Circuit completion : is it necessary?

  • Thread starter AlchemistK
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In summary: In your example, one of the terminals of the battery (most likely the negative) was (edit: probably) fastened to the chassis framework of the car to use it as an intentional return path back to the source (the battery). It found it's way back to the battery through the metal parts of the car... It didn't drain itself into the ground through the tires, I assure you.
  • #36
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 traveling 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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  • #37
AlchemistK said:
Could we please get at one correct conclusion? someone said the bulb will glow.

I don't see how the bulb can glow if there isn't a difference in potential between the two terminals of it. Which, as far as I know, there shouldn't be if you only connect one side of a battery to it.
 
  • #38
The light will not light if we're talking a battery and a light bulb.

That is the one correct conclusion. The other conclusion is incorrect, by definition.

:rofl:
 
  • #39
Don't think of the source of charge as a battery, think of it as a simply charged body.

Suppose we remove the variable of Earth and everything else and conduct the experiment in ideal situations, would your touching a current carrying wire (remember your not touching anything else) make you experience a shock?
 
  • #40
Well jumping from a battery scenario to a static electricity scenario completely changes the entire conversation.

You don't typically talk about circuits with static electricity. You're more likely to be talking about discharge then. Studiot just hit on that pretty nicely
 
  • #41
Yeah, but one more thing, don't people say that birds don't get killed while perching on live wires because they don't complete the circuit? Shouldn't charge flow into the birds ?
 
  • #42
AlchemistK said:
Don't think of the source of charge as a battery, think of it as a simply charged body.

Suppose we remove the variable of Earth and everything else and conduct the experiment in ideal situations, would your touching a current carrying wire (remember your not touching anything else) make you experience a shock?

Not if the electrons had nowhere to go to and the conductor was electrically neutral. (Which a current carrying conductor is)
 
  • #43
AlchemistK said:
Yeah, but one more thing, don't people say that birds don't get killed while perching on live wires because they don't complete the circuit? Shouldn't charge flow into the birds ?

The resistance of a bird is much much higher than the conductor itself. There really isn't any reason the current would want to flow through the bird just to get back to the same conductor when it could simply keep on moving through the conductor.
 
  • #44
It wouldn't be that all the current would just pass by, even if it has a greater resistance, there would still be some current flowing though the bird.
 
  • #45
There is no potential between the bird's first leg and his second leg. No current flows. I suppose in theory you might get some nano amps through the bird but its insignificant.
 
  • #46
Why is there not a potential difference? the wire is positively charged and the bird is neutral. Voila!
 
  • #47
AlchemistK said:
Why is there not a potential difference? the wire is positively charged and the bird is neutral. Voila!

The wire is not positively charged.
 
  • #48
sorry i meant negatively. My bad. Slip of...keys.
 
  • #49
Its not negatively charged either
 
  • #50
@.@ its not? but its got electrons flowing through it.
 
  • #51
There is no "charge" on the wire at all... Current is flowing through that wire and returning on the neutral wire (usually several feet below the "hot" wire or wires).

There is no "charge" on either wire. There is a potential difference between the hot wire and the neutral, but they aren't "charged" in the static electricity sense. Just like there is no "charge" on the positive terminal of a battery. Just like there is no "charge" on the negative terminal of a battery.

A thundercloud overhead is "charged" and it will "discharge" to the ground when the potential gets high enough and we will see a bolt of lightning between the cloud and the ground.
 
  • #52
Wait, we say something is charged if it has more positively charged particles than negatively charged ones or vice versa. If we connect two oppositely charged bodies, by say a wire, the charge will be transferred, to make the system more uniformly charged, if not neutral.
So, actually, the negative mobile particles, electrons for example, get transferred to the wire, making it negatively charged, and then to the positive body(or even a neutral one) making the system more electrically stable.
Indicating that the wire is charged at some unit time.
 
  • #53
I can't explain it any better than http://en.wikipedia.org/wiki/Electric_charge" [Broken] :

"Static electricity and electric current are two separate phenomena, both involving electric charge, and may occur simultaneously in the same object. Static electricity is a reference to the electric charge of an object and the related electrostatic discharge when two objects are brought together that are not at equilibrium. An electrostatic discharge creates a change in the charge of each of the two objects. In contrast, electric current is the flow of electric charge through an object, which produces no net loss or gain of electric charge. Although charge flows between two objects during an electrostatic discharge, time is too short for current to be maintained."

(Edit: bolded a portion of the quote for emphasis)
 
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  • #54
Alright, maybe there is no NET change in charge but what oh charge at a specific time?
 
  • #55
Drakkith said:
That is DC. And what capacitance are you referring to?
Capacitance between other lead of battery and the bulb.
The point is, it is not steady state right at the moment when you connect something, there are AC components. You can do a Fourier transform on t<0 : f(t)=0, t>=0 : f(t)=1 edit: actually not a good idea, just do Fourier on a step, when you turn something on, then turn something off.

Also, the leads of battery can be at any voltages, the battery only ensures potential difference. E.g. 1.5v battery can have leads at 0v and 1.5v or at -1.5v and 0v, or at 100v and 101.5v . If you just let battery sit with equal leakage from both terminals to ground it will be at +0.75v and -0.75v but it will take a while in practice and you are unlikely to have equal leakage. That is why when you connect positive terminal of one battery to negative terminal of another battery through lightbulb (and leave the other terminals disconnected), there won't be continuous current (but there may be transient when you connect due to parasitic capacitances). That's why you can connect batteries in series and get more voltage.

The OP's question is not because he doesn't know current won't flow continuously, it's because he knows that some charge has to be moved, and he is totally correct in that. You guys however really just confuse him. The charge is very small though.
 
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  • #56
AlchemistK said:
Yeah, but one more thing, don't people say that birds don't get killed while perching on live wires because they don't complete the circuit? Shouldn't charge flow into the birds ?
There's already a completed circuit on live wires, assuming that something(s) at the end of the transmission lines is using the electricity supplied by the wires. The birds provide a high resistance and very short parallel path to the wire and only receive a tiny amount of currrent. It is possible to draw current from such wires with a transformer and/or antenna like setup (it's also illegal), and there's enough strength in the electrical field from those high voltage live wires to light up florescent bulbs stuck in the ground.

http://gadgets.boingboing.net/2009/04/10/uk-farm-grows-1301-f.html

batteries
The amount of charge on the terminals of a battery is very small, and that tiny amoung charge quickly dissipates if you connect the terminal to a large plate or Earth ground without completing the circuit. If some large conducting plate were altnerately and rapidly switched between the two terminals, eventually the battery could be drained, but it would take a very long time. Unlike a capacitor which can have a high amount of charge on it's plates a battery has a low amount of charge on its terminals relies on an inernal chemical reaction to produce current, which requires circuit completion.

battery draining in car
The primary source of leakage is probably the rectifier and/or any capacitors in the alternator used to charge the battery when the engine is running. I'm not sure this is an issue with modern cars.

circuit completion
Current can flow if you have a device that can generate a charge at one end, and another device that can drain that charge at the other end. Unless this is done in a vacuum like outer space, eventually the charge will flow back through the Earth and/or air, so there is eventual circuit completion, but it's not required to produce the original current flow.
 
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  • #57
Dmytry said:
Capacitance between other lead of battery and the bulb.
The point is, it is not steady state right at the moment when you connect something, there are AC components. .

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

Dmytry said:
Also, the leads of battery can be at any voltages, the battery only ensures potential difference. E.g. 1.5v battery can have leads at 0v and 1.5v or at -1.5v and 0v, or at 100v and 101.5v .

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.

Dmytry said:
If you just let battery sit with equal leakage from both terminals to ground it will be at +0.75v and -0.75v but it will take a while in practice and you are unlikely to have equal leakage.

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...


Dmytry said:
That is why when you connect positive terminal of one battery to negative terminal of another battery through lightbulb (and leave the other terminals disconnected)

what other terminals? There are two terminals on a battery and two terminals on a light bulb... if you connected this up as stated, which terminals are disconnected?

Dmytry said:
The OP's question is not because he doesn't know current won't flow continuously, it's because he knows that some charge has to be moved, and he is totally correct in that. You guys however really just confuse him. The charge is very small though.

Saying that a charge will move through a light bulb when you connect only one lead of a battery to it does not make it so... Your explanation is that a wire and a lightbulb somehow make up a capacitor that can charge itself, but I'm not sure this is an accurate statement.

rcgldr said:
The amount of charge on the terminals of a battery is very small, and that tiny amoung charge quickly dissipates if you connect the terminal to a large plate or Earth ground without completing the circuit. If some large conducting plate were altnerately and rapidly switched between the two terminals, eventually the battery could be drained, but it would take a very long time. Unlike a capacitor which can have a high amount of charge on it's plates a battery has a low amount of charge on its terminals relies on an inernal chemical reaction to produce current, which requires circuit completion.

This is an interesting statement. Do you have a good source for information on this? I have spent considerable time in the past looking for information about charge buildup from voltage sources and I haven't been able to find a thing...
 
  • #58
rcgldr said:
The amount of charge on the terminals of a battery is very small, and that tiny amoung charge quickly dissipates if you connect the terminal to a large plate or Earth ground without completing the circuit.

Evil Bunny said:
This is an interesting statement. Do you have a good source for information on this? I have spent considerable time in the past looking for information about charge buildup from voltage sources and I haven't been able to find a thing.

Take a look at post #21 in this thread:

https://www.physicsforums.com/showthread.php?p=3116670&postcount=21
 
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  • #59
rcgldr said:

That's what I thought, so if you hook up a light bulb to just one terminal, some charge will flow to the light bulb, and on the other battery terminal the charge just stays on so that the potential between that end of the battery and the bulb terminal is equal to the voltage of the battery. But will this cause the bulb to flash when it just receives current from one terminal?

Also, if you connect a voltmeter to one end of the battery, and the other end of the voltmeter to the ground, then charge should flow from the battery to the ground and a voltage should be recorded in the voltmeter. But this would imply that a 12 V battery should register 6V when connecting just one terminal to the ground, which doesn't seem correct. Is it because the battery does not replenish the charge on that terminal until it can accept the same amount of charge on the other terminal, so that there is not enough charge for a sustained current? So initially the voltmeter would record 6 volts but will quickly drop to zero as the terminal runs out of charge?
 
  • #60
RedX said:
Also, if you connect a voltmeter to one end of the battery, and the other end of the voltmeter to the ground, then charge should flow from the battery to the ground and a voltage should be recorded in the voltmeter. But this would imply that a 12 V battery should register 6V when connecting just one terminal to the ground, which doesn't seem correct.
It isn't. As mentioned in that post, the effective charge is miniscule, less than 10-11 Coulombs, and so will be the voltage.
 
  • #61
rcgldr said:
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 [tex]V_+ [/tex] and [tex]V_- [/tex] as the voltages between the positive terminal and the ground, and the negative terminal and the ground, respectively, then [tex]V_+-V_-=12 [/tex] volts, and why not assume symmetry to get [tex]V_+=V_-=6 [/tex] volts?
 
  • #62
RedX said:
But the voltage between the two charges on the terminals of the battery is 12 V. So if you define [tex]V_+ [/tex] and [tex]V_- [/tex] as the voltages between the positive terminal and the ground, and the negative terminal and the ground, respectively, then [tex]V_+-V_-=12 [/tex] volts, and why not assume symmetry to get [tex]V_+=V_-=6 [/tex] 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.
 
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  • #63
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.
 
  • #64
Evil Bunny said:
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.
 
  • #65
rcgldr said:
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? :confused:

A: Not more than one, for then they can't agree on what will happen! :rofl:
 
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  • #66
AlchemistK said:
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.
 
  • #67
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?
 
  • #68
AlchemistK said:
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?
 
  • #69
rcgldr said:
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.

RedX said:
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.

RedX said:
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.
 
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  • #70
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.
 
<h2>1. Is circuit completion necessary for a circuit to function?</h2><p>Yes, circuit completion is necessary for a circuit to function properly. Without a complete circuit, electricity cannot flow and the circuit will not work.</p><h2>2. What happens if a circuit is not completed?</h2><p>If a circuit is not completed, electricity will not flow and the circuit will not work. This can result in a device or system not functioning properly or at all.</p><h2>3. How can I tell if a circuit is complete?</h2><p>You can tell if a circuit is complete by using a multimeter to measure the voltage and resistance of the circuit. If there is a complete path for electricity to flow, the multimeter will show a reading. You can also visually inspect the circuit for any breaks or disconnected components.</p><h2>4. Can a circuit be completed in different ways?</h2><p>Yes, there are different ways to complete a circuit. For example, a series circuit is completed when electricity flows through each component in a single path, while a parallel circuit is completed when electricity flows through multiple paths. Additionally, circuits can be completed using different materials such as wires, conductive metals, or even water.</p><h2>5. Why is circuit completion important in electronics?</h2><p>Circuit completion is important in electronics because it allows for the flow of electricity and enables devices to function. Without a complete circuit, electricity cannot flow and devices will not work. Additionally, understanding circuit completion is essential for troubleshooting and repairing electronic devices.</p>

1. Is circuit completion necessary for a circuit to function?

Yes, circuit completion is necessary for a circuit to function properly. Without a complete circuit, electricity cannot flow and the circuit will not work.

2. What happens if a circuit is not completed?

If a circuit is not completed, electricity will not flow and the circuit will not work. This can result in a device or system not functioning properly or at all.

3. How can I tell if a circuit is complete?

You can tell if a circuit is complete by using a multimeter to measure the voltage and resistance of the circuit. If there is a complete path for electricity to flow, the multimeter will show a reading. You can also visually inspect the circuit for any breaks or disconnected components.

4. Can a circuit be completed in different ways?

Yes, there are different ways to complete a circuit. For example, a series circuit is completed when electricity flows through each component in a single path, while a parallel circuit is completed when electricity flows through multiple paths. Additionally, circuits can be completed using different materials such as wires, conductive metals, or even water.

5. Why is circuit completion important in electronics?

Circuit completion is important in electronics because it allows for the flow of electricity and enables devices to function. Without a complete circuit, electricity cannot flow and devices will not work. Additionally, understanding circuit completion is essential for troubleshooting and repairing electronic devices.

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