A question about electrical ground (with diagrams!)

Ok, so I've been wrestling with the concept of ground for the past two weeks. I'm in an electrical engineering program at college, and let me preface this by saying I have no difficulty dealing with ground in a practical sense and understand it's general function very well. That being said, I've posed a few hypothetical scenarios about ground to a few of my professors and none of them can really give me a straight answer.

So the crux of the issue is whether ground acts as an infinite source/sink of electrons, or whether it acts as a return path for current. Based on my musings, the two seem to be mutually exclusive.

Take hypothetical scenario A. So we have a DC power source, let's say it's a battery, but it could really be any power supply, not necessarily a battery. If you were to hook one end of the supply up to ground, and the other end to a long wire up to the moon with a light bulb on the end, and grounded it on the moon, would current not flow? For the purpose of this hypothetical, let's assume the moon is at 0v, or earth ground potential. My thoughts are that it would work because electricity flows from a higher potential to a lower potential, but if electricity needs a complete circuit, then this would seem to be impossible. Here's a crappy little diagram:

Ok, so if scenario A doesn't work, than does scenario B work? In this case the power supply is using two points on earth ground to complete the circuit.

If neither of these two would work, than it seems like earth ground has absolutely no use, so one of them must work. If current doesn't in fact flow THROUGH ground, then ground must be an infinite source/sink for current. But if scenario A doesn't work because it needs a return path, than scenario B must work because it's using earth as a ground return.

If scenario B would work, then it would seem to follow that scenario C would not work, but it's my understanding that it would, because otherwise how would you ever get electrocuted by making a connection to ground?

General feedback has said the first and third options are incorrect, yet isn't the whole point of grounding in the first place to prevent shock hazard? If ground needs a return path in order to conduct, then what's the point of grounding circuits in the first place? It would seem like it only allows people to be electrocuted more easily since essentially the surface of the planet becomes a return point for any electric current, whereas without grounding at all electricity can never escape the circuit except to bridge to another point in the same circuit.

Furthermore, if the ground DOES act as a return path, how does it know where to travel to? If nearly all points connected to ground anywhere are at 0V, how does the current see a return path. Assuming both actual earth ground wires would measure 0V with reference to ground, so the potential difference between the respective grounding wires would be 0V, across some amount of resistance so no current would flow.

Someone clarify this for me if you can. I've run these past several people including two analog electronic instructors, an electrical physics instructor, several classmates and my Dad who knows quite a bit about electricity and everyone is giving me conflicting answers or is just plain confused by it altogether.

tldr: Does current travel through the actual ground or does the ground simply sink and source current when appropriate?

Autechre

Don't derail this, I want answers!

I like the drawings.

I'm not 100% following you, but surely the difference in electric potential causes current to flow so you'll always be electrocuted as in case C?

Well case C can't work because there is nothing supplying electrons to the negative wire. This is because the circuit is open so the only place electrons could come from on the other end of the generator is the air.

i think it's possible to test scenario B, don't try C

i think A is the same as B

you need a circuit for electricity to flow thru, no circuit,no flow

im not an electrician - i've circuit bent some keyboards... other than that i know nothing

  On 4/3/2012 at 11:19 PM, thehauntingsoul said:

Well case C can't work because there is nothing supplying electrons to the negative wire. This is because the circuit is open so the only place electrons could come from on the other end of the generator is the air.

if you have enough charge won't electricity flow thru air? lightning?

Yes but we'll assume that it's less voltage than that or touching the wire would be the least of your concerns.

  On 4/3/2012 at 11:23 PM, volg4 said:

you need a circuit for electricity to flow thru, no circuit,no flow

but A is not a complete circuit, rather a starting point and a destination. A circuit needs to arrive back at its starting point.

i think the one to concentrate on then is B, this is testable in the real world

2 nails, some wire, a battery and a bulb, what happens? i think you wont see any light from the bulb

why do you think the bulb would light in scenario A or B if ther is not circuit? if the earth and moon are both 0v then is there no potential difference?

i think you would break the bulb

  On 4/3/2012 at 11:39 PM, volg4 said:

i think the one to concentrate on then is B, this is testable in the real world

2 nails, some wire, a battery and a bulb, what happens? i think you wont see any light from the bulb

 

why do you think the bulb would light in scenario A or B if ther is not circuit? if the earth and moon are both 0v then is there no potential difference?

 

i think you would break the bulb

there's such a thing as ground potential. not all grounds are the same. 'true' ground is not necessarily the same as, say, the ground from an iron pipe in your house, because of static electrity and some other factors too. the difference between grounds is why you can get amp hum if your amp and your decks are grounded to different sources.

  On 4/3/2012 at 11:39 PM, volg4 said:

why do you think the bulb would light in scenario A or B if ther is not circuit? if the earth and moon are both 0v then is there no potential difference?

Because there is a potential difference, from the 50V power source to the 0V ground on the moon. The 0V on earth is supplying electrons for the negative terminal of the battery.

it's been a while, but though the earth is conductive, it's a very poor conductor, so by ohm's law (v=ir), you'd have a very low current, which would not illuminate the bulb.

even as i write that, i'm not sure if i have it correct.

edit: that doesn't seem right dammit.

2nd edit: the more i think about it, the more i think that it's not so much that it's a poor conductor, but that it's a huge conductor, with no simple path (of least resistance) for the current to take. so instead of following the conductor in its circuit, the current would disperse in all directions, with limited (or virtually no) current flowing through the circuit and the bulb.

anyone?

I suppose the issue isn't the amount of current that would flow but IF the current would flow. If it needs to be a complete circuit as many have told me, then it would seem that the current would not flow at all, but if that's the case what's the point of even using ground? It seems like grounding a circuit in the first place is a bad idea, because it turns earth into a huge conductor. Assuming scenario A doesn't work, if the circuits were all closed and not using ground as a return then you could not get electrocuted by making a ground connection.

I don't know but i think sink.

  On 4/4/2012 at 1:44 AM, thehauntingsoul said:

I suppose the issue isn't the amount of current that would flow but IF the current would flow. If it needs to be a complete circuit as many have told me, then it would seem that the current would not flow at all, but if that's the case what's the point of even using ground? It seems like grounding a circuit in the first place is a bad idea, because it turns earth into a huge conductor. Assuming scenario A doesn't work, if the circuits were all closed and not using ground as a return then you could not get electrocuted by making a ground connection.

at the core of the earth is a big massive ball of spinning liquid iron. a giant electromagnet.

Ok well suppose A doesn't work then.

In that case, what's the point of using ground as a return? It would seem to me that if the ground actually serves as a return path, and NOT an infinite source/sink of current, then grounding circuits just makes them more dangerous, because you're always standing on a potential return path so a single point of contact would create a fault and shock you.

If you have an entirely self contained circuit, it would seem to follow from this line of logic that the current could not escape the circuit when you touch a hot wire, even if you were grounded because ground would not be serving as a return since the initial power source is not grounded, so this is essentially my point in scenario C that you could not be electrocuted by this sort of set up unless you bridge the circuit in two places.

So why use ground at all when it would be safer just to use an actual return wire? You would have to make contact in two different places to be shocked, which seems far less likely.

Clearly there's something I'm not fully grasping here...

  On 4/4/2012 at 3:38 AM, thehauntingsoul said:

Ok well suppose A doesn't work then.

 

In that case, what's the point of using ground as a return? It would seem to me that if the ground actually serves as a return path, and NOT an infinite source/sink of current, then grounding circuits just makes them more dangerous, because you're always standing on a potential return path so a single point of contact would create a fault and shock you.

 

If you have an entirely self contained circuit, it would seem to follow from this line of logic that the current could not escape the circuit when you touch a hot wire, even if you were grounded because ground would not be serving as a return since the initial power source is not grounded, so this is essentially my point in scenario C that you could not be electrocuted by this sort of set up unless you bridge the circuit in two places.

 

So why use ground at all when it would be safer just to use an actual return wire? You would have to make contact in two different places to be shocked, which seems far less likely.

 

Clearly there's something I'm not fully grasping here...

I don't think a circuit needs to be already grounded in order to give you a shock. It's YOU that is providing the grounding when it shocks you, the current flows through you to the ground. I may be wrong but I'm pretty sure electrocution isn't the result of current flowing through the earth from one ground point to the other (you). I think it's different to closing a circuit.

Ah, here we go (from wikipedia):

  Quote

Electrical circuits may be connected to ground (earth) for several reasons. In mains powered equipment, exposed metal parts are connected to ground to prevent contact with a dangerous voltage if electrical insulation fails. Connections to ground limit the build-up of static electricity when handling flammable products or when repairing electronic devices. In some telegraph and power transmission circuits, the earth itself can be used as one conductor of the circuit, saving the cost of installing a separate return conductor.

For measurement purposes, the Earth serves as a (reasonably) constant potential reference against which other potentials can be measured. An electrical ground system should have an appropriate current-carrying capability in order to serve as an adequate zero-voltage reference level. In electronic circuit theory, a "ground" is usually idealized as an infinite source or sink for charge, which can absorb an unlimited amount of current without changing its potential. Where a real ground connection has a significant resistance, the approximation of zero potential is no longer valid. Stray voltages or earth potential rise effects will occur, which may create noise in signals or if large enough will produce an electric shock hazard.

I think the common ground is just an approximation of what the neutral potential is that the cases of devices, kitchen sinks, etc are supposed to have so that there is no potential difference or voltage between them that could be discharged suddenly causing shocks and harm to humans or devices. There is of course slight variation because the earth and the materials used in buildings, etc have some capacitance. And Earth is basically an infinite sink simply because it is so huge that you can't really change it's potential artificially by connecting it to any voltage source. And it's a very poor conductor.

Though there are some cases where you can use the Earths conductivity to form fucking big antennas (60km in length) for extremely low frequency communication but you have to drive it with huge power. Check this for example http://en.wikipedia.org/wiki/Communication_with_submarines#Extremely_low_frequency

Though after pondering the first image I'm a little bit intrigued now how far apart the Earth and Moon grounds actually are. I'm gonna go harass some physicists with this.

OK so these last three posts seem to indicate that ground is a sink/source and not a conductor.

So doesn't it follow that scenario A would work provided moon ground is a lower voltage than the power supply?

My problem here is if ground is used as a return path, why is it used in the first place. Conversely, if ground is a sink, why would scenario A not work?

What? Scenario A wouldn't work because DC flows from positive to negative (actually it's negative to positive, but let's not get into that), and requires a closed circuit to do so.

I'm not going to get into anything else about ground because it's been a while since I studied electronics, and I've forgotten a lot. But I do know that it doesn't quite work the way you seem to think it does.

tldr. Confusing doodles.

  On 4/4/2012 at 2:54 PM, thehauntingsoul said:

OK so these last three posts seem to indicate that ground is a sink/source and not a conductor.

 

So doesn't it follow that scenario A would work provided moon ground is a lower voltage than the power supply?

 

My problem here is if ground is used as a return path, why is it used in the first place. Conversely, if ground is a sink, why would scenario A not work?

I don't see why it would not work. For some time at least.

Take a lightning: the cloud is in different potential than the ground. It's not a complete circuit. Still the lightning is the current flowing between them because they are in hugely different potentials.

If you could tap a conductor with a bulb in between the cloud and the ground it would blow the bulb. The current would run until the cloud was in the ground potential, at that point the cloud's relative charge would have been depleted.

I also found out that during Solar Energetic Particle (SEP) events the potential difference between the day and the night side of the moon can be 4.5kV! That's because the day side gets bombarded with charged particles. So it's no way constant.

Also one physics guy I talked to said that the Earth ground is variable too and only way we could have a real common ground would be to cover the Earth with superconductive material.