Circuit completion : is it necessary?

[RedX]

It isn't. As mentioned in that post, the effective charge is miniscule, less than 10^-11 Coulombs, and so will be the voltage.

But the voltage between the two charges on the terminals of the battery is 12 V. So if you define V_+ and V_- as the voltages between the positive terminal and the ground, and the negative terminal and the ground, respectively, then V_+-V_-=12 volts, and why not assume symmetry to get V_+=V_-=6 volts?

 

[rcgldr]

But the voltage between the two charges on the terminals of the battery is 12 V. So if you define V_+ and V_- as the voltages between the positive terminal and the ground, and the negative terminal and the ground, respectively, then V_+-V_-=12 volts, and why not assume symmetry to get V_+=V_-=6 volts?

Although not a great analogy, think of a battery as a capacitor with a small amount of charge on each plate but with a very large distance between the plates in order to achieve that 12 volts. Now imagine a long grounded wire that spans the distance between the plates (without touching the plates). Since the charge on the plate at each end of the capacitor is small, the voltage between each plate and each end of that long grounded wire is also small.

The issue here is voltage is a potential that is affected by distance within a field, or the equivalent of distance in the case of a battery. When you measure the voltage between either terminal of a battery and some common ground, the equivalent of the distance component is much smaller between the terminal and the common ground, so the voltage is much less, even though the voltage between the terminals is relatively high.

 

[AlchemistK]

I think the problem arising here is that a battery has too little voltage, so let's just take some other high voltage source into consideration.

 

[Dmytry]

Why is there a capacitance between a light bulb and a wire? This is something new to me... Never heard of it.

never heard of parasitic capacitances? They're very small.
The thing is, OP's actually smart, he thought of parasitic capacitances himself (those are what holds the charges on battery leads etc), he just doesn't know the word.

Again... I'm not real sure what you're talking about. A voltage is a measurement between two points so if you're saying 1.5 Volts between the terminals... Where is the 100V measurement coming from? one of the battery teminals and what else? The ground? I don't mean to repeat myself, but if you put one meter lead on either of the posts of that 1.5V battery, you will not read a voltage until you put that other lead on the other post of the battery. if you put that lead anywhere else your meter will read 0 volts.

But that was electric potential I was speaking of.
http://en.wikipedia.org/wiki/Electric_potential

100 volts to what ever. Ground that is under your feet, or 'ground' that is the ground bus, or the box that the circuit was built in.

Where are these numbers coming from? What is this "leakage" you are talking about and what are these 0.75v measurements? leakage rates? Please explain...

suppose you have battery laying somewhere. You may have few hundred gigaohm resistance from each lead of battery to the ground, parasitic resistances.
The actual circuit would be - each end of battery connected to the ground with very high value resistor. If you use a voltmeter that has very high internal resistance (essentially infinite), you can measure the potential difference between lead and ground.

 

[RedX]

Although not a great analogy, think of a battery as a capacitor with a small amount of charge on each plate but with a very large distance between the plates in order to achieve that 12 volts. Now imagine a long grounded wire that spans the distance between the plates (without touching the plates). Since the charge on the plate at each end of the capacitor is small, the voltage between each plate and each end of that long grounded wire is also small.

The issue here is voltage is a potential that is affected by distance within a field, or the equivalent of distance in the case of a battery. When you measure the voltage between either terminal of a battery and some common ground, the equivalent of the distance component is much smaller between the terminal and the common ground, so the voltage is much less, even though the voltage between the terminals is relatively high.

This just seems like it violates Kirchoff's rule that the sum of the voltages should be zero. If you have an imaginary path from one terminal of the battery to one end of the grounded wire, through the grounded wire to the other end of the grounded wire, and through that end of the grounded wire to the other terminal, then it should equal the voltage drop of the battery. The potential through the grounded wire is zero since it's a conductor. So the sum of the two voltage drops from terminal to ground wire should equal 12 V. So at the very least one of the drops has to be 6V (it could be 10 and 2V, 11 and 1V, etc, with the minimum drop 6 and 6V).

In any case if you have a single charge above a conducting plate, then the potential above the plate can be gotten by reflecting that charge behind the plate. So if you model the battery as two charges separated by a distance, then there reflections behind the surface of the Earth should allow you to calculate the voltage. So for example say the plus and minus charges of the battery are at (x,y)=(-1,4) and (1,4) respectively. Then if you pretend there are also plus and minus charges at (x,y)=(-1,-4) and (1,-4) respectively, then you can calculate the potential at y>=0.

In any case Kirchoff's rule should apply.

So...

Q: How many physics students does it take to change a light bulb?

A: Not more than one, for then they can't agree on what will happen!

 

[Dmytry]

I think the problem arising here is that a battery has too little voltage, so let's just take some other high voltage source into consideration.

Well i think that'd really confuse everyone... batteries are easier and the principle is same regardless of voltage. Basically, the battery works by making the potential difference between it's leads be what's specified.
Very little charge is stored on the leads; you can say that a lot of charge is stored inside the battery chemically, but this charge is stored in a balanced way - you can not take out just the positive or just the negative charge alone.
The potentials themselves can be anything. For example, you can put battery on insulated table, and then put electrostatic charge on it (using plastic bag for example). That won't change the charges stored inside battery, you won't really charge the battery in the sense in which the battery charger does. However, this way you can make potentials on the leads be e.g. 10 000 v and 10 012 v (to ground). Then if you connect one to ground, current will flow for a short time. It's really same as if you had piece of metal in place of battery, you can put charge on it, you can get charge off it, etc. It's very confusing to put net charge onto battery lol.

You need to distinguish between two equal opposite charges 'stored' inside the battery, and the net charge of the battery.

 

[AlchemistK]

Can the power supply in our house also be treated like a battery?
Then we shouldn't get a shock if we put one finger in the live wire socket until we put another finger in the neutral wire socket?

 

[RedX]

Can the power supply in our house also be treated like a battery?
Then we shouldn't get a shock if we put one finger in the live wire socket until we put another finger in the neutral wire socket?

The power supply can continuously push current through you. The battery won't push more current through you until you can get rid of the charges on the other lead of the battery.

My guess is you'll probably die if you stick your finger in the live wire socket, although I'm not completely sure since if the socket has a ground fault interrupter, wouldn't it trip?

Also, what would happen if the neutral wire weren't grounded/earthed? Then would you die if you touch the neutral wire? What if the neutral wire were only connected to the transformer and there was no connection between the neutral wire and the ground?

 

[rcgldr]

Although not a great analogy, think of a battery as a capacitor with a small amount of charge on each plate but with a very large distance between the plates in order to achieve that 12 volts. Now imagine a long grounded wire that spans the distance between the plates (without touching the plates). Since the charge on the plate at each end of the capacitor is small, the voltage between each plate and each end of that long grounded wire is also small.

This just seems like it violates Kirchoff's rule that the sum of the voltages should be zero.

In my example, that grounded wire is never connected to either plate, but is placed much closer to the plates than the distance between the plates. Since voltage is equal to electrical intensity x distance, the much smaller distance means the voltage between either plate to the wire is much less than 1/2 the voltage from plate to plate.

If a charged particle traveled from either plate to the grounded wire, it gains much less energy than if the wire wasn't there and the charged particle traveled from one plate to the other.

Also, what would happen if the neutral wire weren't grounded/earthed? Then would you die if you touch the neutral wire? What if the neutral wire were only connected to the transformer and there was no connection between the neutral wire and the ground?

In most places in the USA, the neutral wire is center tapped from the last step down transformer, and it's not grounded directly, but is supposed to be within 10 volts of Earth ground due to grounding at previous transformer stages. In my home I see .3 to .5 volts between "netrual" and the actual grounded 3rd pin on the 110 volt outlets in my home.

 

[sophiecentaur]

It's all a matter of Capacity.
Imagine the battery and bulb connected by a large capacitor in series. You would have a series RC circuit. At switch on, you would find that current would flow (lighting the bulb a bit) until the Capacitor was charged. Leaving out the huge Capacitor gives you a minute capacitor (the gap between the two wire ends). Only a minute current will flow for a very short time.

 

[RedX]

In most places in the USA, the neutral wire is center tapped from the last step down transformer, and it's not grounded directly, but is supposed to be within 10 volts of Earth ground due to grounding at previous transformer stages. In my home I see .3 to .5 volts between "netrual" and the actual grounded 3rd pin on the 110 volt outlets in my home.

According to this picture at hyperphysics:

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/imgele/gfault.gif

the neutral wire is grounded at the centertap, and to the same "neutral tie block" as the ground wire.

I agree that it makes no sense. We've already established that 10V is not dangerous (touching the terminals of a car battery), so why not just connect the ground wire to the neutral wire instead of driving a pin into the ground to connect to the ground wire? Can't you just connect the ground wire to the neutral wire instead?

 

[Drakkith]

I agree that it makes no sense. We've already established that 10V is not dangerous (touching the terminals of a car battery), so why not just connect the ground wire to the neutral wire instead of driving a pin into the ground to connect to the ground wire? Can't you just connect the ground wire to the neutral wire instead?

The mains power lines that provide power for homes and business runs on AC electricity. The two lines that run into your house are both the positive AND negative lines at the same time. When the AC cycle is at one phase one line is + and the other -, and when the cycle reverses so do the lines. The ground is simply there for any accidents so that the current will flow to ground instead of through your toaster and into you.

 

[Drakkith]

It's all a matter of Capacity.
Imagine the battery and bulb connected by a large capacitor in series. You would have a series RC circuit. At switch on, you would find that current would flow (lighting the bulb a bit) until the Capacitor was charged. Leaving out the huge Capacitor gives you a minute capacitor (the gap between the two wire ends). Only a minute current will flow for a very short time.

The capacitor would never charge unless the battery was connected at both terminals. The chemical reaction that takes place to provide electrons cannot occur if only one terminal is connected. There is NOT a charge imbalance on either terminal of a battery. The cells that make up a battery are connected by something that let's charges flow between the half cells, so that when the reactions on the anode and cathode take place the removed charges can be replaced, avoiding a difference in charge that would stop any further reactions. THAT is why you will not have leakage from one terminal to ground or to anywhere else. That would cause an imbalance in charges, as either the anode or cathode isn't reacting and taking or replacing charges in the electrolyte.

 

[Dmytry]

Can the power supply in our house also be treated like a battery?
Then we shouldn't get a shock if we put one finger in the live wire socket until we put another finger in the neutral wire socket?

you don't get much of a shock unless you're connected to ground well enough. but yes.
Actually that is used sometimes when working on equipment in the lab / doing repairs. Isolation transformer. Then you don't really have live and neutral.
The problem is that when you try it for entire house, eventually some device is going to connect either wire to the ground due to a fault, or worse yet, some device is going to leak high voltage into it, raising both lines to very high voltage + the line voltage, which is going to damage the insulation somewhere. Also, in case of lightning strike. Or if the transformer that steps down several kv to the line voltage for the house fails, developing connection between the several kv part and the 240v (120v if you are in US) part.
In those cases, if the neutral is grounded, the current from overvoltage on live would go through the winding in the transformer onto neutral.

 

[sophiecentaur]

@DRAKKITH
My RC idea assumed that the circuit was complete, of course. That was just an extreme example to establish the principle. The fact is that whenever you connect a wire to one end of a battery there will be some small charge flow; some extra electrons will move from one end to the other until the Potential is at a minimum again. The amount of charge that will actually flow is determined by the Capacity that is introduced across the battery terminals.

 

[rcgldr]

According to this picture at hyperphysics:

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/imgele/gfault.gif

the neutral wire is grounded at the centertap, and to the same "neutral tie block" as the ground wire. I agree that it makes no sense. We've already established that 10V is not dangerous (touching the terminals of a car battery), so why not just connect the ground wire to the neutral wire instead of driving a pin into the ground to connect to the ground wire? Can't you just connect the ground wire to the neutral wire instead?

Apparently some of the current flows back through the neutral and its Earth ground points (in addition to the breaker panel, also in transmission transformers). What I don't get is if current is flowing back through the neutral and Earth ground, and if the ground wires are also tapped into the earth, then why is there a neutral to ground voltage of up to 10 volts when measured at an outlet, and also why are the "neutral" wires insulated, while the ground wires are bare copper wires?

 

[Evil Bunny]

Can the power supply in our house also be treated like a battery?
Then we shouldn't get a shock if we put one finger in the live wire socket until we put another finger in the neutral wire socket?

AC voltages, if we ignore induction, radiation, capacitive coupling, etc., will behave exactly the same way as this battery we've been talking about. If you "freeze" any instant in time, we can analyze it in the exact same way as we do a battery... The difference is that the voltage potential is constantly changing magnitude and polarity, so to discuss it in terms of a circuit you need to kind of "freeze" it first. But it behaves the same way

And you need to be careful about sticking your finger in the live wire socket because in residential power distribution, we "ground" the neutral wire... this means that the ground you are standing on is already connected to the neutral wire back at the box. But what you're thinking is exactly correct. If we didn't "ground" the neutral back at your service entrance, you could stand there and hold on to the live wire all day standing in a puddle barefoot and you would not get shocked. The only reason current will travel through you at your house (and the reason you should NOT do this) is because they intentionally connected the neutral to the ground. You would complete the circuit back to the source through the ground because they set it up that way.

All that being said... I've done this myself and nothing happens. The resistance of your body and the ground is so high that it is essentially an open. I don't recommend doing it just because you never know how resistive your body is and how close you are to the ground rod... plus if you're dealing with higher voltages, this could be extremely dangerous. My experience was with 120V. Too many variables, so nobody should be doing it. You could kill yourself so don't try it!

The power supply can continuously push current through you. The battery won't push more current through you until you can get rid of the charges on the other lead of the battery.

both sources would continue to push current through you... this is ohm's law. it comes down to the available voltage and it's ability to "push" the current through the high resistance of your body. The battery, however, would drain very quickly compared to the AC power source at your house.

My guess is you'll probably die if you stick your finger in the live wire socket, although I'm not completely sure since if the socket has a ground fault interrupter, wouldn't it trip?

Also, what would happen if the neutral wire weren't grounded/earthed? Then would you die if you touch the neutral wire? What if the neutral wire were only connected to the transformer and there was no connection between the neutral wire and the ground?

If we didn't ground anything, then you could touch the neutral all day without danger. You caould also touch the "hot" or "live wire" all day without any danger. You aren't completing the circuit. As I explained above... the only danger we have messing around with power lines at our house is due to the fact that we intentionally "ground" the neutral wire at just about every power pole (in the US anyway) in the country and again at every service entrance to every house. There is a good reason for this that could take up an entire new thread, but for the sake of this conversation, the ground is completing the circuit (EDIT FOR CLARIFICATION: when you touch a powerline at your house, not under normal operating conditions) because the power company intentionally designed the distribution system that way.

 

[Drakkith]

@DRAKKITH
My RC idea assumed that the circuit was complete, of course. That was just an extreme example to establish the principle. The fact is that whenever you connect a wire to one end of a battery there will be some small charge flow; some extra electrons will move from one end to the other until the Potential is at a minimum again. The amount of charge that will actually flow is determined by the Capacity that is introduced across the battery terminals.

And where did that extra charge come from?
I quote from Dalespam, post 3.

AlchemistK, I think you may have a misconception about how batteries work. Batteries are electrically neutral, they do not have a "higher charge relative to the bulb". What they have is a potential difference between the terminals. If you touch only one end of the bulb to one of the battery terminals then you have not put the potential difference across the bulb, so no current.

Both terminals of a battery are neutral with respect to everything else. Inside the battery there are chemicals that want to react with each other. However, without a way for electrons to flow from one electrode to another, those reactions cannot take place, as it would result in a difference in potential inside the battery.

This is not like a capacitor. You don't have a buildup of charge on one side and a depletion on the other. And quite frankly, if you are saying that an absolutely miniscule amount of charge builds up, such as like 3 electrons, come on...ignore it and move on. That most definitely does NOT have any noticeable effect on any of the examples here. I'm sure i can get a difference in charge just by touching my glass of milk, but that doesn't mean that me and my milk act like a capacitor or battery or anything. The answer to the OP's original post is still "No, the bulb wouldn't glow."

 

[AlchemistK]

If we didn't ground anything, then you could touch the neutral all day without danger. You caould also touch the "hot" or "live wire" all day without any danger. You aren't completing the circuit.

would the result vary if DC current is used?

 

[Evil Bunny]

For example, you can put battery on insulated table, and then put electrostatic charge on it (using plastic bag for example). That won't change the charges stored inside battery, you won't really charge the battery in the sense in which the battery charger does. However, this way you can make potentials on the leads be e.g. 10 000 v and 10 012 v (to ground). Then if you connect one to ground, current will flow for a short time. It's really same as if you had piece of metal in place of battery, you can put charge on it, you can get charge off it, etc. It's very confusing to put net charge onto battery lol.

You need to distinguish between two equal opposite charges 'stored' inside the battery, and the net charge of the battery.

Talk about confusion! Now we're injecting static electricity into the mix and trying to discuss it alongside the basic circuit theory of a 12V battery as a voltage source... Why make this so much more complicated than it is? Maybe I'm wrong, but I find it extremely hard to believe that the OP was referring to a static discharge off the battery that is unrelated to the voltage potential between it's posts.

Here is a question that I think is related to this discussion... Let's ignore static electricity and talk about only the voltage potential from the battery:

If we connected a copper rod (let's say 3cm diameter and 1 meter long) on it's end to the negative pole of a 12 V battery that was sitting on a well insulated platform and we encapsulated the positive pole of the battery (in an effort to rid ourselves of any stray capacitance), would electrons "rush" up to the tip of that rod to "equalize" the charges of the negative pole and the copper rod?

 

[AlchemistK]

If we connected a copper rod (let's say 3cm diameter and 1 meter long) on it's end to the negative pole of a 12 V battery that was sitting on a well insulated platform and we encapsulated the positive pole of the battery (in an effort to rid ourselves of any stray capacitance), would electrons "rush" up to the tip of that rod to "equalize" the charges of the negative pole and the copper rod?

From what i know about conduction of charges, yes.

http://www.physicsclassroom.com/class/estatics/u8l2c1.gif

 

[Drakkith]

If we didn't ground anything, then you could touch the neutral all day without danger. You caould also touch the "hot" or "live wire" all day without any danger. You aren't completing the circuit. As I explained above... the only danger we have messing around with power lines at our house is due to the fact that we intentionally "ground" the neutral wire at just about every power pole (in the US anyway) in the country and again at every service entrance to every house. There is a good reason for this that could take up an entire new thread, but for the sake of this conversation, the ground is completing the circuit because the power company intentionally designed the distribution system that way.

I don't think all of this is correct. The neutral wire is also a conductor. I have put my finger in an outlet as a child and most definitely know it shocked me. Touching either of the conductor wires, IE the hot or neutral wires, can result in being shocked as far as I know.

Also, the ground wire has nothing to do with completing the circuit. From wikipedia:

Ground or Earth in a mains (AC power) electrical wiring system is a conductor that provides a low impedance path to the Earth to prevent hazardous voltages from appearing on equipment (the terms "ground" (North American practice) and "earth" (most other English-speaking countries) are used synonymously here). Normally a grounding conductor does not carry current.

Neutral is a circuit conductor (that carries current in normal operation), which is connected to Earth (or ground) generally at the service panel with the main disconnecting switch or breaker.

To go back to the bird on a power line example, the reason the birds don't get shocked is because there is nowhere for the current to flow if it tries to go through the bird. The line is the same potential effectively everywhere (on the scale of a bird sized length of cable), and since there is nowhere for the current to go except through the line the bird is safe. Now, if the bird touches a hanging line, that's a different story that might involve toasted bird...as most likely you have two different potential lines connected by a bird.

 

[Dmytry]

Talk about confusion! Now we're injecting static electricity into the mix and trying to discuss it alongside the basic circuit theory of a 12V battery as a voltage source... Why make this so much more complicated than it is? Maybe I'm wrong, but I find it extremely hard to believe that the OP was referring to a static discharge off the battery that is unrelated to the voltage potential between it's posts.

Here is a question that I think is related to this discussion... Let's ignore static electricity and talk about only the voltage potential from the battery:

If we connected a copper rod (let's say 3cm diameter and 1 meter long) on it's end to the negative pole of a 12 V battery that was sitting on a well insulated platform and we encapsulated the positive pole of the battery (in an effort to rid ourselves of any stray capacitance), would electrons "rush" up to the tip of that rod to "equalize" the charges of the negative pole and the copper rod?

My impression is that OP is trying to unify his knowledge of electrostatics and charges with his knowledge of basic circuitry (batteries etc). That is a perfectly reasonable thing to do. Yes that is a little bit complicated and confusing, but that is what OP is confused about and that's what he wants clarified, not basic circuitry with perfect batteries.

Here, basic circuitry is not considered any more or less advanced than basic electrostatics, albeit the textbooks can often neglect to explain the relation between two, leading to the confusion like that of OP.

 

[Evil Bunny]

the neutral wire is grounded at the centertap, and to the same "neutral tie block" as the ground wire.

I agree that it makes no sense. We've already established that 10V is not dangerous (touching the terminals of a car battery), so why not just connect the ground wire to the neutral wire instead of driving a pin into the ground to connect to the ground wire? Can't you just connect the ground wire to the neutral wire instead?

The neutral is the path back to the source (transformer).
The "ground wire" is not meant to carry any current under normal conditions. It is there to give a low resistance path back to the source in the event of a fault. If the "hot" wire of your stove became disconnected and touched the metal framework of the stove, then you came along and touched it, you would receive a shock (because the system is grounded). This "ground wire" (a horrible choice of names that causes confustion) is used to bond all the metal parts in your house together and send the current back to the source on a big fat low resistance copper wire instead of through you. The hope is that this low resistance path back to the source would open an overcurrent protection device and turn that circuit off.

The National Electric Code requires that you connect this "ground" and the "neutral" together at the service entrance and not at every connection point in the house. The reason you measure a small voltage between ground and neutral is because of voltage drop on the wire.

 

[Drakkith]

From what i know about conduction of charges, yes.

http://www.physicsclassroom.com/class/estatics/u8l2c1.gif

Again, a battery does not have an excess (or buildup) of charges like your picture shows.

 

[Drakkith]

If we connected a copper rod (let's say 3cm diameter and 1 meter long) on it's end to the negative pole of a 12 V battery that was sitting on a well insulated platform and we encapsulated the positive pole of the battery (in an effort to rid ourselves of any stray capacitance), would electrons "rush" up to the tip of that rod to "equalize" the charges of the negative pole and the copper rod?

No. There is no difference in potential or charges.

 

[Drakkith]

Alright...i see.

So the bulb should light in the above case if instead of a battery, a negatively charged body is used?

To finish off your original questions, YES. If you connected a source of negatively charged particles to your light bulb, they would discharge into your circuit and light bulb until the charges equalized. Would this light up the bulb? Depends on the amount of current you had flowing and such.

The difference between this and the battery is that a battery is NOT a source of extra built up charges. Static electricity is however. An extreme example of this is Lightning. If lightning struck one terminal of your bulb you can bet your singed face that the bulb "lit up". (And more)

 

[Evil Bunny]

I don't think all of this is correct. The neutral wire is also a conductor. I have put my finger in an outlet as a child and most definitely know it shocked me. Touching either of the conductor wires, IE the hot or neutral wires, can result in being shocked as far as I know.

Also, the ground wire has nothing to do with completing the circuit. From wikipedia:

You were most likely shocked because you got your finger across both terminals, but like I said.. there are a lot of variables and the possibility exists. I've done it as an adult and got nothing off of either post individually and gotten a shock when I got across them...

Anyway, the only way one would get shocked by touching only one of the terminals is if the current traveled through you and went through the ground to find it's way back to the source.

To be clear... I know the ground wire has nothing to do with completing the circuit under normal conditions. It's there to complete the circuit only if something goes wrong. If that point didn't come through in my explanations, I apologize because it is an important one.

 

[AlchemistK]

So the only problem is that a battery requires electrons to return back to the other terminal to show any significant movement of charges.
Any voltage source that does not show this property?

 

[Evil Bunny]

If we connected a copper rod (let's say 3cm diameter and 1 meter long) on it's end to the negative pole of a 12 V battery that was sitting on a well insulated platform and we encapsulated the positive pole of the battery (in an effort to rid ourselves of any stray capacitance), would electrons "rush" up to the tip of that rod to "equalize" the charges of the negative pole and the copper rod?

No. There is no difference in potential or charges.

OK thank you. This is the point I have been trying to make throughout this entire thread and it is being met with great resistance.

I believe (but I'm not certain) that this gets to the bottom of what the OP was asking...