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Circuit completion : is it necessary?

by AlchemistK
Tags: circuit, completion
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Vespa71
#19
Apr26-11, 09:20 PM
P: 45
Quote Quote by AlchemistK View Post
...the battery still has a higher charge relative to the bulb, so electrons must still go to the bulb..
I shall humbly offer my layman's understanding of the issue. Your argument, if I understand correctly, is that a stream of electrons (in or out), from battery-terminal potential-equalizing, should give light to the bulb for at least a short while, when the bulb is connected by one terminal to one from the battery.

I would compare the bulb to a hydraulic engine. If you open for pressurized liquid to enter engine, while the exit flow-valve of engine is closed, you would get pressure in the engine, but no work would be done, as that depends of flow. There would be some compression of the liquid though, and this may allow some insignificant movement of the engine-rotor, as the pressures equalize through the engine.

How eletrical charge compresses/expands in a lightbulb glow-wire is not known to me, but low volt DC will only make one, small adjustment of electron-numbers. A high volt ac-terminal would make the electrons compress and expand repeatedly, and faster, thus maybe make a glow of duration in a sensible (low-volt) bulb.

Magnetically induced AC in short circuit glow-wire will however make a bulb glow, entirely without terminals. Much better (and safer) party-trick. Maybe shortcircuit a normal bulb and lay it on an induction cooker-plate. General warnings, of course
Vespa71
#20
Apr26-11, 09:45 PM
P: 45
Quote Quote by DaveC426913 View Post
So you're suggesting I cannot get a shock by grabbing the positive terminal of a car battery while standing barefoot in a puddle of water.
Leaking from a cars battery occurs not because (-) is in the body of the car, but because (+) leaks to body, unintentionally.

And if you get a shock, negative pole would not be carried through the water and bare feet, but the body of the car. The car is insulated from the ground, normally, with rubber wheels.
DaveC426913
#21
Apr26-11, 10:21 PM
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Quote Quote by Vespa71 View Post
Leaking from a cars battery occurs not because (-) is in the body of the car, but because (+) leaks to body, unintentionally.

And if you get a shock, negative pole would not be carried through the water and bare feet, but the body of the car. The car is insulated from the ground, normally, with rubber wheels.
Are you too suggesting that, if I grabbed the positive terminal of a car battery while standing barefoot in a puddle of water, I would not get a shock? Is that what you're saying?

Because that's what Evil Bunny is claiming in post 11:

Quote Quote by Evil Bunny View Post
You cannot drain a battery without completing a circuit from the positive terminal to the negative terminal.

Connecting a wire from one terminal of your battery to the earth, even if we drove a 10-foot ground rod into the earth and hooked the wire to it, would not drain anything from the battery.
RedX
#22
Apr26-11, 10:47 PM
P: 969
Quote Quote by DaveC426913 View Post
Are you too suggesting that, if I grabbed the positive terminal of a car battery while standing barefoot in a puddle of water, I would not get a shock? Is that what you're saying?
Would the potential between the positive terminal and the ground be 6 volts (assuming a 12 volt battery)?
Drakkith
#23
Apr27-11, 03:11 AM
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Quote Quote by DaveC426913 View Post
Are you too suggesting that, if I grabbed the positive terminal of a car battery while standing barefoot in a puddle of water, I would not get a shock? Is that what you're saying?

Because that's what Evil Bunny is claiming in post 11:
My co-worker next to me works on cars all the time and says that he touches the terminals on many occasions and NEVER gotten a shock from them. Even touching both terminals at the same time. Perhaps the resistance of the human body is too high?
Evil Bunny
#24
Apr27-11, 05:10 AM
P: 237
Quote Quote by DaveC426913 View Post
So you're suggesting I cannot get a shock by grabbing the positive terminal of a car battery while standing barefoot in a puddle of water.
You absolutely would not get shocked... not one little bit.

Put your volt meter leads on the positive post and the puddle of water.

0 volts I promise you.
Evil Bunny
#25
Apr27-11, 05:12 AM
P: 237
Quote Quote by RedX View Post
Would the potential between the positive terminal and the ground be 6 volts (assuming a 12 volt battery)?
if "the ground" means where you bolted the negative post to the metal car.. then you'd measure 12 volts across the positive and ANYTHING metal in the car.

If "the ground" means the actual ground you're standing on then you would measure 0V between that and the positive terminal.
Evil Bunny
#26
Apr27-11, 05:23 AM
P: 237
The danger from cars comes from getting your screw driver across the positive and another metal part of the car... This is because they attached the negative terminal to the metal frame of the car in order to intentionally create a chassis ground. They have intentionally created a return path with the metal parts of the car... this return path is returning the charge back to the source (the battery). Nothing is leaving the battery and ending up in the earth.
Dmytry
#27
Apr27-11, 05:39 AM
P: 505
Quote Quote by AlchemistK View Post
It is known that a voltage must be applied around a closed circuit to make current pass through it. So thats why a bulb doesn't glow without connecting both terminals, right?

But, suppose only one terminal of the bulb is attached to a battery and the circuit is closed, the battery still has a higher charge relative to the bulb, so electrons must still go to the bulb, to even out the charge, so why doesn't the bulb glow,even if it is for a small time period?....or does it?
It would heat filament a little... try doing it with neon bulb, it will actually glow if you apply the voltage to one lead. Especially noticeable with AC voltage, so the lamp would appear to be lit continuously.
Try it, hold neon bulb by the glass and connect one of leads to mains phase. VERY carefully not to kill yourself. Works well here (240v mains) . The bulb has capacitance.
Dmytry
#28
Apr27-11, 05:42 AM
P: 505
Quote Quote by Evil Bunny View Post
The danger from cars comes from getting your screw driver across the positive and another metal part of the car... This is because they attached the negative terminal to the metal frame of the car in order to intentionally create a chassis ground. They have intentionally created a return path with the metal parts of the car... this return path is returning the charge back to the source (the battery). Nothing is leaving the battery and ending up in the earth.
Precisely. You must always take off any jewellery when working with car as the jewellery may get electrically heated, and you must disconnect negative terminal first AFAIK, so that you don't risk to short out with a wrench when you disconnect positive terminal.

Quote Quote by Drakkith View Post
My co-worker next to me works on cars all the time and says that he touches the terminals on many occasions and NEVER gotten a shock from them. Even touching both terminals at the same time. Perhaps the resistance of the human body is too high?
Yep, the skin resistance is too high, in particular. It is also voltage dependent. At 12 volts, its many tens thousand ohms, so the current is fraction of milliamp. That is why battery terminals are not protected, and your 120v or 240v mains are.

However, I recall hearing that some guy killed himself with 9v battery, by piercing the skin with electrodes.
Evil Bunny
#29
Apr27-11, 06:38 AM
P: 237
We are introducing capacitive coupling and induction into this conversation now and I hope it's not confusing the OP.

Yes, when we move into the realm of AC induction the electromagnetic fields will induce voltages into wires that are not physically connected to any voltage source... this is completely different than what was discussed in the OP.

If we get back to the DC circuit and the light bulb, and forget about capacitive coupling or induced voltages... We are talking about the very basics of electricity.

The laws of physics dictate that we need a complete circuit, a closed loop, for current to flow. If we are talking about a basic 12 volt battery, the loop must start at one terminal... and end up at the other terminal... If we start at one terminal and end up at some other point (like the earth, for example, or a mud puddle, or the metal frame of a car), then we have not closed the loop. We have not made it back to the other terminal. No current flowed.

If you somehow made current flow out of a post on a 12 volt battery without having it return on the other post, then I think you should probably start writing it up...
Dmytry
#30
Apr27-11, 07:42 AM
P: 505
Quote Quote by Evil Bunny View Post
We are introducing capacitive coupling and induction into this conversation now and I hope it's not confusing the OP.

Yes, when we move into the realm of AC induction the electromagnetic fields will induce voltages into wires that are not physically connected to any voltage source... this is completely different than what was discussed in the OP.

If we get back to the DC circuit and the light bulb, and forget about capacitive coupling or induced voltages... We are talking about the very basics of electricity.

The laws of physics dictate that we need a complete circuit, a closed loop, for current to flow. If we are talking about a basic 12 volt battery, the loop must start at one terminal... and end up at the other terminal... If we start at one terminal and end up at some other point (like the earth, for example, or a mud puddle, or the metal frame of a car), then we have not closed the loop. We have not made it back to the other terminal. No current flowed.

If you somehow made current flow out of a post on a 12 volt battery without having it return on the other post, then I think you should probably start writing it up...
He's not discussing DC though... he's asking what will happen when he connects the battery to the bulb, which is not a steady state, and he is absolutely right that the current will flow until potentials equalize, and it really is the case that neon bulb will lit up for a short time when he connects it. You can say in this case that the circuit is completed by capacitance between other battery terminal and the bulb.
Evil Bunny
#31
Apr27-11, 08:10 AM
P: 237
How is connecting a battery to a bulb not considered DC?

Not sure how you can have any capacitance between a battery post and a light bulb, but I suppose it could be a possiblity...
Drakkith
#32
Apr27-11, 08:32 AM
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Quote Quote by Dmytry View Post
He's not discussing DC though... he's asking what will happen when he connects the battery to the bulb, which is not a steady state, and he is absolutely right that the current will flow until potentials equalize, and it really is the case that neon bulb will lit up for a short time when he connects it. You can say in this case that the circuit is completed by capacitance between other battery terminal and the bulb.
That is DC. And what capacitance are you referring to?
AlchemistK
#33
Apr27-11, 09:05 AM
P: 158
Alright, so the bulb WILL glow, even if it is for an undetectable period of time. Well since the bulb can glow, we can also get a shock, by just touching a high voltage source, without completing the circuit.

Right?
Evil Bunny
#34
Apr27-11, 09:27 AM
P: 237
No. Bulb won't glow. You won't get a shock if only one post of the battery is involved.
AlchemistK
#35
Apr27-11, 09:28 AM
P: 158
Could we please get at one correct conclusion? someone said the bulb will glow.
Studiot
#36
Apr27-11, 09:33 AM
P: 5,462
When I saw the title of this thread I knew trouble was brewing.

And as always we have all sorts of red herrings introduced.

The simple fact is that if you move even one single unit of charge from a to b you have, by definition a current.

The fifth picture in this article shows a classic where a man's hair is standing on end because of proximity to a Van De Graaff generator.

http://en.wikipedia.org/wiki/Van_de_Graaff_generator

Capacitance, inductance, resonance or any other ants is not involved.

Some charge passes from the generator ball to the man by travelling through the resistive path formed in the air between him and the generator ball.

This is by definition a current.

No circuit is involved or completed.

Because the man is insulated the charge does not proceed further and spreads to his extremities, causing his hairs to separate.

If you bring your knuckles close to the ball you can actually feel the small charges jumping the gap and impinging upon your flesh.


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