Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Nailing down charge separation due to a battery

  1. Dec 15, 2009 #1
    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.
  2. jcsd
  3. Dec 15, 2009 #2
    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".
  4. Dec 15, 2009 #3
    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.
  5. Dec 15, 2009 #4
    Last edited by a moderator: Apr 24, 2017
  6. Dec 15, 2009 #5
    My mistake.
  7. Dec 17, 2009 #6
    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.
  8. Dec 18, 2009 #7
    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.
  9. Dec 18, 2009 #8
    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.
  10. Dec 18, 2009 #9
    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.
  11. Dec 18, 2009 #10
    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?
  12. Dec 18, 2009 #11
    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.
  13. Dec 18, 2009 #12
    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.
  14. Dec 18, 2009 #13
    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.
  15. Dec 18, 2009 #14
    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.
  16. Dec 19, 2009 #15
    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.
  17. Dec 19, 2009 #16


    User Avatar
    Science Advisor
    Gold Member

    Last edited by a moderator: Apr 24, 2017
  18. Dec 30, 2009 #17
    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.
    Last edited by a moderator: Apr 24, 2017
  19. Dec 30, 2009 #18


    User Avatar
    Science Advisor
    Gold Member

    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?
  20. Dec 30, 2009 #19
    What method of measuring the potential do you have in mind?
  21. Dec 30, 2009 #20


    User Avatar
    Science Advisor
    Gold Member

    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?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook