Understanding Electrical Ground & How It Works

[Mårten]

I do not understand the concept of electrical ground, or why it works. Let me explain.

In elementary school physics, one learns that for a current to take place, you need a battery with positive and negative poles. This creates a voltage that makes charges run from plus to minus, because pluses would like to be with minuses. So therefore, if you connect wires and a lamp, you can make the lamp shine.

This is how I learned that an electric current arise. So I cannot understand, how there could be a current from, for instance the plus pole of the battery to the Earth - there are no minuses in the Earth that would attract the pluses, the Earth is neutral (as opposed to the minus pole on the battery that is negative).

 

[chroot]

Voltage is a purely relative concept. A 9V battery, for example, is called a 9V battery because it produces a potential difference of 9V between its terminals. You are free to refer to the terminal voltages individually as, say, 3V and 12V, or 19V and 28V.

The term "ground" is used to denote a reference potential, and all other voltages in the circuit will be measured with respect to it.

In many systems, the potential of the Earth itself is a reasonable reference potential, since we can all agree upon it (it's under all our feet). However, in many electrical systems -- those powered by batteries, for example -- the "ground" potential has nothing to do with the actual Earth at all.

- Warren

 

[turbo]

Ground is useful for both AC and DC devices. When I was a kid there was a telegraph line from town to a remote logging camp and IIR, there was just a single copper wire -the return path was ground. In a more complicated DC circuit, it is often useful to reference parts of the circuit to ground. In an AC (household) circuit, the neutral leg is referenced to ground, and the two 120V hot legs vary with respect to that ground reference. The step-down transformer that feeds your house takes transmission-line voltage and steps it down to 240V. The secondary coil is center-tapped, and this is where the neutral line to your entrance panel originates. At the electrical entrance to your house there will be a local ground driven into the ground or clamped to incoming water pipes etc, and a heavy ground wire is run from this ground to the entrance panel and is clamped to the same buss as the incoming neutral wire.

Edit: Warren jumped in ahead of me. He"s quick.

 

[tiny-tim]

… This creates a voltage that makes charges run from plus to minus, because pluses would like to be with minuses
…
So I cannot understand, how there could be a current from, for instance the plus pole of the battery to the Earth - there are no minuses in the Earth that would attract the pluses, the Earth is neutral (as opposed to the minus pole on the battery that is negative).

Hi Mårten!

"pluses would like to be with minuses" … no … you're imagining that there are positive charges at one end of the battery, and that they are attracted to negative charges at the other end.

All the charges are negative, aren't they? They're all electrons!

The electrons drift "downstream" along the potential difference.

Electrons don't need a "positive" plate to be attracted to … they only need to feel a greater pull in one direction than the other (which is what a voltage gradient is).

So between a plus pole of a battery and the earth, there is a potential difference (though only roughly half that from the plus pole to the minus pole).

 

[Mårten]

Hi Mårten!

"pluses would like to be with minuses" … no … you're imagining that there are positive charges at one end of the battery, and that they are attracted to negative charges at the other end.

All the charges are negative, aren't they? They're all electrons!

The electrons drift "downstream" along the potential difference.

Hi tiny-tim and you others!

I thought it was an old convention to go from plus to minus, and that it didn't matter, even though it really is electrons that runs...

So between a plus pole of a battery and the earth, there is a potential difference (though only roughly half that from the plus pole to the minus pole).

Okey. So I've now done some experiments. I have three 1.5 V batteries in series (=4.5 V), a wire connected to the minus terminal on the last battery, and the other end of the wire connected to the metal on the side of a small 3 W bicycle lamp. I connect the bottom of the lamp to the plus terminal of the first battery, and the lamp is shining nicely and brightly. (I've also tried the lamp with just one battery (1.5 V), and it shines okey, not much, but it shines clearly). Now I try the following:

1) I connect the bottom of the lamp to a big metal shelf here. Since there should be a potential of 2.25 V on the minus terminal of the last battery in the series, and the shelf should have 0 V potential, there ought to be a current - but no light. Why?

2) (Not recommended) I connect the bottom of the lamp to the grounding pin in my AC outlet in the wall (it's a new house, so I know the grounding is properly installed here). But, still no light. Why?

 

[chroot]

1) I connect the bottom of the lamp to a big metal shelf here. Since there should be a potential of 2.25 V on the minus terminal of the last battery in the series, and the shelf should have 0 V potential, there ought to be a current - but no light. Why?

Because there is no continuous path for current to flow from one terminal of your battery pack to the other. Also, there's no reason why the shelf should be considered to have "0V potential." As I've said, voltage is a purely relative quantity. You cannot say "an object has a potential of 0V," only that "an object has a potential of 0V with respect to some other object."

2) (Not recommended) I connect the bottom of the lamp to the grounding pin in my AC outlet in the wall (it's a new house, so I know the grounding is properly installed here). But, still no light. Why?

Again, because there's no continuous path for the current to flow.

- Warren

 

[atyy]

I do not understand the concept of electrical ground, or why it works. Let me explain.

In elementary school physics, one learns that for a current to take place, you need a battery with positive and negative poles. This creates a voltage that makes charges run from plus to minus, because pluses would like to be with minuses. So therefore, if you connect wires and a lamp, you can make the lamp shine.

This is how I learned that an electric current arise. So I cannot understand, how there could be a current from, for instance the plus pole of the battery to the Earth - there are no minuses in the Earth that would attract the pluses, the Earth is neutral (as opposed to the minus pole on the battery that is negative).

Let's think of a slightly simpler situation, dealing with a capacitor, instead of a battery.

A capacitor is just two metal plates separated by air.

If you put 10 positive charges on one plate, and 10 negative charges on the other, they will stay there, because the charges cannot flow through the air. If you now connect the plates with a metal wire, then the 10 positive charges will flow to the other plate and neutralize the 10 negative charges, exactly as you conceived.

Now what happens if you put 10 positive charges on plate 1, and zero negative charges on plate 2? If you don't connect the plates, nothing will happen. If you connect the plates, will the positive charge flow from plate 1 to plate 2? Yes - you don't need negative charge on plate 2 for the charge on plate 1 to flow. Positive charges like to get as far from each other as possible. So if you give them a way to distribute themselves 5 on each plate, they will try to do that. Thus 5 charges will flow from plate 1 to plate 2 once you connect the plates with metal wire. The first charge to go will get to plate 2 quickly. The second charge to get there will take a little longer, because the first charge that just got to plate 2 will push him back a bit, but he will still get there, because the 8 remaining charges on plate 1 are pushing him away much harder than the 1 charge on plate 2. You can make up the rest of the story yourself.

The main point about the Earth is not that its potential is zero. Potential itself is meaningless. If you have a 9 V battery, you can call the negative terminal 10987 V, in which case the positive terminal will be 10987+9 V, and that will be completely correct; you can also call the negative terminal -926.765 V, in which case the positive terminal will be -926.765+9 V, and that will also be completely correct. Earth is usually given a potential of 0 V, but that's convention, it contains no physics, it is like deciding whether to call your negative terminal 10987 V or -926.765 V.

The important part about Earth is that it is like a very, very, very, very, very big plate with zero charge. So if you connect plate 1 with 10 charges to earth, the first charge to go will go quickly, just like if you connect it to plate 2. Unlike when you connect it to plate 2, the second charge to go will not take a longer time to reach the earth, because the first charge has lots of room on the earth, so much room that it will not push back on the second charge. In fact, there is so much room on the earth, that all 10 charges will be gone from plate 1 to the Earth - quite unlike having 5 charges remaining on plate 1 when you connect it to plate 2. Furthermore, there is so much room on the earth, that although there are 10 more charges on the earth, we can still pretend that the Earth has zero charge. That is the important idea about Earth - it always has zero charge, no matter how much you charge add to it. It's obviously an approximation, but it's an excellent one as long as you don't have access to a Death Star.

 

[atyy]

Since you can never change the number of charges in the earth, you can never change its potential.

This means that whatever you connect to Earth will have the same potential as Earth very quickly. Let's adopt convention and call Earth's potential 0 V. With that convention, the plate with 10 charges has a positive potential. But once you connect it to earth, the charge on the plate quickly becomes zero, just like earth, so the plate's potential will be 0 V, just like earth.

 

[tiny-tim]

… capacitors confuse me …

The important part about Earth is that it is like a very, very, very, very, very big plate with zero charge. So if you connect plate 1 with 10 charges to earth, the first charge to go will go quickly, just like if you connect it to plate 2. Unlike when you connect it to plate 2, the second charge to go will not take a longer time to reach the earth, because the first charge has lots of room on the earth, so much room that it will not push back on the second charge. In fact, there is so much room on the earth, that all 10 charges will be gone from plate 1 to the Earth - quite unlike having 5 charges remaining on plate 1 when you connect it to plate 2.0

Also, there's no reason why the shelf should be considered to have "0V potential." …

Hi atyy! Hi chroot!

capacitors confuse me.

Aren't you ignoring the electrostatic force in the capacitor?

And is the potential of the shelf relevant?

An excess of Q electrons on one plate will repel Q electrons from the other plate.

For example, if you have a compete circuit with a battery and two capacitors in series, the emf of the battery will (loosely speaking) force Q electrons from the outer plate of one capacitor all the way round the circuit onto the outer plate of the other capacitor.

But the emf isn't connected to the inner plates, so it can't influence them and yet we know that the inner plates will have charges ±Q also.

This is because the Q on each outer plate repels Q electrons from one inner plate to the other.

Now charge up a capacitor, disconnect it, and connect the plus plate plate to the earth, through a lamp.

Do Q of the inexhaustible pool of electrons in the Earth rush over to fill up the Q holes?

No, because the other plate still has Q electrons, and so it will still repel any electrons trying to fill those holes.

No electrons will flow, and the lamp will not light.

And the potential of the Earth is irrelevant.

I'm honestly not sure how a battery works , but I assume the potential pressure inside it is something like that "inside" a capacitor, and so connecting only one terminal to the Earth will not allow electrons to flow from the Earth because the other terminal will still repel them.

But where does potential difference come into this?

Afterthought: connect two capacitors of different capacitances in a circuit. Then connect a battery across opposite sides of the circuit, until the capacitors are fully charged. Now disconnect the battery.

The unequal charges on the two capacitors will now even out, with charge moving from one capacitor to the other.

This is despite the fact that both + plates are at the same potential, and so are both - plates, at all times.

So charge moves, and current flows (for a short time) without any potential difference. Have I got that right?

 

[atyy]

Hi atyy! Hi chroot!

capacitors confuse me.

Aren't you ignoring the electrostatic force in the capacitor?

The capacitor is just a pair of metal plates. If there is no charge on the metal plates, there is no electrostatic force. Electrostatic forces are forces between charges - it's equivalent to the idea that positive charges repel positive charges, whereas negative charges and positive charges attract each other. In general, the force between charges is not purely electrostatic, but the electrostatic approximation is good as long as the charges aren't giving off high frequency electromagnetic radiation.

 

[atyy]

Now charge up a capacitor, disconnect it, and connect the plus plate plate to the earth, through a lamp.

Do Q of the inexhaustible pool of electrons in the Earth rush over to fill up the Q holes?

No, because the other plate still has Q electrons, and so it will still repel any electrons trying to fill those holes.

No electrons will flow, and the lamp will not light.

OK, I guess this is the situation where you have 10 positive charges one plate of a capacitor, and 10 negative charges on the other plate. If you connect the positive plate to a bulb, and the bulb to the earth, the inexhaustible pool of electrons will rush up and neutralize the positive plate. Or if you prefer, you can think that the positive charges want to get as far away from each other as possible, so they will all go into the huge room that Earth provides. Of course, the positive charges like to be near the negative charges on the other plate, but then they have to be all squeezed together on one small plate. They don't like 10 negative charges on the other plate that much to resist the infinite room that Earth supplies for them to get away from other positive charges. So the bulb will light (in principle, to get this effect experimentally you may need more than 10 charges)- but only very briefly, because the current will stop once the positive charges have gone into the earth.

 

[tiny-tim]

Hi atyy!

If there is no charge on the metal plates, there is no electrostatic force.

Yes, but I was talking about a charged capacitor, with one plate connected to Earth …

surely the charge on that plate will be determined only by the electrostatic force from the (fixed) charge on the other plate …

and so it won't change, whatever the potential of the earth?

 

[Mårten]

Hi all!

Thank's for some very thorough replies! I'm getting closer...

Let me first say this, which I forgot in the first post here: In my elementary school physics understanding, I also learned that you cannot have a current, if you don't have a closed circuit. So that's also something that has confused me, since when you have a ground in the circuit, it (the circuit) doesn't seem to have to be closed in order to make a current run. Now:

1) I connect the bottom of the lamp to a big metal shelf here. Since there should be a potential of 2.25 V on the minus terminal of the last battery in the series, and the shelf should have 0 V potential, there ought to be a current - but no light. Why?

Because there is no continuous path for current to flow from one terminal of your battery pack to the other. Also, there's no reason why the shelf should be considered to have "0V potential." As I've said, voltage is a purely relative quantity. You cannot say "an object has a potential of 0V," only that "an object has a potential of 0V with respect to some other object."

I think I understand the relativity here pretty well. But - as atty said - 10 positive charges on one plate, and zero on the other plate, will make some of the ten charges to flow to the other plate. So why doesn't my excess of charges on the terminal of the battery flow to the other plate (i.e. the shelf, or the ground pin in my outlet)? Is it some property of the battery here that needs to be considered to explain this? Tiny-tim, you seemed to say that this experiment should produce a current from the battery terminal to the ground, but you may have changed your mind?

 

[atyy]

But where does potential difference come into this?

Afterthought: connect two capacitors of different capacitances in a circuit. Then connect a battery across opposite sides of the circuit, until the capacitors are fully charged. Now disconnect the battery.

The unequal charges on the two capacitors will now even out, with charge moving from one capacitor to the other.

This is despite the fact that both + plates are at the same potential, and so are both - plates, at all times.

So charge moves, and current flows (for a short time) without any potential difference. Have I got that right?

I think the unequal charges on the capacitors will not even out, so there will be no current flow. The capacitance kind of measures how big the metal plates of the capacitor are. So if you have two capacitors with different capacitances, they have plates of different sizes. So let's say one plate has capacitance 10 and the other plate has capacitance 3. When you charge the capacitors up with a battery, you will put 100 charges on the bigger capacitor, and 30 charges on the smaller capacitor. When you disconnect the battery, the 100 charges will not rush to even out the 30 charges on the smaller capacitor, because the plates of the big capacitor have 10/3 times more room than the plates of the small capacitor, so 100 charges on the big plate to 30 charges on the small plate already corresponds to all 130 charges being as far away from each other as possible.

You will find that capacitance also depends on distance, so it could be that the plates of the big and small capacitor have the same size, but the plates of the big capacitor are further apart than the plates of the small capacitor.

How is this related to potential difference? Potential difference measures the tendency for charge to move in a certain direction. So if you have one + charge and one - charge, there will be a potential difference between them, since they will want to move together. But potential difference also depends on distance, since if the + and - charges are very far from each other, their mutual attraction will be smaller, and the potential difference between them will also be smaller. This is why it is possible to change capacitance by adjusting either the area of the metal plates, or the distance between the plates.

 

[atyy]

Hi atyy! Yes, but I was talking about a charged capacitor, with one plate connected to Earth …

surely the charge on that plate will be determined only by the electrostatic force from the (fixed) charge on the other plate …

and so it won't change, whatever the potential of the earth?

Oh, I see what you mean. If you have negative charge on plate 1, and no charge on plate 2, then you connect plate 2 to earth, the negative charge will attract positive charge from the Earth to plate 2.

OK, in this case, I would predict that if you have negative charge on plate 1, and no charge on plate 2, then you connect plate 2 through a bulb to the earth, the bulb will light when positive charge moves from the Earth to plate 2.

Which is the opposite of what I thought earlier ...

Well, at least I'm predicting both times that the bulb will light;)

I think we have to be careful by what we mean by 10 positive charges, or 10 negative charges ...

 

[tiny-tim]

Hi Mårten!

Tiny-tim, you seemed to say that this experiment should produce a current from the battery terminal to the ground, but you may have changed your mind?

ooh … I'd forgotten that!

Yes … I think I have changed my mind … subject to what everyone else says …

I can see that if you connect an overhead supply ("high-tension") cable to the earth, a current will certainly flow, because the cable is kept at a large potential difference to earth. But is that only because the "battery" at the power station is still connected?

If you disconnect that battery, will the potential difference drop to zero (unlike the charge on the plate of a capacitor)?

So does current flow through the Earth from a domestic power circuit because the power source is still in a circuit (a different one), and able to maintain a potential difference, but not from one terminal of a battery through your shelf because the power source (the battery itself) is not in any circuit?

I think the unequal charges on the capacitors will not even out, so there will be no current flow.

Hi atyy!

Yes, you're right … no charge will flow … I wasn't thinking straight. Sorry!

… I would predict that if you have negative charge on plate 1, and no charge on plate 2, …

But can you start like that? Doesn't the electrostatic force mean that the charges on the two plates of the same capacitor are always equal (and opposite), whatever happens?

 

[atyy]

can[/I] you start like that? Doesn't the electrostatic force mean that the charges on the two plates of the same capacitor are always equal (and opposite), whatever happens?

OK, I think you are right that if you charge up a capacitor then connect it to a bulb then to ground, nothing will happen. But I'm somewhat confused about this. Am off to get some sleep, see you guys later!

 

[Mårten]

Hm, I'll try to organize a little bit here of what we have concluded...

If we have a charged metal plate and connect it to a metal shelf, a wire or the earth, the charge on the plate will be evenly distributed on the new system that contains e.g. the metal plate and the metal shelf. This new system is still charged, but because the system has a bigger volume, the charge density is less. If the system happens to be the metal plate and the earth, the charge will be also evenly distributed, but because the Earth is so enormously big, the charge distribution will be so low, so the amount of charges on the metal plate part of this new system, will almost be zero. Now, all this is ultimately due to Coulomb's law. - Have I understood the concept of grounding correct so far?

If we talk about capacitors and batteries, it seems to be a little bit more complicated. The plus plate of a capacitor will maybe not lead current to the Earth (if connected to the earth), since the negative charges on the other plate of the capacitor holds the plus charges in its grip. This due to electrostatic induction between the plates. Is that correct? Something similar applies to batteries.

Now, when is the electrostatic induction between the metal plates of the capacitor, or between the plus and minus parts inside the battery (or whatever happens in the battery) less than the tendency for the charges to be evenly distributed in its conductor where they resides? If you connect one of the metal plates to the earth, the electrostatic induction seems to win. If you connect one of the metal plates to the other metal plate, the electrostatic induction seems to lose. - So why then?

 

[atyy]

OK, I would like to change some of my answers. Here are my latest guesses. I am making my guesses under the approximation that the Earth has an infinite supply of free positive and negative charges, and that no matter how much you add or subtract from it, the Earth remains uncharged. So anything you connect to it will end up having the same potential as the earth. Thus there cannot be charge on anything that you connect to earth.

1) Charge a capacitor, disconnect the battery. There are 10 + charges on plate 1, and 10 - charges on plate 2. Connect plate 2 via a bulb to earth, and the 10 - charges should flow to earth, lighting the bulb momentarily, and finally leaving plate 2 uncharged.

2) Charge a capacitor, disconnect the battery. There are 10 + charges on plate 1, and 10 - charges on plate 2. Now we take away plate 2, and replace it with plate 3 which is uncharged. Connect plate 3 via a bulb to earth, and nothing should happen.

So what about the rule tiny-tim cites about the charges on opposite plates of a capacitor always being the same? Perhaps this is a rule from circuit theory, and only works under conditions in which the opposite plates are connected within some closed circuit?

I know there are times when circuit theory fails. For example, in circuit theory, you can change the capacitance of one capacitor without changing the capacitance of the other capacitor. Since changing the capacitance of one capacitor is equivalent to moving one metal plate about, that means that moving one conductor about doesn't affect the capacitance of the other capacitor. But there are times when this is not true. When you tune a radio, you are adjusting a capacitor in the radio. As we all know, if you tune the radio perfectly, then move away from it, the radio can become slightly untuned. This is because you are a conductor, and your position relative to the capacitor in the radio affects its capacitance, and hence the tuning of the radio. So that's an example where the rule from circuit theory doesn't work.

BUT I am not sure whether my latest guesses are just wrong, or whether we are actually bumping against a limitation of circuit theory here.

 

[tiny-tim]

cramming work?

Doesn't the electrostatic force mean that the charges on the two plates of the same capacitor are always equal (and opposite), whatever happens?

So what about the rule tiny-tim cites about the charges on opposite plates of a capacitor always being the same? Perhaps this is a rule from circuit theory, and only works under conditions in which the opposite plates are connected within some closed circuit? …

This electrostatic force has really been bugging me …

I think you're right, atty, and exact equality occurs only in a "circuit", and then only from conservation of charge.​

I think this is how it works:

Imagine an isolated metal sphere N, already holding a negative charge, of 100 electrons.

Those 100 electrons will be repelling each other, and will try to get as far away from each other as possible, so they'll be fairly uniformly distributed around the sphere.

Attach a metal wire to N … only one or two of the electrons will go into the wire, because the wire is small, and any more would be repelled back onto the sphere.

Connect the wire to the earth, E … now all 100 electrons will be able to go to E, because E is so large (or will 1 electron remain on N, simply because it has no reason to move? ).

In other words, the electrons move from N to E not because of any potential difference, but because they repel each other, and E is a great place to hide! ​

Now start again, with the same negatively charged sphere N and a positively charged sphere P with 200 "holes".

Bring them close together … the closer they are, the more the electrons on N will be attracted to the holes on P, so the electrons and holes will no longer be uniformly distributed.

Connect the wire to N as before, and then connect E to the wire.

This time, the 100 electrons will still be repelling each other, but they will also be attracted to the 200 holes on P. So not all will want to get away.

How many will stay on N? Will any more arrive from E?

I don't know …

Perhaps we can work it out like this … suppose P and N (in their final position) were originally charged, in the same circuit, so as to have 200 holes and electrons respectively.

So the 200 electrons on N could have gone a long way away (to the battery, which we'll assume is as good as infinitely far away, so far as electrostatic force is concerned). But they didn't, 'cos their natural repulsion for each other was exactly balanced by their attraction for the holes on P plus the "pressure" from the battery.

Now disconnect the battery, and connect N to E … the "pressure" has gone, but the attraction from P is still there!

So some of the electrons on N will go to E, but some will remain crammed on N, attracted by P.

So the questions seem to be,
i] what is the work is done by moving a charge Q though a voltage V, compared with the work done by cramming a charge Q (made up of many mutually repelling electrons) into a small volume? and
ii] what is the electrostatic attraction compared with the battery "pressure"?

At this point, I have no idea how to calculate either.

Though it seem obvious that the smaller the area A of the plate, the more the cramming work … and capacitor plates are very small compared with metal spheres … and that a "positive feedback" means that the larger Q is, the more the electrostatic attraction is "helping".

I've never even seen cramming work mentioned … but it seems an essential part of the operation of a capacitor.

Dimensionally, one would expect it to be something like Q²/ε√A (which is much larger than the standard work done, Q²d/εA).

Or is it insignificantly tiny? ​

 

[Mårten]

1) Charge a capacitor, disconnect the battery. There are 10 + charges on plate 1, and 10 - charges on plate 2. Connect plate 2 via a bulb to earth, and the 10 - charges should flow to earth, lighting the bulb momentarily, and finally leaving plate 2 uncharged.

Are you sure the 10 + charges on plate 1 will not attract the 10 - charges on plate 2 via electrostatic induction, so much that this current to the Earth never takes place?

I think we need som expert help here - help!

There are in particular two questions I'd like to know the answer of:

1) Is my interpretation above correct, that when you ground a charged object A, then the charges are spread evenly over the earth-and-object-A-system so that there are practically no charges left on A (but actually the Earth has then been a little bit more charged, but it's so extremely little, so you can say that Earth is still uncharged)?

2) About continuous path for the current to flow: Is it really needed in all situations? I thought one of the points in grounding was that a complete circuit (a closed loop) is not necessary for the current to flow. Isn't it possible to have a charged plate from which you let current flow to Earth through a wire slowly so that you can light a lamp in between for some seconds?

 

[Mårten]

In other words, the electrons move from N to E not because of any potential difference, but because they repel each other, and E is a great place to hide!

Exactly! Can we get this confirmed from an expert? (Although, I would say that the potential difference is a way of expressing that the electrons repel each other. )

So some of the electrons on N will go to E, but some will remain crammed on N, attracted by P.

So the questions seem to be,
i] what is the work is done by moving a charge Q though a voltage V, compared with the work done by cramming a charge Q (made up of many mutually repelling electrons) into a small volume? and
ii] what is the electrostatic attraction compared with the battery "pressure"?

At this point, I have no idea how to calculate either.

Does it really need to be calculated? Maybe it's obvious (at least point i] above) for an expert that one of the "works" will be more preferable, or how to put it...

Is there a way to make a pf mentor pay attention to this thread, so she/he can help us?

 

[tiny-tim]

Is there a way to make a pf mentor pay attention to this thread, so she/he can help us?

Patience!

They keep a special eye out for mass hysteria such as this!

 

[Integral]

Atty and Tiny-Tim,
You guys are confusing me. Please take a break. There is no need or help to bring capacitors into the game. We are trying to understand grounding here please keep it simple.

Marten,
A battery is a self contained EMF, for it to function both terminals must be in the circuit. All potentials are with respect to the terminals of the battery. If you wish to establish a external reference point for your battery, you must connect your battery to it. So if you want to use a shelf (if it is a conductor), or the ground terminal as a reference for your circuit you must connect one end (it does not matter which) of your battery to the reference point. So if you tie the negative terminal to the shelf, you can then connect the positive terminal to your light bulb, the other side of the bulb to the shelf. Now you have a complete circuit. BOTH sides of the battery are in the circuit you have replaced a wire with the shelf.

The power company establishes Earth ground as a reference point for your household power. So you can get current flow from the hot lead to ground, because Earth ground is an established reference point.

 

[atyy]

1) Is my interpretation above correct, that when you ground a charged object A, then the charges are spread evenly over the earth-and-object-A-system so that there are practically no charges left on A (but actually the Earth has then been a little bit more charged, but it's so extremely little, so you can say that Earth is still uncharged)?

2) About continuous path for the current to flow: Is it really needed in all situations? I thought one of the points in grounding was that a complete circuit (a closed loop) is not necessary for the current to flow. Isn't it possible to have a charged plate from which you let current flow to Earth through a wire slowly so that you can light a lamp in between for some seconds?

Well, sorry about bringing up capacitors! It's funny what's confusing - I've always found batteries confusing, because they involve chemistry - whereas capacitors are simple, because you can see where everything is going.

Anyway, my answers:

1) Yes, that's correct. But it's a slightly different definition of ground from what Integral is using. Integral's definition is essentially "where you wish to set your zero of potential".

2) No, a "complete circuit" is not always required (or we can use a more refined definition of "complete circuit").

The example I'm going to give is lightning. Here the charged plate is a storm cloud. The Earth is again approximated as something that has an inexhaustible supply of free positive and negative charge that always remains uncharged. Connecting the metal plate to ground is what happens when so much charge accumulates on the cloud, that the air momentarily becomes "conducting", and the charge on the cloud is discharged to the Earth via the air (not sure what the real charge carriers are in this case, whether lightning is from ground to cloud or vice versa, but it doesn't matter for what we're discussing). I've never tried the experiment, but I think if you place a bulb in the path of a stroke of lightning, the bulb will light.

Going back to what Integral said, you cannot always assume that the entire Earth is at a uniform potential. Actually you cannot even assume that the "ground" in different sockets have the same potential. Electrical experiments often plug all "grounds" into one socket, to make sure that that there aren't two reference potentials by mistake (which will cause lots of trouble, since the theory only allows one reference potential to be chosen).

 

[atyy]

I've never even seen cramming work mentioned … but it seems an essential part of the operation of a capacitor.

Yes, there is such a thing as cramming work. No one calls it that, they say the work done in charging a capacitor. In circuit theory, you can calculate it by charging a capacitor by putting it in series with a resistor and battery. You know the formulas for power provided by a battery and the power dissipated by a resistor, so you can find the corresponding energies by integrating those over time. The difference between those two energies should be the energy stored in the capacitor. Cramming work obviously depends on the shape of the capacitor. Once the shape of a capacitor is specified, you can calculate its capacitance. The capacitance contains all the information about shape needed for circuit theory, so once you know the capacitance, there is no need to refer back to the shape.

 

[stewartcs]

This is a very convoluted thread…what was the original question again?

CS

 

[Mårten]

A battery is a self contained EMF, for it to function both terminals must be in the circuit.

Hm, the question is why. Is it due to how the chemistry inside it works?

So if you want to use a shelf (if it is a conductor), or the ground terminal as a reference for your circuit you must connect one end (it does not matter which) of your battery to the reference point.

Would be interesting to see what happens if you connect one terminal to the Earth in the soil right here, and the other terminal in the soil some meters away. Would the Earth really be a good conductor there? There's a lot of air and strange dust in the soil, maybe it won't be such a good conductor so my lamp will shine.

The power company establishes Earth ground as a reference point for your household power. So you can get current flow from the hot lead to ground, because Earth ground is an established reference point.

The question here is: Is this an example of a complete circuit (where the soil in the Earth leads all the electrons back to the power station), or is it an example of the phenomenon when you have a lot of charges of the same kind together, they want to go as far away from each other as possible due to Coulomb's law? In principle, what happens at the power station? Is it that they there build up a huge amount of charges of the same kind, which then "falls down" all the way to my house, through my lamps, and then down to the Earth - and again: because the charges want to be as far away as possible from each other?

Well, sorry about bringing up capacitors! It's funny what's confusing - I've always found batteries confusing, because they involve chemistry - whereas capacitors are simple, because you can see where everything is going.

Well, I think I agree there... Except that capacitors have some difficulties due to the other plate they contain...

1) Yes, that's correct. But it's a slightly different definition of ground from what Integral is using. Integral's definition is essentially "where you wish to set your zero of potential".

2) No, a "complete circuit" is not always required (or we can use a more refined definition of "complete circuit").

Okey!

 

[stewartcs]

Hm, the question is why. Is it due to how the chemistry inside it works?

An electric field exists between the two terminals of the battery. The chemical energy is transformed into electrical potential energy. It is self-contained since it is not connected to anything else. So unless you physically connect the battery to the earth, you will not have a path for current to flow.

Would be interesting to see what happens if you connect one terminal to the Earth in the soil right here, and the other terminal in the soil some meters away. Would the Earth really be a good conductor there? There's a lot of air and strange dust in the soil, maybe it won't be such a good conductor so my lamp will shine.

The Earth is generally a somewhat good conductor of electricity due to the electrolytic content. IIRC it is about 25 ohms per foot or something along that line (not sure on that number off the top of my head).

It will make your light shine as long as the distance isn't that large. The current that flows through the Earth is called fault current in most electrical systems as it is undesired.

The question here is: Is this an example of a complete circuit (where the soil in the Earth leads all the electrons back to the power station), or is it an example of the phenomenon when you have a lot of charges of the same kind together, they want to go as far away from each other as possible due to Coulomb's law? In principle, what happens at the power station? Is it that they there build up a huge amount of charges of the same kind, which then "falls down" all the way to my house, through my lamps, and then down to the Earth - and again: because the charges want to be as far away as possible from each other?

Utility providers ground the system intentionally to mitigate lightning strikes and other surges. It serves no other purpose.

CS

 

[chroot]

Hm, the question is why. Is it due to how the chemistry inside it works?

Yes, batteries use oxidation/reduction reactions, which involve the transfer of electrons. The electrons are forced to move from one terminal to the other, via a circuit, in order to complete the reaction.

Would be interesting to see what happens if you connect one terminal to the Earth in the soil right here, and the other terminal in the soil some meters away. Would the Earth really be a good conductor there? There's a lot of air and strange dust in the soil, maybe it won't be such a good conductor so my lamp will shine.

The Earth is a very poor conductor. It is used as a reference for voltages, NOT as a conductor.

The question here is: Is this an example of a complete circuit (where the soil in the Earth leads all the electrons back to the power station), or is it an example of the phenomenon when you have a lot of charges of the same kind together, they want to go as far away from each other as possible due to Coulomb's law? In principle, what happens at the power station? Is it that they there build up a huge amount of charges of the same kind, which then "falls down" all the way to my house, through my lamps, and then down to the Earth - and again: because the charges want to be as far away as possible from each other?

Integral mislead you in his previous post. You can get current to flow from the hot conductor to the ground conductor in your household wiring because the ground conductor is tied to the neutral conductor in your breaker box. The power company does not pass current through the earth.

Charges in a conductor will move as far away as possible from each other, but this is not really relevant to the flow of current in a wire. Current flows in a wire because there exists a difference in potential between one end and the other. A basketball rolls downhill for the same reason.

- Warren

 

[Mårten]

Thanks for patience and all answers...

Charges in a conductor will move as far away as possible from each other, but this is not really relevant to the flow of current in a wire. Current flows in a wire because there exists a difference in potential between one end and the other. A basketball rolls downhill for the same reason.

But wait for a moment here... What is potential? Isn't that electric potential energy per unit charge? And a high magnitude of the potential energy for a specific charge (e.g. +) (relative to some point at some distance) means that you have a charge very close to a gathering of some other + charges, and that the electric field between your charge and these other charges then is strong (pointing away from each other). And therefore your charge will run away fast. So this will create a current (as long as we also have some EMF continously filling up new charges in the charge gathering). At least, this is my theory now of how it works after been studying the answers in this thread thoroughly. Or have I missed something here with the concept of potential?

 

[atyy]

What is potential?

Let's consider again my favourite example with capacitors.

Let's put 1 + charge on plate A, and 1 - charge on plate B. They would like to be with each other. In circuit theory, they can't get together because the two plates are not connected by a metal wire (outside of circuit theory, if you bring the plates close together enough, or put zillions of pairs of +/- charges instead of just 1 pair, then the charges will express how much they like to be with each other by jumping across the plates without a metal wire connecting them, and you will see this as a spark).

Anyway, in circuit theory, the charges would like to be with each other. In more technical language, we say that the separation of charges creates an electric field between the plates, and the electric field indicates the direction in which the charges would move if the plates are connected by a wire. The important thing to note is that the electric field is present in between the plates where there is no charge present.

An electric field is a *vector* field, which we imagine as an arrow attached to every point in the space between the plates indicating the direction and magnitude of the electric field. In general, the arrows at different points can have different directions and magnitudes. By some magic, it turns out we can calculate all the right answers if we replace the vector field with a *scalar field*, which we imagine as only a magnitude - no direction! - attached to every point in the space between the plates. This scalar field is called the electric potential. The electric potential, although it is just a number, encodes the directional information in the electric field in the following way: the direction in which charges move is in the direction of lower potential, ie. down the potential difference. So now you can kinda see why only potential differences have meaning, whereas potentials themselves don't - if you raise the potential at every point between the plates by the same amount, the potential difference between any two points remains the same, and hence the directional information remains the same. Anyway, the point here is that just as there is an electric field between the plates where there are no charges, there is an electric potential between the plates where there are no charges.

In almost all physical situations, every electric field or electric potential difference is ultimately created by some charge separation. However, since the field or potential can exist in regions where there are no charges, it makes sense to consider them as the actors in those regions. This is why chroot said that the electrons are moving down a potential difference.

One minor point, the magic of replacing the vector electric field with the scalar electric potential only works when you have steady currents. Strictly speaking, this magic doesn't work when you are charging a capacitor, or have alternating current - however, the approximation of an electric field by an electric potential remains very good as long as the current doesn't change too quickly.

 

[schroder]

The Earth is a very poor conductor. It is used as a reference for voltages, NOT as a conductor.



Integral mislead you in his previous post. You can get current to flow from the hot conductor to the ground conductor in your household wiring because the ground conductor is tied to the neutral conductor in your breaker box. The power company does not pass current through the earth.

- Warren

These statements are not quite correct. The power generated by the electric company is always referenced to earth. The Earth is a very good conductor of electricity! The resistance of any conductor varies inversely with the cross sectional area of the conductor. What has a bigger cross section than the earth? In the early days of power generation and distribution, no ground or neutral wire was used in the transmission system. Sometime later it was determined that not all Earth references at the distribution centers had equal ground potential and certainly not all final users (homes) had equal ground potential which caused an unequal and very dangerous distribution of voltages. So all national electrical codes mandated that the power companies shall provide a separate neutral lead to ensure the equal distribution of voltage to all end users. But at all large commercial sites and power stations the National Electrical Code still mandates that a separate “safety ground to earth” be established and maintained at a resistance of not greater than 25 ohms. Sometimes electrodes need to be driven deep into the Earth to achieve this , but it can be achieved just about anywhere on the earth.

 

[atyy]

Isn't that electric potential energy per unit charge?

Yes. So back to the capacitor with 1 + charge on plate A and 1 - charge on plate B. Let's assume that they are confined to those plates (no sparks). There is an electric field between the plates, ie. if you put another + charge near plate A (let's call this the test charge), it will be repelled from the + charge on on plate A, and will be attracted to the - charge on plate B. If the test charge is allowed to move in the electric field from plate A towards plate B, the electric field will be doing work. Since we can describe an electric field by an electric potential, we can also say that the electric potential is doing work.

Since potential is arbitrary. Let's set the potential at plate B to be zero.
Let the potential at plate A be V.
So the potential difference between the plates is V.
Let the test charge be q.
Then the work done by the electric field/potential in moving the test charge from plate A to plate B will be qV.
So the work done per unit charge will be qV divided by q = V.
That's why the electric potential is the electric potential energy per unit charge, as you said.

Minor point: There's a conventional double minus sign, which I'm ignoring.

More important: I've assumed that the + charge on plate A really is stuck there, that it doesn't move towards - charge on plate B although it is attracted to it. I've also assumed that the placement of a test charge near the + charge on plate A doesn't cause the + charge on plate A to move, even though it is repelled by the test charge.

 

[chroot]

The Earth is a very good conductor of electricity!

The Earth is a very poor conductor of electricity when compared to, for example, a copper wire. Your statements later in your post regarding differences in ground voltage are a good example of how poor a conductor it is.

- Warren

 

[schroder]

The Earth is a very poor conductor of electricity when compared to, for example, a copper wire. Your statements later in your post regarding differences in ground voltage are a good example of how poor a conductor it is.

- Warren

You need to clarify your statement. If you are talking about a cubic meter of “earth” as compared to a cubic meter of copper then certainly the copper is a much better conductor. However, when the term “earth” is used in reference to the Earth itself, it is the best possible conductor of electricity because of the cross sectional area, as I pointed out. It is necessary to establish a proper connection to this earth, however. Sometimes that requires driving electrodes deeply into the ground, or possibly treating the ground with chemicals or water. But once a good gound is established it is very possible to transmit electricity from one place to another without the need for a separate ground or neutral wire. As I have already mentioned, that was the practice in the early days of power transmission but because the ground references at the various distribution points were not all properly done and verified it soon became apparent that a separate neutral wire must be provided. I suggest you do some research on this.

 

[stewartcs]

You need to clarify your statement. If you are talking about a cubic meter of “earth” as compared to a cubic meter of copper then certainly the copper is a much better conductor. However, when the term “earth” is used in reference to the Earth itself, it is the best possible conductor of electricity because of the cross sectional area, as I pointed out.

The resistance of the ground rod to the Earth is also a function of the soil's resistivity as well as the length and radius of the ground rod.

But once a good gound is established it is very possible to transmit electricity from one place to another without the need for a separate ground or neutral wire.

Although this is quite true, it is not how electrical distribution systems are really intended to work under normal conditions. The Earth ground is mainly used when fault conditions occur. Utility companies don't depend on fault current to transmit power.

There is also a slight benefit to the grounds although it is not really an intended design per se. The repeated grounding points throughout the electrical distribution system (power transmission lines for example) help reduce power line losses by essentially creating a parallel network with the transmission line.

CS

 

[schroder]

Although this is quite true, it is not how electrical distribution systems are really intended to work under normal conditions. The Earth ground is mainly used when fault conditions occur. Utility companies don't depend on fault current to transmit power.

There is also a slight benefit to the grounds although it is not really an intended design per se. The repeated grounding points throughout the electrical distribution system (power transmission lines for example) help reduce power line losses by essentially creating a parallel network with the transmission line.

CS

Yes, I agree with this. I did not mean to imply that today’s power transmission systems use Earth ground to transmit power. I mentioned it in an historical context as well as a physics context to establish that it is possible. Besides the indirect benefits you mentioned, the main advantage today of proper grounding is the safety function, which I would like to point out by a specific example.
Suppose you are supplying three-phase 4000 Volt ac supply to a machine and the source and feeder resistance is 15 ohms. Now one of the phase wires in this machine touches to the motor frame which has a 10 ohm resistance to Earth ground. This will place about 2,300 volts on the frame of the machine with reference to ground, resulting in a flow of 92 amperes between the machine frame and ground. Now if a technician should happen to come along with his test cart, which is itself grounded, and touch the frame of this machine he will be subjected to roughly 920 Volts, which is more than sufficient to kill him instantly.
But if the machine was better grounded, say with a resistance to Earth of only one ohm, the voltage shock would be reduced to about 92 Volts and he will most probably live to correct the fault. There are also in place ground fault circuit breakers, but I am not taking those into consideration in this discussion. Proper grounding is one of the least appreciated and most misunderstood disciplines in electrical engineering and a thorough discussion of this would involve many more posts by myself and others.

 

[stewartcs]

Yes, I agree with this. I did not mean to imply that today’s power transmission systems use Earth ground to transmit power. I mentioned it in an historical context as well as a physics context to establish that it is possible. Besides the indirect benefits you mentioned, the main advantage today of proper grounding is the safety function, which I would like to point out by a specific example.
Suppose you are supplying three-phase 4000 Volt ac supply to a machine and the source and feeder resistance is 15 ohms. Now one of the phase wires in this machine touches to the motor frame which has a 10 ohm resistance to Earth ground. This will place about 2,300 volts on the frame of the machine with reference to ground, resulting in a flow of 92 amperes between the machine frame and ground. Now if a technician should happen to come along with his test cart, which is itself grounded, and touch the frame of this machine he will be subjected to roughly 920 Volts, which is more than sufficient to kill him instantly.
But if the machine was better grounded, say with a resistance to Earth of only one ohm, the voltage shock would be reduced to about 92 Volts and he will most probably live to correct the fault. There are also in place ground fault circuit breakers, but I am not taking those into consideration in this discussion. Proper grounding is one of the least appreciated and most misunderstood disciplines in electrical engineering and a thorough discussion of this would involve many more posts by myself and others.

I agree that grounding (not to mention bonding) is not very well discussed in Electrical Engineering courses - most unfortunate.

CS

 

[transgalactic]

we were told in our starting course of EE
that electrisity as we see it at home
has nothing to do with EE

further more we were told that
todays electrical engeneer doesn't have to know what's triple faze current

 

[stewartcs]

we were told in our starting course of EE
that electrisity as we see it at home
has nothing to do with EE

further more we were told that
todays electrical engeneer doesn't have to know what's triple faze current

That's most unfortunate that you were told that because it is not true. Two of the EE's that work with me specialize in power systems. If they didn't know anything about three-phase systems they wouldn't be employed.

CS

 

[Mårten]

For anyone who passes by this thread, I'm adding the following helpful link which sorts things up and explains quite a lot about grounding:

http://www.ese.upenn.edu/rca/instruments/misctutorials/Ground/grd.html

(Thanks Redbelly for the tip!)