only carries fault currents like short-circuit currents. So really, most of the time when we are saying ground in any circuit (AC or DC) we should be saying neutral.

THe difference? Something like the metal outside of your toaster might be connected to ground while the current actually powering the toaster flows between hot and neutral. THe intent is that if a neutral or hot wire goes loose inside and touches the outside of the toaster and you touch it, the current (mainly from the hot line) won't flow through the metal case, through you, and into the ground. It will flow through the ground line which is a lower resistance to the ground than you are and save your life (and also helps for detecting faults in your house since it should normally be zero current in the ground line). You can't just connect the metal case to neutral to do the same thing (well you can in a perfect world) because current is actually flowing through here during normal operation and even though it's "supposed" to be at the same voltage as ground, this often makes it not so and the neutral could actually be at a dangerously high voltage.

You could almost imagine it like neutral is the voltage of the physical ground of the power plant (where is staked into as a reference)) far far far away from you and is the voltage your live line is referenced to. And that the ground in your plug is the voltage of the ground right at your house. They are not necessarily the same. Atmosphereic charge build-up on the long transmissions lines, the fact that it carries current over a long distance, and the large distance in the soil between the power plant has the potential to make the neutral be dangerously high voltage relative to the ground you are standing on at your house.

Yeah, sometimes I wish I could quickly doodle out quick little animations with a pencil and post the animation on here.

 

oops i should have said One wire is LIVE and the other NEUTRAL... sorry there....

Third we say Earthing or Ground

 

We do that about 99.99999% of the time here. THe only time we ever say neutral is when we are talking about the difference between neutral and ground on the AC mains...that is, if someone even bothers to point it out.