Why the earth is zero potential?
In what context do you ask this? Are you asking why AC power systems use the local ground potential as the zero-point?
Why the earth is zero potential (0 V)? In electrical point.
It is potential differences that matter, not potential values.
However, one might object to that and wonder:
Why should the Earth's potential be constant?
The short answer to that is that any addition of electric charge down there won't usually affect it (the ground is very large).
However, in extremely rare cases, like massive dumps of charge, the Earth potential might change somewhat on a local scale. And that is a bother for elictricians..
Electrical potential is defined only in terms of differences, that is, the potential difference between two points can be calculated, but there is no absolute definition of the electrical potential at one point (this is because the physically observable field, the electric field, is given by a derivative of the potential, so only differences are physical).
The Earth is often chosen to be the zero point for convenience, much like choosing the origin of a coordinate system. It is not the only possible choice, and in fact the potential is often to be zero at r = infinity, so that the potential at any distance closer to a source charge at r = 0 will be negative.
EDIT: Sorry, I posted this response a few milliseconds after arildno posted his. (hers?)
Earth-grounding is a means of establishing a reasonably-consistent zero point for the transmission of AC power. Please note that ground potential is not the same everywhere, and can swing widely with lightning strikes nearby, more slowly with magnetic storms, etc, and there may be other events that can induce currents in the ground so that ground potential is changed. Dairy farms are often afflicted with stray voltage problems that shock cows and make them nervous and unproductive. Imagine that you're a cow that is going to be milked, and as you are led into the milking stall, your wet nose touches the pipe-frame of the stall and you get a nice shock! Such problems can be caused by these differentials in ground potential. They are generally addressed by improving the ground-reference integrity of the neutral line to local ground, and may require that the primary and secondary neutral/ground reference at the transformer be separated. Ground zero-point is not the same everywhere.
You can also have AC power that is not referenced to earth ground and has a zero-point that is not at ground potential.
Edit: This is a REALLY responsive forum. I got claim-jumped by two other members before I could compose what I hope was a cogent explanation.
It is truly unimaginable. I can't understand how something is zero potential or whatever. It must have some property which makes the ground zero potential. And how then the Fe have lets say 1volts e. potential, Cu 0.3 volts, or whatever. Sometimes voltage is defined as electron's pressure.
Once again, potential is not absolute, it's relative. You can pick any point and call it "zero" and then measure all other voltages in reference to that point. The Earth is a convenient choice, since it does not change for most practical purposes.
There is no reason to say that one material (e.g. Fe) has a different potential than another (Cu). You can charge up any conductive material to whatever voltage you like (within reason), if you have means to do so, e.g. a generator.
If the ground if doesn't have 0 potential, do you know what will happen, if I connect R with 0? Look on this http://img106.imageshack.us/img106/2797/68664788cd4.jpg" [Broken]The whole system will burn out. So it means, that the ground is zero potential, in the most of the cases, but the question is why?
In an AC power system, ground is CHOSEN as the zero-point because it is a relatively stable reference. Ground may not be at the same electrical potential at the generating station and at a step-down transformer outside your house, but it is close enough for purposes of commercial AC transmission. Ground is not some magical zero-potential electrical state - it is a relatively stable reference that is accessible everywhere, so the power companies exploit that.
If the ground is not zero potential, why when I connect R and 0, the R will burn out?
If ground is not zero potential, then the potential of R will be higher, or the potential difference between them will be smaller.
It's a matter of choice.
The power company has designed earth ground to be the zero, you are forced to follow the same convention.
You might consider that the resistance of ground (dirt if you will) approaches zero when the current spreads out. For most intents and purposes it is works just like any other conductor.
Think of ground as a piece of wire.
If I get piece of ground and put it in pan tile, and than make electric circuit, will it work as same as, if I make electric circuit with the ground?
Do you understand the concept of parallel resistance?
The ground works like that.
This is why ground planes tend to involve long pieces of metal conductor buried in the ground.
I don't know what do you mean. Please explain. Thanks.
If you have two 100 ohm resistors in parallel then the effective resistance is 50 ohms.
If you have four 100 ohm resistors in parallel then the effective resistance is 25 ohms.
Do you understand that?
I'm not sure.
This is a basic principal in electricity.
Perhaps one of your instructors can help you select a book, in your language, that will help you understand.
Ok, I checked my dictionary and understand what do you mean by the efficiency. So what next?
Yes. Can you give me the end of your analogy?
Separate names with a comma.