Is the Earth really electrically neutral (0.00000000V), or do we just say it is?

[JoeN]

For any normal circuit it wouldn't matter.  And how would we know anyway?  Voltages are all relative, right?  Might the Earth not be truly neutral - it has some net charge one way or the other?  Are all heavenly bodies neutral - or might there be a potential of difference between the Earth and the Moon or Venus?

Inquiring minds want to know.

[hamster_nz]

See... http://vixra.org/pdf/1008.0071v1.pdf

It is generally assumed, that the naked Earth bears a large negative electric
charge, Qs
, generating a vertical electric field at its surface. In the fair-weather area the
magnitude of this electric field is about – 100 V/m, corresponding to a charge Qs =
4??0 rs
2E = – 4.5×105 C (rs = 6371 km) at Earth’s surface (see, e.g., Uman [1]). However,
an almost equal amount of positive charge, is distributed throughout Earth’s nearest
atmosphere. In this study it is attempted to deduce Earth’s residual charge QE, up to an
altitude of about 70 km.

[/quote]

[Skimask]

 Monster will makes some test leads for that...

[JoeN]

Monster will makes some test leads for that...

I just bought a 1000' spool tonight.  How much more do I need?   

[Skimask]

Monster will makes some test leads for that...

I just bought a 1000' spool tonight.  How much more do I need?   

Best wait for the next indiegogo campaign.

That PDF linked above made for an interesting read though.

[T3sl4co1l]

Potentials in the positive MV to GV range occur (or are presumed to) up in space.  This corresponds to solar wind knocking away electrons preferentially.  Likewise, the Sun itself has a pretty high voltage (in terrestrial terms).  Neither matters much for overall matter or charge balance, because such a field pretty quickly overcomes the energy pushing it (i.e., MeV of particles in solar wind) and the vast majority of matter remains neutral.

An ion thruster works by ionizing a propellant gas (usually xenon) and accelerating it with as much voltage as possible (usually in the 100k to several meg range).  This would leave the craft negative, however, and the charged ions would eventually circle around and return (after travelling a pretty good distance I suspect).  Therefore, they also provide an electron gun, of similar voltage level and equal current flow, to shoot electrons into the ion wake and maintain neutrality.  Whether the gas ever deionizes, who knows, but as long as it remains neutral (plasma or gas), it's fine.

There is such a concept as potential at infinity, which corresponds to, taking the average over the rest of the universe.  This would be "ground" in the most fundamental sense.  In almost every other situation, however, "ground" is whatever is a convenient reference.  Sometimes it's the Earth's surface, sometimes it's a nearby metal plane or enclosure.  Sometimes it's the inside of an enclosure, which externally is at some massive potential against something else!

Voltage is only ever meaningful as a difference.

Tim

[jeroen79]

Isn't earth considered 'neutral' because it is a really big ball of matter that you can freely draw current from or into it without having a significant impact on it's charge or potential?

[John Coloccia]

It's the other way around.  Earth isn't considered neutral.  In US power distribution (not sure how it is elsewhere), the "Neutral" wire is simply the center tap on a transformer.  That's a crappy name for it...it's not "neutral" in any sense of the word.  When the wire gets to your house, it's attached to the Earth.  That references the secondary of the transformer to Earth, as opposed to simply floating.

So it's "neutral" that is considered to be at Earth potential, not the other way around

Earth is convenient to use because:

1) we're all attached to it, and ideally we're all sitting at Earth potential, whatever that is, so it makes sense to use it as our reference
2) It's really big and can happily supply and absorb however many electrons you wish, so it's a good place to dump fault currents

[Shock]

Dr Martin Uman (University of Florida)

"so there is constantly current flowing into the earth from thunder storms and flowing out where there is not thunder storms,  and in that process an electric field is generated in the atmosphere. We walk around the voltage between your head and toes is 200 or 300 volts and noone notices that because you grew up in it."

To the best of my knowledge..

Neutral is actually the return path of current flow in a circuit. In many countries the neutral side of mains is earth referenced to prevent dangerous DC charges from building up on floating mains lines from the effects generated in atmosphere and storms.

So the earth is earth (not neutral), confusing terms regarding neutral/ground/earth/chassis are often interchanged which makes it initially a little harder to grasp.

Negatively charged would be a better term to describe earth.

[Kjelt]

We walk around the voltage between your head and toes is 200 or 300 volts and noone notices that because you grew up in it."

My Fluke doesn't notice it either when I put it on ACV and hold the probes on the floor and the other one floating 2m in the air, what is he talking about?

[EEVblog]

My Fluke doesn't notice it either when I put it on ACV and hold the probes on the floor and the other one floating 2m in the air, what is he talking about?

You've got to stick your tongue on it.

[Mechatrommer]

We walk around the voltage between your head and toes is 200 or 300 volts and noone notices that because you grew up in it."

...when I put it on ACV...

there's your problem. it should be DCV. anyway i disagree there is 200V differential between head to toe.

[opty]

My Fluke doesn't notice it either when I put it on ACV and hold the probes on the floor and the other one floating 2m in the air, what is he talking about?

Possibilities:
1) 10Meg input Fluke might still be to low to measure the voltage?
2) He might be referring to situation when you are passing under power lines?
3) ...

btw. I can clearly see around 200VAC 50Hz on my body when I check on oscilloscope...

[tszaboo]

Voltage by definition is charge difference. Talking about the charge difference of one point is pointless. Please don't confuse people with US mains, it is an abomination by any standards.

[SL4P]

Dr Martin Uman (University of Florida)
"so there is constantly current flowing into the earth from thunder storms and flowing out where there is not thunder storms,  and in that process an electric field is generated in the atmosphere. We walk around the voltage between your head and toes is 200 or 300 volts and noone notices that because you grew up in it.".

That's a cool reference...
I remember seeing a documentary back in 70s or 80s explaining the problem for those tall transmission towers and skyscrapers - and lightning charged clouds...
Basically what I recall is that proportional to the density of the charge in the cloud mass, there is a 'shadow' of opposite charge that tracks across the ground below the clouds... when it meets a tall structure (or fool standing out in the open), the charge takes that opportunity to equalise.
Accompanied by a large flash, and burnt things!

So in that model, there is no universal level of charge that floats the whole planet at one time.  Just a lot of very large capacitors !

[Gyro]

My Fluke doesn't notice it either when I put it on ACV and hold the probes on the floor and the other one floating 2m in the air, what is he talking about?

There _is_ an atmospheric potential gradient, and it _is_ in the ballpark of 200V/m depending on the weather. The reason you can't see it - It's very high impedance of course. You can't just stick a meter probe of negligible contact area up in the air and expect to measure it with a typical DVM. It's static, and it's DC not AC.

To measure it you need a well insulated plate or wire grid of significant area and measure it with an electrometer. There have been several project published over the year. Meteorologists studying atmospheric electricity do it all the time, one experiment (I think, in the French Alps) has a wire strung across a valley between two peaks.

It's common for radio Hams to have a shorting switch and/or discharge tubes to ground on their antennas

In thundery weather the potential gradient rises to some major levels until...

Now if you want AC then look up Schumann resonance, an ELF resonance excited by lightning discharges around the globe.

[TimFox]

When I was in high school, I visited the local university physics department and the professor had an experiment in progress to measure the electrostatic field in the atmosphere at different locations.  The apparatus was a pair of electrodes with one rotating rapidly.  If I remember correctly, the top electrode was three 60-degree segments with 60-degree gaps, and the lower electrode had six 60-degree segments with short gaps.  In order to get a huge input impedance on the amplifier for the AC voltage produced by this sensor, he used an acorn triode (955) with the input connected between the plate and cathode and the output from the grid-cathode circuit.  He showed me how to clean the glass carefully to avoid parasitic resistance.

[rolycat]

There is a (naturally) brilliant discussion of atmospheric electricity in chapter 9 of the second volume of The Feynman Lectures on Physics.

These are rather wonderfully now available for everyone to read free of charge, courtesy of Caltech.

[Gyro]

These are rather wonderfully now available for everyone to read free of charge, courtesy of Caltech

Wow, good find, thanks!

Chapter 9 covers it all with the clarity that we mere mortals could only dream of 


P.S. Just realized I've actually got the book he references sitting on the bookshelf (Chalmers, 1957, Pergamon Press 63/- net). What are the chances 

[AG6QR]

Voltage by definition is charge difference. Talking about the charge difference of one point is pointless.

That's one common practical definition, and from that, it seems there would be no absolute reference for zero volts.  Voltage would always be relative.  You'd only be able to measure voltages between two points.  All voltmeters would need two leads.

But an electroscope shows that, in fact, there IS such a thing as an absolute reference for zero volts, even though it may be hard to measure precisely.  A gold leaf electroscope is a crude sort of voltmeter, with only one terminal.

https://en.wikipedia.org/wiki/Electroscope

The two leaves of a gold leaf electroscope will separate if there is a positive voltage on them, and they'll also separate if there is a negative voltage on them.  Since they're conductive, they'll each have the same voltage, and since like charges repel, if there is a non-zero net charge on them, they will repel.  The two leaves will have no repulsive force between them only if they've got zero volts on them.

Zero volts would be when the body as a whole has no net charge, that is, the same number of protons as electrons.

In practice, an electroscope can't measure small voltages.  It requires voltages in the range of thousands of volts to produce a visible indication.  It's much easier to precisely measure relative voltages than absolute voltages, and for almost all electronics purposes, the only thing that matters is relative voltage.

But sometimes it's worth remembering that there's a bit more to the story.

[Marco]

For any normal circuit it wouldn't matter.  And how would we know anyway?

Measuring the force between two earthed metal plates in vacuum?

[Gyro]

The two leaves of a gold leaf electroscope will separate if there is a positive voltage on them, and they'll also separate if there is a negative voltage on them.  Since they're conductive, they'll each have the same voltage, and since like charges repel, if there is a non-zero net charge on them, they will repel.  The two leaves will have no repulsive force between them only if they've got zero volts on them.

That's a tricky one. I agree 100% with the premise, they will only deflect when there is a net charge, positive or negative, but surely it must be relative to something - such as the outer case, assuming there is one? or it's surroundings.

To take an example - take a discharged electroscope, put it in a faraday cage (mesh so you can see through it) at the same potential and observe that it still shows no deflection. Now raise the Faraday cage to several kV (enough to ensure that the electroscope can reliably detect). i suspect that the electroscope will remain flat despite it's absolute potential (or potential relative to Earth) having changed.

Not sure how that works.

[Sigmoid]

So... if I understand correctly, if I set up an iron pole that reaches from ground level to 4 meters above, then what I have is a voltage source of 800V connected across the pole, with an extremely high internal resistance?

[John Coloccia]

Think of it like this...if you put 1000V on one end of a 4m piece of glass, and ground the other end, and then start probing around, what might you expect to measure?  Air is a much worse conductor than glass.  There's a voltage difference for sure, but there aren't any charge carriers available to do anything useful.

[rolycat]

So... if I understand correctly, if I set up an iron pole that reaches from ground level to 4 meters above, then what I have is a voltage source of 800V connected across the pole, with an extremely high internal resistance?

No. Why would an iron pole have an extremely high internal resistance?

Take a look at the Feynman reference given earlier for a lucid explanation of potential distribution.