Nailing down charge separation due to a battery

[FireBones]

Hi all,
I'm trying to nail very precisely the effects of a battery in terms of charge separation. In particular: when and what points are excess charges seen in a typical circuit powered by a battery.

A. Prior to wires being attached to the battery, you just have the battery itself. My understanding is that in modern batteries, there is NO EXCESS CHARGE whatsoever at either pole (at this point)...which allows batteries to live for a long time on a shelf without "leaking."

Is this true? Are both poles neutral prior to a wire being attached?

B. Now, attach the wires, but do not create a circuit. Do the free electrons in the wires allow the chemical half-reactions at each cell to proceed? Is there now any net excess charge in either wire?


C. Once you complete the circuit, chemical reactions proceed and there is small excess charge at each end of the battery (and, indeed, in the wires coming out from each one). That is the case that is clear...but my question is whether there is any charge separation at all in the batteries prior to the circuit being formed.

 

[Topher925]

I've never seen the terms applied "excess charge" or "charge separation" applied to anything beyond static electricity. Where exactly are you getting these terms from?

A. If you have a battery, any kind of battery, without wires attached you will have an electric potential across the terminals. This is ultimately due to a difference in fermi levels of the materials used to create the battery.

By the term "leak" do yo mean self discharge? If so, self discharging is caused by a variety of mechanisms which are dependent upon the chemistry used. Some chemistries suffer from ion crossover, and for some other chemistries self discharging its not really well understood.

B. If you just attach wires but do not complete a circuit, no reaction will take place (ideally) and there will be no transport of charge.

C. Again, I don't really understand what you mean by "charge separation".

 

[FireBones]

I've never seen the terms applied "excess charge" or "charge separation" applied to anything beyond static electricity. Where exactly are you getting these terms from?

Not to be rude, but by 'charge separation" I mean just that...

A battery has a net charge of 0. However, if it is part of a closed circuit the half reactions in each cell cause the "+"pole to have a slight imbalanced of positive charge and the "-"pole to have a slight imbalance of negative charge. These charges, of course, quickly distribute themselves along the wires connected to the terminals in such a way as to facilitate the flow of electrons through the circuit.

My question refers to modern, everyday batteries, and I have read that they do not actually have this charge separation until the circuit is closed. I'm trying to pick that apart. I believe, in fact, that a poster on this forum made that remark somewhere...I'll try to find the post.

 

[FireBones]

The place where I saw someone mention this on this forum is https://www.physicsforums.com/showpost.php?p=2076674&postcount=2"

I'll ask the poster to comment on this thread.

 

[Adjuster]

My mistake.

 

[DrZoidberg]

Of course there is an excess charge at the poles even when no wires are attached. Without a charge there wouldn't be a voltage.

 

[FireBones]

Zoidberg, there doesn't need to be a voltage prior to wires being connected.

The claim, I believe, is that the chemical processes that create the separation of charge only occur (in modern batteries) once the wires are attached, or perhaps only once a complete circuit is made and reactions become spontaneous due to the simultaneous provision of a receptacle for the excess electrons produced at one pole and a source of electrons for the reactions at the other.

I've already referenced a poster who indicates that a quirk of modern batteries, the charge separation does not persist after a circuit was broken, and I'm hopeful he will chime in.

 

[Topher925]

Zoidberg, there doesn't need to be a voltage prior to wires being connected.

This is incorrect. If you have any type of Galvanic cell, you will have a voltage potential across the electrodes just as you will have an exchange current at the electrodes.

 

[FireBones]

This is incorrect. If you have any type of Galvanic cell, you will have a voltage potential across the electrodes just as you will have an exchange current at the electrodes.

You misinterpret my post. When I was saying a voltage is not "needed" until a circuit is created, I was not claiming that it was possible for a battery to have no voltage. I was using "need" in the practical sense of what we use batteries for. We do not need a battery to have a voltage until it is wired into a circuit.

As to the question of whether batteries retain their charges when the circuit is broken, that is the question I'm asking... and (as I have already cited), at least some people indicate that in modern batteries it does not occur...suggesting perhaps charge separation only occurs when electrons from the wire are made available or something similar. I'm hopeful the poster I cited will comment more on this.

 

[Adjuster]

Hello Firebones

I really have to ask this:

If a or cell or battery has no initial voltage, how can it react in any way to the connection of an external circuit, let alone wires which are not connected to each other?

With no emf, there would be no tendency for charge to move along the external wires, and so no activation of chemical reactions within the cell.

We know that batteries actually work, so please explain how this can be so?

 

[FireBones]

Hello Firebones

I really have to ask this:

If a or cell or battery has no initial voltage, how can it react in any way to the connection of an external circuit, let alone wires which are not connected to each other?

With no emf, there would be no tendency for charge to move along the external wires, and so no activation of chemical reactions within the cell.

We know that batteries actually work, so please explain how this can be so?

Adjuster, as I've already said a number of times, I'm inquiring about this topic because I (obviously) don' t know everything about it.

Also note that voltage is not the equivalent of emf.

I've already indicated one way how this might occur: the reaction occurring at the positive pole might not occur unless there is a free electron around (a free electron given by a wire...)

Or, perhaps, the half-reactions only occur at a single electrode in the absence of a closed circuit.

Theoretically, of course, there are initial reactions that occur using just the material of the battery that "prime the pump" so to speak...but it appears possible that this might be an oversimplification, and I was just looking for anyone who had specialized knowledge or had read the same thing Archosaur had when he made his comment. I note that "Howstuffworks.com" is ambiguous on this question.

 

[Topher925]

Also note that voltage is not the equivalent of emf.

Yes it is. They are in fact the same thing, actually. The whole reason we even have an open circuit voltage in a battery is because there is a difference in charge. These are pretty basic concepts that I suggest you study in more depth.

 

[Adjuster]

Well if we have to be that precise, I should have asked whether you believe that a cell has zero electro-motive force. Some people would say that for most practical purposes this is the same thing as open-circuit terminal voltage, others would not.

Let's not split hairs by diverting the discussion into these definitions.

 

[FireBones]

Yes it is. They are in fact the same thing, actually. The whole reason we even have an open circuit voltage in a battery is because there is a difference in charge. These are pretty basic concepts that I suggest you study in more depth.

No, actually, they aren't.

There are half a dozen different definitions for emf. Some people equate emf with voltage, but the more formal definition makes a distinction. Read http://en.wikipedia.org/wiki/Electromotive_force#Electromotive_force_and_voltage_difference as well as the larger article on the same term.

Furthermore, as I've already indicated quite clearly, I understand the theoretic answer to this question while also understand that, based on particular remarks I've seen, that there may be mitigating niggles. Your condescension is neither helpful nor addressing the question I've posed.

 

[DrZoidberg]

Modern batteries are not fundamentally different from the old ones. There are chemical reactions taking place at the electrodes which cause a charge separation. e.g. positive zinc ions dissolve in the electrolyte and leave excess electrons in the zinc electrode.
That gives the electrolyte a positive and the zinc a negative charge. That reaction stops when the electric forces between the zinc and the electrolyte become so strong that they prevent further zinc ions from leaving the electrode.
So there is a charge imbalance in modern batteries even when no wires are attached.

 

[dlgoff]

Perhaps you should check out this instead of wikipedia.

When a voltage is generated by a battery, or by the magnetic force according to Faraday's Law, this generated voltage has been traditionally called an "electromotive force" or emf.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elevol.html#c1"

 

[FireBones]

Perhaps you should check out this instead of wikipedia.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elevol.html#c1"

Don, that is a rather curious response since my point is that a variety of sources give a variety of views on this matter. Those various sources are cited on the Wikipedia page. Hence, choosing one source that gives a particular definition and claiming it is somehow proof to the contrary shows a basic lack of reading comprehension.

Getting back to the actual topic, perhaps some of the concern is whether the cathode reaction can create charge separation there in the absence of a source of free electrons. It would be easy to claim that, even without a closed circuit, a partial reduction can occur by stealing metallically bonded electrons from the electrode itself to plate copper ions onto the cathode. The thing is, I haven't actually found any source that claims such a thing occurs.

The article at http://electronics.howstuffworks.com/battery.htm actually makes the cryptic claim "Electrons flow from the battery into a wire, and must travel from the negative to the positive terminal for the chemical reaction to take place. That is why a battery can sit on a shelf for a year and still have plenty of power -- unless electrons are flowing from the negative to the positive terminal, the chemical reaction does not take place."

Such a claim could mean exactly that: that the cathode reaction does not occur unless the circuit is complete. However, it could also mean "after a certain miniscule charge has built up due to the reduction reaction being so chemically favorable"

In any event, I just called a friend of mine who is a full professor of Chemistry at Tarleton State University [and former dean of the entire natural science division], and he said he honestly didn't know, but in his view the chemical reactions simply did not occur at all until the circuit was completed.

 

[dlgoff]

Don, that is a rather curious response since my point is that a variety of sources give a variety of views on this matter. Those various sources are cited on the Wikipedia page. Hence, choosing one source that gives a particular definition and claiming it is somehow proof to the contrary shows a basic lack of reading comprehension.

You were doubting that EMF and voltage were not the same. So I gave you another reference.

In any event, I just called a friend of mine who is a full professor of Chemistry at Tarleton State University [and former dean of the entire natural science division], and he said he honestly didn't know, but in his view the chemical reactions simply did not occur at all until the circuit was completed.

Strange. Then why can you measure a potential if no reaction has occured?

 

[FireBones]

You were doubting that EMF and voltage were not the same. So I gave you another reference.

Strange. Then why can you measure a potential if no reaction has occured?

What method of measuring the potential do you have in mind?

 

[dlgoff]

Any way you want. Do you think the chemical reaction will provide a instantaneous potential when you measure it with say some meter that only [STRIKE]sources[/STRIKE] sinks, say 1microamp?

 

[FireBones]

Two things:
I would assume that any method that measures potential would end up creating a closed circuit, allowing the reactions we are discussing to take place (perhaps too quickly to measure). Remember that the actual charge separation caused by the reactions are extremely, extremely small... so the amount of time necessary for reactions to provide such a difference may be fleeting.

Secondly, even if we could dismiss the above, the measurement of a voltage from one to the other would not prove net charge on both. If, for example, the anode reactions were taking place to produce a net negative charge at the (-) end, but the cathode reactions were not, a potential difference would still be recorded.

To answer the question without closing the circuit, you could measure the attraction of either end to a test charge...but you would need a pretty precise instrument given the size of the charges.

 

[dlgoff]

To answer the question without closing the circuit, you could measure the attraction of either end to a test charge...but you would need a pretty precise instrument given the size of the charges.

Well there you go. You've answered you own question. You have a potential without making a closed circuit.

 

[FireBones]

Except that unless someone makes such a measurement with a suitably precise instrument, the question is still not answered.

 

[Topher925]

The article at http://electronics.howstuffworks.com/battery.htm actually makes the cryptic claim "Electrons flow from the battery into a wire, and must travel from the negative to the positive terminal for the chemical reaction to take place. That is why a battery can sit on a shelf for a year and still have plenty of power -- unless electrons are flowing from the negative to the positive terminal, the chemical reaction does not take place."

And the article is correct. In order for a complete redox reaction to occur in a galvanic cell, there must be a flow of electrons from one electrode to another. That is by definition what a galvanic cell does.

The reason you can measure an open circuit voltage is because you don't need ion or electron transport in order to generate an EMF. The potential created is entirely due to a difference of energy levels between the electrolyte and electrodes in each of the half cells. Any current being generated by the cell will reduce the open circuit voltage, not generate it. You can see this mathematically with the Butler-Volmer Equation.

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

Except that unless someone makes such a measurement with a suitably precise instrument, the question is still not answered.

Such a device was developed in the 1700's. Its called an "electrometer".

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

 

[FireBones]

And the article is correct. In order for a complete redox reaction to occur in a galvanic cell, there must be a flow of electrons from one electrode to another. That is by definition what a galvanic cell does.

And by that definition there is no separation of charge (which is what this thread has been about) since the separation of charge caused by a battery is due to the chemical reactions you are saying do not occur.

Except that unless someone makes such a measurement with a suitably precise instrument, the question is still not answered.

Such a device was developed in the 1700's. Its called an "electrometer".

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

I wasn't claiming the invention didn't exist! I was saying that to answer the question, I'd have to read someone using a electroscope with a sensitivity on the order of 0.1 pC to determine the answer. I don't happen to have an electroscope on my desk.

 

[DrZoidberg]

If you connect a capacitor to a battery, remove it again and then measure the voltage of the capacitor it will be charged to the voltage of the battery.
The terminals of a battery also possesses a capacitance. Therefore they too must get charged even when no wire is connected.
However it's impossible to tell what charge exactly each one of the terminals has.
e.g. the - terminal might be charged to -0.75V relative to Earth and the + terminal would then be charged to + 0.75V rel. to earth.
But that is just one of many possibilities. The - terminal might also be charged to 0V and the + terminal to 1.5V. Or - is charged to 5V and + is charged to 6.5V. Or you rub a comb through your hair, touch it to the battery and get 10,000V and 10,001.5V. As long as the battery is not connected to anything there is an infinite number of possibilities. The only thing we know for sure is that the potential difference between both terminals is always 1.5V.

If you want further proof look at the oxford electric bell.
http://en.wikipedia.org/wiki/Oxford_Electric_Bell
It's powered by a special kind of battery that produces several thousand volts. The circuit is never closed. And yet the terminals are obviously charged otherwise the bell wouldn't work.

 

[FireBones]

If you connect a capacitor to a battery, remove it again and then measure the voltage of the capacitor it will be charged to the voltage of the battery.
The terminals of a battery also possesses a capacitance. Therefore they too must get charged even when no wire is connected.
However it's impossible to tell what charge exactly each one of the terminals has.
e.g. the - terminal might be charged to -0.75V relative to Earth and the + terminal would then be charged to + 0.75V rel. to earth.
But that is just one of many possibilities. The - terminal might also be charged to 0V and the + terminal to 1.5V. Or - is charged to 5V and + is charged to 6.5V. Or you rub a comb through your hair, touch it to the battery and get 10,000V and 10,001.5V. As long as the battery is not connected to anything there is an infinite number of possibilities. The only thing we know for sure is that the potential difference between both terminals is always 1.5V.

If you want further proof look at the oxford electric bell.
http://en.wikipedia.org/wiki/Oxford_Electric_Bell
It's powered by a special kind of battery that produces several thousand volts. The circuit is never closed. And yet the terminals are obviously charged otherwise the bell wouldn't work.

I wish people would stop talking about voltage or potential here. The initial question was about charge (i.e. measured in coulombs, not volts). And net positive or negative charge can be measured at one end without reference to the other.

Thanks for bringing up the Oxford bell, which is close enough to a dry cell to be compelling evidence toward the view that charge separation occurs prior to the completion of a circuit, but I still intend on testing a Duracell with an electroscope.

 

[DrZoidberg]

I wish people would stop talking about voltage or potential here. The initial question was about charge (i.e. measured in coulombs, not volts). And net positive or negative charge can be measured at one end without reference to the other.

But the voltage relative to Earth is directly depending on the charge and the other way around. It's just easier to talk about voltage because I don't know the capacity of the terminals.
If the capacity is e.g. 1pF and the voltage 1.5V then the charge is 1.5pC. However that is the difference in charge not the total charge. As I said there is an infinite number of possibilities. The total charge of the - terminal might be 1000pC and the charge on the + terminal 1001.5pC.

Thanks for bringing up the Oxford bell, which is close enough to a dry cell to be compelling evidence toward the view that charge separation occurs prior to the completion of a circuit, but I still intend on testing a Duracell with an electroscope.

The only way to do that is to connect a large number of Duracells in series to get several hundred volts and then test it with a very sensitive electroscope.

You could also buy an electric field meter. But those things usually don't work with static fields so you need to rotate the battery to get an ac field.

 

[FireBones]

But the voltage relative to Earth is directly depending on the charge and the other way around. It's just easier to talk about voltage because I don't know the capacity of the terminals.
If the capacity is e.g. 1pF and the voltage 1.5V then the charge is 1.5pC. However that is the difference in charge not the total charge. As I said there is an infinite number of possibilities. The total charge of the - terminal might be 1000pC and the charge on the + terminal 1001.5pC.

No, it is not easier to talk about voltage since that is only a measurement that has relevance when comparing two different points, and part of the point of this inquiry is whether net charge is produced at neither, one, or both ends of a battery when there is no closed circuit. That question is far better addressed by looking at net charge, which can be measured in isolation and in an absolute sense without completing the circuit in any way.

The only way to do that is to connect a large number of Duracells in series to get several hundred volts and then test it with a very sensitive electroscope.

That was exactly what I was talking about doing earlier when Topher made his comment (which struck me as a bit asinine, to be honest).

Note that a modern electroscope would be able to measure the expected net charge at the end of a single battery. I'm not sure that the original devices Topher referred to would be able to...but I wouldn't be surprised if they could. Electrostatic electroscopes are surprisingly accurate given their simplicity.