If the positive plate on a charged capacitor will pass current to the negative

[jaydnul]

...plate of a different capacitor, why won't the positive end of a battery (lets say AA) pass current to the negative plate of a different AA battery. Does it have something to do with the chemical reaction that happens inside the battery?

 

[jedishrfu]

The plates are separated enough and the electrons can't jump from one plate to the next hence no effective current flow. Capacitors hold charge.

Here's some more info on capacitors:

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

 

[Ratch]

lundyjb,

If the positive plate on a charged capicitor will pass current to the negative...

...plate of a different capacitor,

What do you mean by that statement? Do you mean that the charge from one plate will leak through the capacitor's dielectic, and arrive at the opposite plate (WRONG!)? Or do you mean that if there is an external conduction path from one plate to the opposite plate, the charge imbalance will equalize (RIGHT!)?


jedishrfu,

...no effective current flow

Current flow literally means "charge flow flow". You should just say "current" or "charge flow"

Capacitors hold charge

No, they do not hold charge. They store energy. A capacitor energized to 1000 volts has the same net charge as it did when it was energized to zero volts.

Ratch

 

[Nevertamed]

i understood the question, actually there would be current from one battery to the other but only for a short time, because there is no continuity (circuit not formed) between them, the electrons would pile up in the "receiving battery", if you want a continue current to exist you must set a path back from the receiving battery to the first one (both would be connected in series).

in an extreme scenario you could imagine that the receiving battery would take all electrons from the first battery (there's a finite amount of electrons), because the chemical reaction will always maintain each poles at a constant potencial level, but once electrons pile up in the receiving battery they will create a strong electric field that would reject and stop the incomming electrons.

 

[jedishrfu]

Thanks for your corrections, Ratch. You are correct, although it is common to say current flow.

With the charge comment, I was really referring to the one plate being more positive (lack of electrons) and the other being more negative (excess of electrons) but you are correct the net charge is zero for the capacitor.

 

[Ratch]

Nevertamed,

Wrong with respect to both a battery and capacitor. According to your reasoning, a bird would not be able to roost on a high voltage wire because of a supposed short transient current it would receive when it first touched the wire. It just doesn't happen that way.

A battery does not accumulate electrons on either pole, and a capacitor does not imbalance the charge on its plates unless there is a voltage difference between them. Just connecting one plate of a capacitor to a voltage source won't do anything, because there would be no voltage difference between the plates.

Ratch

 

[sophiecentaur]

Nevertamed,

Wrong with respect to both a battery and capacitor. According to your reasoning, a bird would not be able to roost on a high voltage wire because of a supposed short transient current it would receive when it first touched the wire. It just doesn't happen that way.

A battery does not accumulate electrons on either pole, and a capacitor does not imbalance the charge on its plates unless there is a voltage difference between them. Just connecting one plate of a capacitor to a voltage source won't do anything, because there would be no voltage difference between the plates.

Ratch

This actually does happen on the highest voltage transmission cables. Birds stick to Intermediate and Domestic voltage cables. (When I say "stick to", I don't mean glue-like)

I don't know how they know to avoid them. Perhaps the local field strength causes them to tingle and so they keep clear of such attractive perches.

 

[Ratch]

sophiecentaur,

This actually does happen on the highest voltage transmission cables. Birds stick to Intermediate and Domestic voltage cables. (When I say "stick to", I don't mean glue-like)

That has to be caused by high voltage induction (affect without touching due to high electric fields), not conduction. I see plenty of birds roosting on domestic power lines without any ill effects.

Ratch

 

[Nevertamed]

Nevertamed,

Wrong with respect to both a battery and capacitor. According to your reasoning, a bird would not be able to roost on a high voltage wire because of a supposed short transient current it would receive when it first touched the wire. It just doesn't happen that way.

A battery does not accumulate electrons on either pole, and a capacitor does not imbalance the charge on its plates unless there is a voltage difference between them. Just connecting one plate of a capacitor to a voltage source won't do anything, because there would be no voltage difference between the plates.

Ratch

Actually there's a little current through the bird, since there is finite resistance in the wire, and finite resistance in the bird, it still forms a parralel circuit. of course the wire's resistance would be meaningless compared with the bird's, and we all know the current always "choose" the easiest path, only a tiny amount of current goes through the bird

 

[sophiecentaur]

sophiecentaur,



That has to be caused by high voltage induction (affect without touching due to high electric fields), not conduction. I see plenty of birds roosting on domestic power lines without any ill effects.

Ratch

Yes - we all do. It must be a matter of Volts per metre and the capacity and length of the bird's body which limits the Voltage to which they a tolerant. I was referring to High Voltage cables - 132kV and above.

 

[jim hardy]

Sophie is correct.

Birds tend to avoid power lines > about 30KV.
I'd guess it tickles their feet.

It'd be the minute capacitive current that charges their body alternately to +/- line peak voltage at line frequency , 50 or 60 hz.

When working such lines hot, linemen wear a special suit that forms a Faraday cage around them.

http://www.hubbellpowersystems.com/lineman/accessories/suit.asp

 

[Nevertamed]

Nevertamed,

Just connecting one plate of a capacitor to a voltage source won't do anything, because there would be no voltage difference between the plates.

Ratch

the thread starter stated " If the positive plate on a charged capacitor will pass current to the negative plate of another capacitor" which is true, because there would be a voltaje between both plates regarless of being part of different capacitors. however there would be only a short current as the voltaje fade away

 

[Nevertamed]

Nevertamed,



A battery does not accumulate electrons on either pole



Ratch

batteries create a constant imbalance of charge, to maintain a DC voltage

 

[sophiecentaur]

Just connecting one plate of a capacitor to a voltage source won't do anything, because there would be no voltage difference between the plates.

Ratch

In fact, there is a small but finite Capacitance between the unconnected end of the capacitor and ground. This will lead to a finite but small charge flow, via the main Capacitor when it's connected. You can regard it as two capacitors in series - one large and one tiny one.

 

[Ratch]

Nevertamed,

the thread starter stated " If the positive plate on a charged capacitor will pass current to the negative plate of another capacitor" which is true, because there would be a voltaje between both plates regarless of being part of different capacitors. however there would be only a short current as the voltaje fade away

Remember, caps do not get charged, they get energized. The positive plate is deficient in electrons. You refer to two capacitors and four plates. So which plates will there be a voltage between? Where is the conduction path for the "short current"? I don't know how to answer your message because I don't know to what you are referencing.

batteries create a constant imbalance of charge, to maintain a DC voltage

What does that mean? How much charge for what voltage? If it were a cap, I could multiply the voltage by the capacitance and give you an answer. But a battery is a electrochemical device, and does not use a electrostatic field to maintain its voltage like a cap does. Both a battery and a cap sustain a voltage, but they do it in very different ways. Besides, a battery is an active device, whereas a cap is not.

sophiecentaur,

In fact, there is a small but finite Capacitance between the unconnected end of the capacitor and ground. This will lead to a finite but small charge flow, via the main Capacitor when it's connected. You can regard it as two capacitors in series - one large and one tiny one.

Your description of the above really confuses me. First of all, ground is nothing special, just a common connection point. The capacitance between one end of the capacitor and the other end is the value of the capacitor. Where and how does the second capacitor come into play? Inquiring minds would like to know.

Ratch

 

[sophiecentaur]

sophiecentaur,

Your description of the above really confuses me. First of all, ground is nothing special, just a common connection point. The capacitance between one end of the capacitor and the other end is the value of the capacitor. Where and how does the second capacitor come into play? Inquiring minds would like to know.
Ratch

OK then. Replace 'ground' with 'the other terminal of the battery'. The effect will then even less but there are still, effectively, two capacitors in series with the battery. (And another small one directly across the battery terminals.) If the battery is supplying an emf, then this network of capacitors will charge up according to Q=CV.
Explanations in terms of equivalent lumped components are often helpful - antenna theory is often usefully approached this way.


btw If you want to be fussy about nomenclature, then I think it would be more desctiptive to say that Capacitors become Polarised, if you don't like 'charged'. (I have not read of the term "Energised" in this context - it is used more int the context of Batteries, I think but I don't think it is defined very rigorously, though people do talk of 'energising a coil' when you switch on the current, I suppose). That just means that there is a displacement (imbalance) of net charge from one side to the other. With no dielectric, the charge needs to be 'taken off' one side and 'put into' the other. When there is a dielectric, the additional charge (giving it higher Capacitance) is due to charges being easily displaced within the dielectric material as its molecules become polarised. You need to move more charges in this case for a given Voltage.

 

[Nevertamed]

Nevertamed,



Remember, caps do not get charged, they get energized. The positive plate is deficient in electrons. You refer to two capacitors and four plates. So which plates will there be a voltage between? Where is the conduction path for the "short current"? I don't know how to answer your message because I don't know to what you are referencing.



What does that mean? How much charge for what voltage? If it were a cap, I could multiply the voltage by the capacitance and give you an answer. But a battery is a electrochemical device, and does not use a electrostatic field to maintain its voltage like a cap does. Both a battery and a cap sustain a voltage, but they do it in very different ways. Besides, a battery is an active device, whereas a cap is not.



Ratch

actually when they get charged they get energized (they get energy from a external source and store it as an electric field between the plates), about the 2 caps and the 4 plates:

we have the first cap: with one plate full of electrons and the other one lacking (after being charged or energized if you like it)

we have a second cap: with one plate full of electrons and the other one lacking (after being charged or energized)

if you set a path (an hypothetical wire) from the first capacitor's full of electrons plate to the second's capacitor lack of electrons plate; there will be a short current until this imbalance disappear (there wuld be a voltage between them, despite the fact the plates involved belong to different caps)

about your second quote:

In a way, a capacitor is a little like a battery. Although i know they work in completely different ways, capacitors and batteries both store electrical energy. A battery has two terminals. Inside the battery, chemical reactions produce electrons on one terminal and absorb electrons on the other terminal, it maintains this imbalance of charge ( charge separation) as the acid keep working in order to offer a constant voltage

 

[sophiecentaur]

I don't understand the need to use a term, other than 'charged' do describe what happens to a battery. Everyone but everyone knows what it means and it's used in every book I have ever read on the subject. The qualifying word 'differentiallly', which might be added in front is omitted because it is such an accepted term.

Yes, of course Capacitors store Energy but the Energy they 'store' is not the same as the Charge as it depends upon the Capacitance value - the two quantities, Charge and Energy are not synonymous. It is the Charge that is used in the context of most circuit calculations but - horses for courses.

The answer to the original question lies with the parasitic Capacitances involved.

 

[Nevertamed]

i like and always use "charge"

 

[Ratch]

sophiecentaur,

OK then. Replace 'ground' with 'the other terminal of the battery'. The effect will then even less but there are still, effectively, two capacitors in series with the battery. (And another small one directly across the battery terminals.) If the battery is supplying an emf, then this network of capacitors will charge up according to Q=CV.
Explanations in terms of equivalent lumped components are often helpful - antenna theory is often usefully approached this way.

I still cannot understand what you are saying because I cannot visualize the circuit you are talking about. Perhaps a schematic?

btw If you want to be fussy about nomenclature, then I think it would be more desctiptive to say that Capacitors become Polarised, if you don't like 'charged'. (I have not read of the term "Energised" in this context - it is used more int the context of Batteries, I think but I don't think it is defined very rigorously, though people do talk of 'energising a coil' when you switch on the current, I suppose).

I don't think "polarization" is a good word to use for energy storage. It sounds too much like a polarized capacitor such as an electrolytic. Energized simply means "charged with energy".

When there is a dielectric,...

A capacitor always has a dielectric.

the additional charge (giving it higher Capacitance) is due to charges being easily displaced within the dielectric material as its molecules become polarised. You need to move more charges in this case for a given Voltage.

Just another way of saying the capacitor is more energized. E=1/2*C*E^2 or E = 1/2*(Q^2)/C

 

[Ratch]

Nevertamed,

we have the first cap: with one plate full of electrons and the other one lacking (after being charged or energized if you like it)

we have a second cap: with one plate full of electrons and the other one lacking (after being charged or energized)

if you set a path (an hypothetical wire) from the first capacitor's full of electrons plate to the second's capacitor lack of electrons plate; there will be a short current until this imbalance disappear (there wuld be a voltage between them, despite the fact the plates involved belong to different caps)

I disagree. There will be no current between two floating caps. Try energizing two caps, and put a ammeter between the two plates to see if there is any current.

In a way, a capacitor is a little like a battery. Although i know they work in completely different ways, capacitors and batteries both store electrical energy. A battery has two terminals. Inside the battery, chemical reactions produce electrons on one terminal and absorb electrons on the other terminal, it maintains this imbalance of charge ( charge separation) as the acid keep working in order to offer a constant voltage

If they work in different ways, then they are not like each other. A battery is an active element, and a cap is not. A battery produces a voltage by chemical means, not by storing energy by accumulating electrons like a cap does. If there is not a conductomg path for the terminals of a battery, the electrons will not accumulate.

Ratch

 

[Ratch]

sophiecentaur,

I don't understand the need to use a term, other than 'charged' do describe what happens to a battery. Everyone but everyone knows what it means and it's used in every book I have ever read on the subject. The qualifying word 'differentiallly', which might be added in front is omitted because it is such an accepted term.

Because it is a wrong descriptive, even if everyone thinks they know what it means.

Yes, of course Capacitors store Energy but the Energy they 'store' is not the same as the Charge as it depends upon the Capacitance value - the two quantities, Charge and Energy are not synonymous. It is the Charge that is used in the context of most circuit calculations but - horses for courses.

Well, that is true. Energy and charge are two different things. Caps don't store any charge, but they do store energy.

The answer to the original question lies with the parasitic Capacitances involved.

Parasitic capacity is usually very small and only of interest at high frequencies. Are you sure you want to get involved with this?

Ratch

 

[Ratch]

Nevertamed,

i like and always use "charge"

Yes, and NASA likes to see their astronauts "walk" in space, too. But tell me. Did you ever ask NASA what would happen if their tether broke. Would the astronaut walk away from his space vehicle?

Ratch

 

[sophiecentaur]

sophiecentaur,
A capacitor always has a dielectric.

How about these? In practice, of course, there is never 'nothing' between the plates of a capacitor but on these jobbies, ε_r is reckoned to be as near as dammit to unity. A 'dielectric' is a material that polarises - not a vacuum.
This MIT link defines Polarisation as charge separation, in any form so my use of the word is justifiable, I think.

I disagree. There will be no current between two floating caps. Try energizing two caps, and put a ammeter between the two plates to see if there is any current.

I will try a diagram, to show you what I mean about the Capacitors involved when I have time - Power Point is OK but slower than writing and ranting for me. You object to the used of the word 'Parasitic' on the grounds that they are only relevant at HF. But that's the whole point. They are small but if they work at HF then they have a finite effect at all frequencies and at DC.
This is hardly worth answering, it's so obvious. I was clearly talking about what would happen at the start of this process - when you add the battery or start to move the (original) wire in a field. If you acknowledge that there will be a charge separation / difference then some current was flowing at one time.

You have clearly nailed this particular flag to your mast and are sticking to it but can you quote me this as a generally used term? 'Correctness' is something that needs a bit more support than just what you are saying. But, again, is this relevant to the main discussion? We can usually describe the World in more than one way. Energy is a pretty good way to describe and predict things (I am a big fan, in general) in Physics but Forces, Masses, Charges and Impedances are often used more conveniently. Perhaps this is why we learn and then use Q=CV so often - despite the fact that V is Energy per Unit Charge.

Energized simply means "charged with energy".

A bit of tautology, (or is it self-referencing?) here, I think. From what I can find, the term 'energised' seems to be used for circuits in which a current is flowing continuously but the word is used very freely and imprecisely so I don't think it is any better than 'Charged', aamof.

 

[Nevertamed]

Nevertamed,
I disagree. There will be no current between two floating caps. Try energizing two caps, and put a ammeter between the two plates to see if there is any current.

Ratch

they are not just 2 floating caps, they are two points with a imbalance of charge between them, one fully negative charged plate (from the first capacitor) and one fully positive charge plate (from the second capacitor), forget the fact both plates belong to different capacitors, they are still 2 points with a voltage present, if you set a path there will be a current until the charge depletes.

 

[Nevertamed]

Nevertamed,





If they work in different ways, then they are not like each other. A battery is an active element, and a cap is not. A battery produces a voltage by chemical means, not by storing energy by accumulating electrons like a cap does. If there is not a conductomg path for the terminals of a battery, the electrons will not accumulate.

Ratch

i didnt say caps and batts are identical, i said "in a way, caps are a lil like a battery" i meant they both have similarities

 

[Nevertamed]

Nevertamed,
Yes, and NASA likes to see their astronauts "walk" in space, too. But tell me. Did you ever ask NASA what would happen if their tether broke. Would the astronaut walk away from his space vehicle?

Ratch

maybe they like speaking metaphorically like lots of people...

 

[sophiecentaur]

sophiecentaur,
I still cannot understand what you are saying because I cannot visualize the circuit you are talking about. Perhaps a schematic?

The 'equivalent circuit' I am proposing is on the attachment and you can read my original ideas with that circuit in mind.
C1 is the capacity between the inner and outer parts of the battery. They are there whether or not there happens to be any chemical in there. The emf and internal resistance r of the battery are shown and so is the finite Capacitance (C2) between the added wire and the case / negative of the battery or ground.
When the emf is turned on (pouring in the electrolyte, perhaps), charge will flow as the arrows show and the two Capacitors will 'charge up'. This means that there is an initial current.
This diagram instantly transfers to the original question because you can leave out the battery and you still have the induced emf and the Capacitor C2. By doubling the length of the wire (the OP) you are increasing C2. My only query about this is how to determine the effective capacity change.
I googled 'equivalent circuit capacitor antenna' and got more hits than I had time to read. They establish the principle that I'm getting at. The fact that antennae are mostly used for AC is not a hinderance because the situation for the DC condition is only arrived at after a change - not DC - in the situation.

 

[Ratch]

sophiecentaur,

How about these?

I thought you would bring them up. It doesn't change anything, however.

In practice, of course, there is never 'nothing' between the plates of a capacitor but on these jobbies, εr is reckoned to be as near as dammit to unity. A 'dielectric' is a material that polarises - not a vacuum.

It does not matter whether a dielectric is a material or not. What matters is whether it has the properity of a dielectric, specifically, whether it has a dielectric constant. A vacuum does. Its dielectric constant is called the "permittivity of free space".

This MIT link defines Polarisation as charge separation, in any form so my use of the word is justifiable, I think.

Yes, caps that are energized also have to be polarized. But, you did not addresses my concern, which is that saying a cap is polarized can easily be mistaken for designating the cap as an electrolytic, even if it is not.

I will try a diagram, to show you what I mean about the Capacitors involved when I have time - Power Point is OK but slower than writing and ranting for me. You object to the used of the word 'Parasitic' on the grounds that they are only relevant at HF. But that's the whole point. They are small but if they work at HF then they have a finite effect at all frequencies and at DC.
This is hardly worth answering, it's so obvious. I was clearly talking about what would happen at the start of this process - when you add the battery or start to move the (original) wire in a field. If you acknowledge that there will be a charge separation / difference then some current was flowing at one time.

I don't think something as elaborate as PP is needed. A simple schematic with all the salient points should suffice. A conduction path is needed at all times for charge to move. That is what I will be looking for.

You have clearly nailed this particular flag to your mast and are sticking to it but can you quote me this as a generally used term? 'Correctness' is something that needs a bit more support than just what you are saying. But, again, is this relevant to the main discussion? We can usually describe the World in more than one way. Energy is a pretty good way to describe and predict things (I am a big fan, in general) in Physics but Forces, Masses, Charges and Impedances are often used more conveniently. Perhaps this is why we learn and then use Q=CV so often - despite the fact that V is Energy per Unit Charge.

What you say in the above paragraph is true, but I am finding it hard to determine what is the point or conclusion.

A bit of tautology, (or is it self-referencing?) here, I think. From what I can find, the term 'energised' seems to be used for circuits in which a current is flowing continuously but the word is used very freely and imprecisely so I don't think it is any better than 'Charged', aamof.

If "energized" means "imbued with energy", would that make a better definition?

Ratch

 

[sophiecentaur]

If "energized" means "imbued with energy", would that make a better definition?

Ratch

This can only be a matter of preference. After all, would you say, in everyday life, that you "climbed a hill" or that you "Increased your Gravitational Potential"? Both would be correct and acceptable in the appropriate context. If we are talking about Charge (which I though we were), then why not allow our experiment to "charge a capacitor"? If we are talking about supercapacitors and road transport then it's arguable that 'energising' them would be appropriate - but did we ever in this World, talk about Energising a Battery? "Energiser" is a trade name for Ever Ready High Power batteries but that, I think, is aimed at describing what they actually do to your toys and not what Ever Ready do to their batteries.

Anyway, I did that diagram (above post). Hope it makes sense for you.

OH - I just spotted what you wrote about permittivity of free space etc.. Dielectric constant is used to say how the effective permittivity is affected by adding a dielectric, surely. If space has a dielectric constant of Unity, then the quantity is totally redundant. But. again, is this really relevant to the price of fish and chips?