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Why does voltage increase when batteries are connected in series?

  1. Apr 8, 2012 #1
    Hey!

    I am looking into how batteries work but I can't understand why -- from a chemical perspective -- voltage increases when they are connected in series.

    Let's say we have two identical batteries: battery 1 at the bottom and battery 2 on top, connected in series. The negative bottom end of battery 1 connects to the positive top end of battery 2 via a wire. The positive top end of battery 1 connects to the negative bottom end of battery 2 directly.

    From what I've read about battery cells, there is a chemical separator in the middle of each battery that prevents electrons to flow directly from one side of the battery to the other. If that's true, then the bottom end of battery 1 and the top end of battery 2 (the only parts of the batteries that connect to the wire) are completely isolated from the direct exchange of electrons that happens at the place where the two batteries connect. Thus, I conclude that in effect, there is only one battery powering the wire, consisting of the bottom end of battery 1 which shares electrons with the top end of battery 2.

    Why is voltage increased when in reality there are only two half-cells powering the circuit?

    Thank you!
     
  2. jcsd
  3. Apr 8, 2012 #2

    NascentOxygen

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    Hi aphyx! http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

    During normal operation, each battery in a bank is oblivious to the presence of the other voltage sources in series with it. Each pushes electrons from one end, and gathers an equal number from whatever conductor is connected to the other end. Exactly where those electrons come from or go to it is not concerned about, as one electron looks and behaves the same as any other. :smile:
     
    Last edited by a moderator: May 5, 2017
  4. Apr 8, 2012 #3
    Thanks so much for your reply, but I still don't understand why voltages get added when it comes to the wire circuit.

    Suppose two 1.5V batteries are connected in series to power a single circuit. The batteries have 4 cells (1 positive and 1 negative per each) and 2 separators in total. The wire, however, connects to just 2 cells (1 positive from battery 1 and 1 negative from battery 2). The other two cells are isolated from the circuit via the separators.

    Since there are only two cells powering the circuit, isn't the potential difference -- between the two points connected by the wire -- just 1.5V? Please explain in layman terms where I wrong.
     
  5. Apr 8, 2012 #4

    vk6kro

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    A cell consists of one positive terminal and one negative terminal with an electrolyte in between them.

    What we call a flashlight battery is really one cell.

    When we put two flashlight cells in a flashlight, this is called a battery but it only works because the two terminals that are not connected to the flashlight are connected to each other.

    You can see it when you put the second cell (maybe a "D" cell) into the flashlight. It touches the top of the first cell.
     
  6. Apr 8, 2012 #5

    NascentOxygen

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    I think you are confusing half-cells. If there are 2 batteries are 2 cells. The transport through the electrolyte in each cell is as ions, not free electrons. The ions can pass through the separator. As the metal case of each cell reacts, the Zn atoms give up electrons and these flow through the external circuit. At the carbon rod, returning electrons join with positive H+ ions and neutralize them.
     
  7. Apr 8, 2012 #6
    Thanks again for your replies, but I obviously haven't understood battery design very well and this confuses me a lot.

    Can you please describe in really simple terms how a battery operates or at least refer me to a source that will help me understand it better?
     
  8. Apr 8, 2012 #7
    I feel your struggle aphyx. Reading this post made me aware that I dont know as much about batteries as I thought I did. Keep reading different sources (wikipedia, textbooks, etc.) and asking great questions like these and i'm sure the whole picture of how the battery truly operates will start to come together. I'ma do the same :)
     
  9. Apr 8, 2012 #8

    jim hardy

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    perhaps that's a misconception ?

    take a look at this tutorial on the humble zinc-carbon flashlight battery. I learn a lot from flashlights.

    http://data.energizer.com/PDFs/carbonzinc_appman.pdf

    page 3 has a diagram. There's nothing in there to block flow of electrons between the zinc can(anode) and carbon center electrode(immersed in cathode solution).
    Be aware that inside the battery negative charge moves from the positive button on top of the carbon rod toward the negative zinc can pushed along by chemistry. Seems backwards at first.

    So an electron is encouraged to enter the carbon electrode and progress into the cathode mix, which allows an equal but different electron to leave via the zinc can.
    It will acquire 1.5 ev (electron volts) in that journey.
    If it goes through another cell it can acquire another 1.5 ev. Like stepping up a ladder .

    Recognize that the individual electrons don't move far at all - the electron that comes out the bottom carrying that added energy is not the same electron that entered the top, but electrons all look the same .
    So we speak of them as entering and leaving as if they had gone all the way through, which they did not.
    That's a simplification for thought experiments and must not be taken literally. They move very slowly but bump their neighbors along like people in a cue.
    It is more correct to refer to movement of "Charge" through the battery and say 1.5 Volt or 1.5 Joule/Coulomb.

    perhaps you were thinking of the between-cell separators in a multicell battery ?

    old jim
     
  10. Apr 8, 2012 #9

    vk6kro

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    Sometimes it is worth forgetting about the inside operation of batteries (and transistors) and just assume the Chemists and others have done their job well, as usual.

    Just regard these things as "black boxes" with certain properties which can be used to do useful things.

    So, all you need to know about a flashlight cell is that it generates 1.5 volts and it has an internal resistance of about a quarter of an ohm.
    You can then predict how it will behave in a circuit.

    If this means that Zinc is being oxidised and something else is preventing bubbles forming on an electrode, well, so be it. You bought the battery and paid for the right to ignore the inner workings and concentrate on what the battery can do for you.
     
  11. Apr 8, 2012 #10
    Forgetting and simply learning what it is and how its applied is well and all if you want to solve a circuit or be successful as an engineer, but I know I and apparently the thread starter are curious as to what the chemist know that we dont, perhaps out of sheer curiosity or a desire for a deeper more intuitive understanding of how and why things work out the way they do. I can only speak for myself of course..
     
  12. Apr 8, 2012 #11

    vk6kro

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    Yes, of course, you should want to know these things, but where do you stop?

    Do you need to know how the wire was made just to wire up a circuit?

    The original question shows the sort of confusion that results from not concentrating on the real problem.

    Incidentally, if you ever try to make a battery that really works, you will see how difficult it actually is. Makers of batteries have done a lot of research that they are not going to tell us about. So, there is not much point in having a rudimentary knowledge of batteries that have no chance of working properly.
     
  13. Apr 8, 2012 #12

    jim hardy

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    now there's practical advice.

    All i'd add is: there's not a barrier inside a single cell to prevent current flow, as was suggested in OP.

    As evidenced by fact you can reverse current direction and charge them, but dont do it unless they're labelled "rechargeable" for they might explode (as warned on the label).
     
  14. Apr 9, 2012 #13
    Thanks for all of you for your insighful replies!

    Still though, are the electrons flowing from the "+" to the "-" as by convention, or do they travel vice-versa? Also, why is there a need for a bridge between the two chemical to complete the circuit, when chemical 1 has extra electrons and chemical 2 has less -- electricity should flow between the two until the batteries are exhausted, no?

    And finally... the reason I started this thread was because I read in an article that voltage gets doubled when connecting 2 batteries in series, but current doesn't. I really don't get how come the electrical potential is doubled if only one cathode and one anode directly power the circuit. If that's not true, how come the current isn't doubled too?

    Lots of confusion here for me. :(
     
  15. Apr 9, 2012 #14

    vk6kro

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    You are talking about this arrangement, seen in Chemistry classes:


    376px-Galvanic_Cell.svg.png

    Normal batteries are not like that, but imagine you had two of these and connected them in series. Can you see that the voltage would be doubled if you measure the combination?
     
  16. Apr 9, 2012 #15

    jim hardy

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    oops i am late as usual..
    Here's some words to go with vk6's great picture. But less than a thousand of trhem.

    This is where it is handy to imagine yourself inside the conductors floating along with the charges. (Silly as it sounds).

    Let's take a humble flashlight - recall i said they can teach a lot.

    Start at the bottom of lowest battery. Ride along with the "conventional current", ie positive charge.
    Walk through the zinc can , then through the electrolyte, then through the carbon rod and out the metal button on top.
    Observe that each unit of positive charge gained 1.5 volts in that journey, ie 1.5 Jouiles/Coulomb, which energy was contributed by the chemistry in the cell..
    Now - does the charge go out into the world to deliver that energy to a load like flashlight lamp, or does it continue upward into a second cell?
    Let's assume it's a two cell flashlight where the charge enters a second cell.
    In traversing that cell it gains an additional 1.5 volts from that cell's chemistry.
    So each charge has gained 1.5 volts twice, for total of 3.

    If you prefer to follow negative charge as in electrons the thinking is the same. They enter the top and exit the bottom, pushed along by chemistry.
    Observe that inside the battery they are pushed toward the zinc can by chemistry.
    Outside the battery they are pushed away from the zinc can by coulombic forces.
    Outside the batteries they give up the energy they acquired inside. In the flashlight, to a small lamp.
    That's in agreement with Kirchoff, who says there are voltage rises and drops and their sum equals zero.

    I'll leave it to an electrochemist to explain half cells.
    You probably already know about them anyway - i'd enjoy a refresher.
     
  17. Apr 9, 2012 #16

    NascentOxygen

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    The "convention" is that electrical current flows from ⊕ to ⊖ and therefore this defines conventional current as being a flow of positive charges. We now know that current in metal conductors is due to electrons, and their path is from ⊖ to zero (or from ⊖ to ⊕, whatever you wish).
    So that the chemicals don't mix, but yet do have contact to allow ion flow. Otherwise, there would be an infinite resistance in the circuit.
    Perhaps it's simply a matter of perspective? If you are operating a tiny toy train from a single cell, and you decide to change that to two cells, then the voltage will double and this will force roughly double the current through your train's motor. The train may operate at a speed of around x4 because it is now being fed four times the power it was designed for. But with its power rating being exceeded, it can be expected to burn out.

    So, yes, this shows that doubling the cells has doubled the voltage and doubled the current. http://img546.imageshack.us/img546/8853/m1272.gif [Broken]
     
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  18. Apr 10, 2012 #17
    This is exactly like the image I was looking at when I read about the Galvanic cell. I can imagine adding two more electrodes in the system and connecting the four electrodes to the voltmeter. This should increase voltage, no?

    OK. So therefore, the cathodes and anodes in each battery do exchange electrons inside the battery and not just across the wire. Is this what you, guys, mean?




    Please correct me if I'm wrong, but the way I understand voltage (i.e. the potential difference) is the amount of eagerness the electrons have to flow from one side of the battery to the other. Current in most "water-current" analogies is referred to as the "amount of water" or "amount of electrons" in electrical terms. Yet, in a formula current is equal to Q/t (charge/time). What is the difference between current and charge?! And where does voltage come in?!

    :(((
     
  19. Apr 10, 2012 #18

    vk6kro

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    This is exactly like the image I was looking at when I read about the Galvanic cell. I can imagine adding two more electrodes in the system and connecting the four electrodes to the voltmeter. This should increase voltage, no?

    Yes, that is right. To connect the cells in series, you have to join a positive and a negative terminal together (but not from the same cell) and then take the output from the other terminals.


    OK. So therefore, the cathodes and anodes in each battery do exchange electrons inside the battery and not just across the wire. Is this what you, guys, mean?

    No. The electrons are generated by taking them from the zinc atoms and allowing them to flow in the outside circuit to the copper ions in solution which then become copper atoms on rhe copper electrode. They don't flow inside the cell.


    Please correct me if I'm wrong, but the way I understand voltage (i.e. the potential difference) is the amount of eagerness the electrons have to flow from one side of the battery to the other. Current in most "water-current" analogies is referred to as the "amount of water" or "amount of electrons" in electrical terms. Yet, in a formula current is equal to Q/t (charge/time). What is the difference between current and charge?! And where does voltage come in?!


    If you like water analogies, charge would be like the amount of water in a bucket and current would be the rate at which it flowed into the bucket.

    Voltage would be like the pressure of the water before it got to the hose used to fill the bucket, which would limit the water flow into the bucket. So, the hose is like resistance.

    But don't take the plumbing analogy too seriously. Water flowing into a bucket is quite different to electric current flowing in a circle.
     
  20. Apr 11, 2012 #19

    sophiecentaur

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    If you want to get to understand 'everything' then you have to take it in easy chunks. Learn about basic circuit theory and it will tell you what happens for emfs in series. Learn about electrical cells and you will learn how they produce an emf across them. Coupling up these two bits of 'understanding' is in no way making a compromise. PD is PD, whatever produces it and PDs follow Kirchoffs laws.
    If you were to take your ideas to their logical conclusion then you would have to invoke Quantum Mechanics if you wanted to understand how your house wiring works.
     
  21. Apr 11, 2012 #20
    Therefore, voltage would be the same as "current" if there is no resistance in the circuit, right? If we remove resistance from the equation what would voltage (=current) represent? The formula Q/t doesn't explain a lot. Would you please elaborate on what current actually means in terms of electrons and energy rather than using a water analogy?





    OK. I think I am starting to understand the terms a little better but I still can't see the logic. Here is a simplified diagram I made of two batteries connected in series. If, like you wrote, there is no flow of electrons inside the batteries, the road is basically blocked for the middle two cells to power the circuit with the voltmeter.

    Again, why would voltage increase? :(

    attachment.php?attachmentid=46126&d=1334170406.gif
     

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