Register to reply

Circuit completion : is it necessary?

by AlchemistK
Tags: circuit, completion
Share this thread:
Evil Bunny
#91
Apr28-11, 07:41 AM
P: 237
Quote Quote by AlchemistK View Post
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?
No. All voltage sources require a return.
Studiot
#92
Apr28-11, 08:06 AM
P: 5,462
voltage sources
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
#93
Apr28-11, 09:42 AM
P: 505
Ahh, and to actually answer the question.
Quote Quote by Evil Bunny View Post
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:
Did you ever understand how 'battery electricity' relates to 'static electricity' ?

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?
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
#94
Apr28-11, 10:27 AM
P: 237
Quote Quote by Dmytry View Post
that is depend to which potential the negative pole was at,
with respect to what?

Quote Quote by Dmytry View Post
what potential the rod was at,
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.

Quote Quote by Dmytry View Post
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 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
#95
Apr28-11, 11:10 AM
P: 158
Quote Quote by Evil Bunny View Post

You keep talking about potential as if you can have it at one point.
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
#96
Apr28-11, 11:36 AM
Sci Advisor
Thanks
PF Gold
sophiecentaur's Avatar
P: 12,130
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
#97
Apr28-11, 12:00 PM
P: 505
Quote Quote by Evil Bunny View Post
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.
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)



The ground is irrelevant. we're on an insulated platform. The positive pole of the battery is irrelevant. we encapsulated it.
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.
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.
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
#98
Apr28-11, 12:08 PM
Sci Advisor
Thanks
PF Gold
sophiecentaur's Avatar
P: 12,130
Quote Quote by Dmytry View Post
1:
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.
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
#99
Apr28-11, 03:20 PM
P: 969
Quote Quote by Evil Bunny View Post
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.
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
#100
Apr28-11, 03:37 PM
P: 969
Quote Quote by rcgldr View Post
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?
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
#101
Apr28-11, 04:53 PM
Sci Advisor
Thanks
PF Gold
sophiecentaur's Avatar
P: 12,130
Quote Quote by RedX View Post

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?
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
#102
Apr28-11, 05:04 PM
P: 237
Quote Quote by RedX View Post
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.
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...


Quote Quote by RedX View Post
I have no idea why neutral wires are insulated. If anything only the hot wires need to be insulated.
They carry just as much current as the hot wires in the branch circuits of your house under normal operating conditions.

Quote Quote by RedX View Post
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.
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".

Quote Quote by RedX View Post
I don't understand why a third prong is needed for ground...
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.
Evil Bunny
#103
Apr28-11, 05:06 PM
P: 237
Quote Quote by sophiecentaur View Post
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???
Yes I do... I will try to explain further later... no time right now... I just touched on it a little bit in my previous post
sophiecentaur
#104
Apr28-11, 05:24 PM
Sci Advisor
Thanks
PF Gold
sophiecentaur's Avatar
P: 12,130
If the neutral were allowed to float anywhere and a low resistance load applied to one of the phases then the volts on the other two phases could end up as 400V instead of 240V. That would not be good. Why does no one else consider the three phase thing in this discussion? There is no major system of electricity generation that does not use three phases so it is a major consideration.
Evil Bunny
#105
Apr28-11, 05:42 PM
P: 237
Quote Quote by sophiecentaur View Post
If the neutral were allowed to float anywhere and a low resistance load applied to one of the phases then the volts on the other two phases could end up as 400V instead of 240V. That would not be good. Why does no one else consider the three phase thing in this discussion?
Most houses (if not all) in the US are wired with single phase. Yes, it originates as three phase (upstream) but we're only using one of the phases. This third prong on the plug receptacles that people are calling the "ground wire" (whose technical term is an "equipment grounding conductor") has absolutely nothing to do with the neutral whatsoever. It is used for bonding the metal parts in your house.
sophiecentaur
#106
Apr28-11, 05:58 PM
Sci Advisor
Thanks
PF Gold
sophiecentaur's Avatar
P: 12,130
Quote Quote by Evil Bunny View Post
Most houses (if not all) in the US are wired with single phase. Yes, it originates as three phase (upstream) but we're only using one of the phases. This third prong on the plug receptacles that people are calling the "ground wire" (whose technical term is an "equipment grounding conductor") has absolutely nothing to do with the neutral whatsoever. It is used for bonding the metal parts in your house.
I know that you're only using one of the phases but other users are using the other two phases and the neutral is common to all users and the system has to work as a whole. This neutral conductor is sure to be near Earth potential, for symmetry reasons. If you look at massive electrical distribution lines there is one tiny 'fourth conductor' which is strung from pylon to pylon.

As you say, the reason for a local ground / earth is to give you something to hang all exposed metal onto. This means that you can never get many volts between yourself, your feet and anything you are likely to touch as long as you are not actually poking about inside some equipment.
rcgldr
#107
Apr28-11, 06:01 PM
HW Helper
P: 7,108
Quote Quote by sophiecentaur View Post
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.
Could a moderator split up this thread into two threads then?
sophiecentaur
#108
Apr28-11, 06:02 PM
Sci Advisor
Thanks
PF Gold
sophiecentaur's Avatar
P: 12,130
Quote Quote by rcgldr View Post
Could a moderator split up this thread into two threads then?
A lobotomy?


Register to reply

Related Discussions
Simple circuit completion counter Electrical Engineering 5
Incomplete and completion Differential Geometry 2
Completion of a metric space Calculus 7
What exactly the completion of a space is Calculus & Beyond Homework 25
Completion problem Introductory Physics Homework 7