Circuit completion : is it necessary?

Drakkith

No. There is no difference in potential or charges.

Drakkith

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

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 travelled 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

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...

Evil Bunny

No. All voltage sources require a return.

Studiot

Have a care with the use of this term.

I think you mean a general device which supplies electrical energy.

However the term voltage source has a much narrower and more precisely defined meaning in circuit theory.

Incidentally much confusion arises when special definitions from circuit theory are introduced into a discussion about physics and vice versa.

Circuit theory is much more specialised and limited than Physics and in particular makes certain assumptions for ease of analysis and calculation, that are not realisable in the real world .

Dmytry

Ahh, and to actually answer the question.
Did you ever understand how 'battery electricity' relates to 'static electricity' ?
that is depend to which potential the negative pole was at, what potential the rod was at, and what stray capacitance both the positive and the negative pole have vs the rod and the ground. The charge that would flow equals to capacitance between the rod and both of the battery poles, multiplied by the difference in potentials. If the capacitance is zero, the charge will be zero.

The OP is clearly trying to find a relation between his knowledge of 'static electricity' and 'battery electricity', into some sort of unified understanding. Assuming that parasitic capacitances are zero does not permit unified understanding.

The situation is such that e.g. 2 A*h battery has a charge of 7 200 Coulomb stored chemically inside.
The stray capacitance of, say, 1pF at potential difference of 12v stores 0.00 000 000 012 Coulombs.

That is why it is very common to neglect stray capacitances when discussing batteries.

edit: whoops mixed up mAh vs Ah

Evil Bunny

with respect to what?

with respect to what?

You keep talking about potential as if you can have it at one point. You can't. Potential at one point is with respect to another point... In this case we're talking about the potential between the rod and the negative pole. This potential is ZERO VOLTS in this case. No current flowed. No light bulb lit.

The ground is irrelevant. we're on an insulated platform. The positive pole of the battery is irrelevant. we encapsulated it.

You're trying to invent capacitance where it doesn't exist.

The answer to the question in post #1 is the bulb absolutely will not light. No way it can. This stray capacitance that may or may not exist will in no way provide enough current for any glow to occur.

AlchemistK

Potential is taken at one point, its the potential difference which is relative.

The electric potential at a point in an electric field is defined as the work done in moving a unit charge from infinity to that point.

sophiecentaur

This thread has bi-polar disorder, I think. On the one hand we have the transformer / live/ neutral discussion and, on the other hand, we have the connect a battery and what happens discussion. Whilst it's all 'Electricity', they two ideas are interfering with each other. It's OK for someone who knows about these things but, for someone fresh, it makes things much harder than necessary.

The power engineering discussion is made complicated by the fact that the neutral we all know and love in the UK is fairly different from the Neutral that goes into many US homes. Essentially, the Neutral which a three phase distribution system takes very little current, when the three phases are loaded equally. This keeps the Neutral volts pretty near Earth all the time. The Neutral that goes into the US home, as I understand, comes from a local transformer which is centre tapped to give both a 110V and 220V supply. It is, inherently, not balanced and, depending on the lopsidedness of a typical domestic load during the day, you could expect some much higher volts on a Neutral. You really don't want lots of earth currents sloshing around your domestic pipes and steel frames as it can cause embarrassing Hum, for a start. Current in the Neutral cable runs right next to the Live current and there is much less magnetic field generated. So - using the Neutral for return makes sense - else, why not have a single line supply to the home and rely on Earth as a return? Think of the copper you'd save!

Dmytry

1: Check electric potential on wiki.
http://en.wikipedia.org/wiki/Electric_potential
2: I was speaking of difference of potentials. Not a single result depended on the absolute potential (which i suppose is defined as energy of unit charge brought in from far outer space)
how do you encapsulate something to totally eliminate the capacitance?
I'd really love to know, because i am building some electronic circuit where I have to calculate capacitance of every piece of wire.
I told already that an incandescent lightbulb won't light up, but if the voltage is, say, 200v, and if it is a neon lamp, and if capacitances are realistic, it will lit up briefly and visibly (in a dark room).

sophiecentaur

Exactly. And every piece of wire you hang on the battery will acquire a small charge. It's hard to reduce stray capacitance to less than one or two pF.

I find it bizarre that we are discussing picoCoulombs on the one hand and Mains supply currents on the other.

RedX

I'm not sure about this. If the neutral wire weren't grounded, I think you would still get shocked if you simultaneously touched the neutral wire (or the hot wire) and a piece of metal plumbing that goes to the ground. But because the neutral wire is grounded, you can touch the neutral wire and a piece of plumbing and be fine. You seem to be saying that if the neutral wire weren't grounded, then the only way to get shocked is to touch the hot wire and the neutral wire at the same time (you could touch the hot or neutral wire [but not both] and the plumbing at the same time and have no fear), which is consistent with your one-terminal battery connection position, but I don't think that's right.

Also when you say you can't get shocked by touching just the hot wire, I'm doubtful about that too. I understand there is a lot of resistance between you and the ground, but I hear a lot of warnings about not sticking your finger in the socket. The current would have to travel through you, through the floor, and through metal piping which sounds like a lot of resistance, but it only takes a little current to kill you.

RedX

I have no idea why neutral wires are insulated. If anything only the hot wires need to be insulated.

The fact there is 10 volts between the neutral and ground suggests that maybe you are right and neutral only connects to the center tap, where it has been reduced to 10V with respect to the ground by previous transformers. But according to hyperphysics, the neutral and ground wire are physically tied together at a location and driven into the ground before the center-tap location.

I don't understand why a third prong is needed for ground. Can't you just plug the chassis of an appliance directly to neutral? Or is that what is done? Because I notice some appliances only have two prongs. Does this mean the chassis of the appliance shares the same connection to neutral as the appliance itself, i.e. two wires connect to the neutral prong?

sophiecentaur

This, as I have already said, is because the neutral is a three phase invention. Letting it float is good for keeping the transmissions on each of the three phases balanced. Anybody disagree with that???
Three phase generation is a great invention but it does generate a lot of conceptual problems.

Evil Bunny

Ok... let's simplify the circuit.

You have a battery and a resistor. You connect one end of the resistor to one terminal of the battery. Does anything happen? No. No complete circuit. No current flow.

Now replace the battery with an AC source. No grounds, no plumbing, nothing but an AC source with two terminals. Now.... connect one end of that same resistor to only one terminal of the AC source. Does anything happen? No. No complete circuit. No current flow.

Now replace the resistor with yourself... see my point now?

Introducing "ground" into the conversation complicates things and confuses people...


They carry just as much current as the hot wires in the branch circuits of your house under normal operating conditions.

The neutral is connected to the center tap. It's also connected to a ground rod at your service entrance and at the utility pole. Again... the reason you measure a small voltage between the "ground wire" and the neutral in your house is because of voltage drop. You are only allowed to connect the neutral and the "ground wire" together at the service entrance. You are not allowed to connect them together anywhere else in your house (per the NEC in the US). The further away from the service entrance you get, the greater the voltage reading will be between neutral and the "ground wire".

For safety reasons.. In the US there never used to be a 3rd prong and everything worked just fine, except that under fault conditions metal appliances had voltage present with respect to ground and it was potentially dangerous.

They added a "ground wire" (third prong) that bonded all the metal things in your house together and tied it together with the neutral (at the service entrance) to create a parallel path back to the source. The reason they did this was in case of a fault, this current would travel back along the low resistance "ground wire" back to the source instead of through your body. If you simply tied everything to the neutral, then under "normal conditions" you would have current flowing through the neutral AND through all of the metal things in your house. This is not a desirable situation.