Why the earth is zero potential?

In summary, the concept of zero potential is relative and chosen for convenience in electrical systems. Ground is often used as the zero-point due to its relatively stable reference, but it may not be the same potential everywhere. This choice of zero potential allows for the calculation of potential differences, which are what truly matter in electrical systems. In rare cases, the potential of the Earth may change locally due to large amounts of charge, causing issues for electricians. However, the Earth is still a convenient choice for the zero-point.
  • #1
Physicsissuef
908
0
Why the Earth is zero potential?
 
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  • #2
In what context do you ask this? Are you asking why AC power systems use the local ground potential as the zero-point?
 
  • #3
turbo-1 said:
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.
 
  • #4
For convenience.

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..
 
  • #5
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?)
 
  • #6
His, usually.
 
  • #7
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.
 
  • #8
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 let's say 1volts e. potential, Cu 0.3 volts, or whatever. Sometimes voltage is defined as electron's pressure.
 
  • #9
arildno said:
His, usually.
:tongue:
 
  • #10
Physicsissuef said:
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 let's 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.
 
  • #11
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?
 
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  • #12
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.
 
  • #13
turbo-1 said:
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?
 
  • #14
If ground is not zero potential, then the potential of R will be higher, or the potential difference between them will be smaller.

arildno said:
For convenience.
belliott4488 said:
Electrical potential is defined only in terms of differences
belliott4488 said:
Once again, potential is not absolute, it's relative.
turbo-1 said:
ground is CHOSEN as the zero-point

It's a matter of choice.
 
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  • #15
Physicsissuef said:
If the ground is not zero potential, why when I connect R and 0, the R will burn out?

The power company has designed Earth ground to be the zero, you are forced to follow the same convention.
 
  • #16
Physicsissuef said:
If the ground is not zero potential, why when I connect R and 0, the R will burn out?
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.
 
  • #17
NoTime said:
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?
 
  • #18
No.
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.
 
  • #19
NoTime said:
No.
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.
 
  • #20
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?
 
  • #21
NoTime said:
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.
 
  • #22
Physicsissuef said:
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.
 
  • #23
NoTime said:
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?
 
  • #24
Physicsissuef said:
Ok, I checked my dictionary and understand what do you mean by the efficiency. So what next?

Translation problem?
 
  • #25
Yes. Can you give me the end of your analogy?
 
  • #26
As far as I am aware the Earth has two potential fields: 1. Gravitational, 2. Magnetic.

The properties of both these fields vary as a function of latitude, elevation, local crustal geology, deep Earth activity (eg happenings in the outer core), and extra-planetary activity (many of which things vary with time).

Saying the Earth has zero potential can be quite convenient in certain applications, although, as already mentioned, it is completely arbitrary. Fortunately for geophysicsts (like me), this arbitrariness is of no consequence, as it is the gradients of these potential fields which are of interest.
 
  • #27
Physicsissuef said:
Yes. Can you give me the end of your analogy?

If each cubic cm of dirt has a Resistance of 100 ohms what is the Resistance of the number contacting 10 meters of ground plane?
Consider that this situation has 3d geometry so each row out connects with additional parallel chunks of dirt.
 
  • #28
NoTime said:
If each cubic cm of dirt has a Resistance of 100 ohms what is the Resistance of the number contacting 10 meters of ground plane?
Consider that this situation has 3d geometry so each row out connects with additional parallel chunks of dirt.
Do you want to say, that the Earth have no resistance?
 
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  • #29
Physicsissuef,

Please make an effort to ask a complete question. Your drawing is nearly indeciferable, could you make an effort to make a clear drawing and ask a specific question about your difficulty.

I will further ask that other members refrain from responding until we have a clarification of the question.

Have you read my post #15?
 
  • #30
Integral said:
Physicsissuef,

Please make an effort to ask a complete question. Your drawing is nearly indeciferable, could you make an effort to make a clear drawing and ask a specific question about your difficulty.

I will further ask that other members refrain from responding until we have a clarification of the question.

Have you read my post #15?
Yes, I read it, but can't understand how they managed to get zero. You say that the Earth potential can be manipulated?
 
  • #31
NoTime, Integral?
NoTime: You want to say that the Earth have no resistance to current?
Integral: You say that everyone can manipulate with the potential of the earth? And If so how?
 
  • #32
PLEASE STATE YOUR QUESTION IN A CLEAR AND CONCISE MANNER>


The power company sets Earth ground as a reference point. They do NOT manipulate it they call it zero and measure the produced voltage with respect to it. Your home wiring is the same.
 
  • #33
I would just like to point out that although most people correctly state that the Earth is assigned a zero potential for convenience, there are real consequences.

Electrical systems of both ships at sea and spacecraft require special considerations. Working on electrical systems on a metal ship that is in intimate contact with ground requires care, and maintaining electrical 'coherence' between the space station and the space shuttle as the shuttle moves from the Earth into orbit is not trivial.
 
  • #34
Why Zero, Simply because its under our feet and its convenient. The Earth has a magnetic field this effects Electrical equipment and conductors around us, but as how to define it as Zero simply Joining to Disimular Conductors together makes them Equal or Zero potential. Same theory as Farendays Cage, you're in the potential Zone so you will not get an electric shock. Step out of this zero potential and your toast. Electricity wants a simple way to Earth and by Earth we mean Zero, become that Zero and it will simply ignore you. Correct me if I am wrong but that's generally how it works.
 
  • #35
Daz87uk said:
Why Zero, Simply because its under our feet and its convenient. The Earth has a magnetic field this effects Electrical equipment and conductors around us, but as how to define it as Zero simply Joining to Disimular Conductors together makes them Equal or Zero potential. Same theory as Farendays Cage, you're in the potential Zone so you will not get an electric shock. Step out of this zero potential and your toast. Electricity wants a simple way to Earth and by Earth we mean Zero, become that Zero and it will simply ignore you. Correct me if I am wrong but that's generally how it works.

also Earth is not in itself Zero it has Resistance it can be used as a conductor. but by wiring things so "earth wires" go to ground (Soil) our electrical conductors equilize with the Ground creating a Zero potential or a place where live connections can fault and go directly to Earth instead of the unlucky guys hand. or basically it just wants to return with little resistance as possiable. (dont get in its way lol)

sorry this is the best i can explain as I am still learning Electrical Theory myself. :)
 

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