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Why does battery serial pole connection plays a role?

  1. Mar 31, 2010 #1
    I just registered as I have been wondering quite a while about a specific question that grew in my head while reading about electronics in school.
    As I have learnt voltage is a difference in potential. So if we have a 9V battery there is a difference of 9V between the two poles of the battery. I can understand that and I can understand that if I connect a wire to the two poles then electrons will tranfer from one pole to the other. In fact I tried that out and the battery got quite hot(suspect because of luck of resistance=high current?).
    Anyway my question is if you have 2 such batteries and you connect the '+' pole of one battery to the '-' pole of the other battery why don't the electrons flow? In my head the difference of potential is the same. But in practice that just doesn't happen. Why?
    Could someone explain me in a not too complicated way? Thanks in adnvance :smile:
  2. jcsd
  3. Mar 31, 2010 #2


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    Last edited by a moderator: Apr 24, 2017
  4. Nov 10, 2010 #3
    No one ?
  5. Nov 10, 2010 #4
    Pithikos described that with a 9V battery there is a difference in potential of 9V and this difference is between the two poles of the battery.When the two batteries are connected together as described the two connected poles reach the same potential but each battery separately still maintains a pd of 9V across its terminals making a total pd of 18V from one unconnected pole to the other but 0V between the connected poles.
  6. Nov 10, 2010 #5


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    The simplest way for you to understand this would be that you need a closed loop in order to get DC current to flow.
  7. Nov 10, 2010 #6
    From my understanding of the original question I think Pithikos knew that a complete circuit was needed and he knew that for a current to flow a potential difference is needed.He was enquiring about why there is no current flow when the two battery poles are connected there being what he thought was an apparent pd between those poles.

    (If there is an initial pd any flow will be brief becoming zero when the potential equalises)
  8. Nov 10, 2010 #7
    Sorry but i don't satisfy with ur answer!
    define of Volt is relate to force of an electrize thing that push an other electrize thing and 0V meaning force to push an other electrize thing to very far.
    In this case the electrize thing is in form of chemical reaction.So that if there's a 9V battery that mean V(+pole) - V(-pole) = 9V => event if poles on separate batteries if you connect (+pole) to (-pole) electron must moving => there's must some current .
    I think the answer is : cause of poles on separate batteries so that chemical reaction will happened in very short time and then energy of chemical reaction not enough to take more electron (valence electron ) from poles.But (-) poles now has double of free electron.that what I guess so
  9. Nov 10, 2010 #8


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    Welcome to PF! A fluid analogy will help here. Think of each battery as a small pump that maintains exactly 9V of "pressure" between its input (+) and output (-) terminals.

    When you connect two pumps together in series, what happens? Their pressures add. The first one produces 9V of pressure, and the next one takes that and boosts it another 9V. The output would be 18V higher in pressure than the input.

    - Warren
  10. Nov 11, 2010 #9
    Hello smarty,
    Remember that a battery is made of cells connected together.Most car batteries have six cells in series and if there was a potential difference between adjacent connected poles currents would flow and the battery would soon run down even when not connected to an external circuit.That doesn't happen.If I understand you correctly I think you are suggesting that a more detailed explanation would consider the chemistry of batteries and if that is the case I think I agree with you.Unfortunately I do not know enough about the chemistry of batteries to make any informed comments.
  11. Nov 11, 2010 #10


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    Voltage is the Energy given to a unit charge. There is a gravitational analogy. If I drop a hammer from a first floor window (that's the one above the ground floor - for the sake of our US readers), it will do a certain amount of damage when it hits the ground. If I go to the second floor window, the hammer will do twice as much damage on reaching the ground. If I drop it from the second floor and it lands on the first floor balcony, it will do the same damage as in the first case. It's all a matter of the Difference in heights and not the 'absolute height'.
    Each flight of steps I go up adds to the Gravitational Potential Energy I give the hammer. Each Cell you add in series to a battery gives the charges a bit of extra energy.
    Batteries only let current out of their + terminals and into their - terminals so they will not provide an internal path for current from the batteries either side to discharge through them. The + terminal will not let go of any electrons; it's too busy grabbing them and pushing them down inside the battery when it can. The current has to flow around the external circuit path and through the load.
  12. Nov 11, 2010 #11
    Let's say you have 2 batteries. Each one has 1.5V.
    The chemical reactions inside the battery can only move electrons from + to - as long as the potential difference is smaller then 1.5V. Otherwise the electric forces will be too strong and prevent the reaction from continuing.
    Usually zinc dissolves at the -pole, positive zinc ions move into the electrolyte and leave their electrons in the zinc metal. But when the electric forces are too high, the zinc ions will be attracted to the -pole and are not able to dissolve.
    To keep the reaction going the potential difference needs to stay below the threshhold. That means electrons need to be removes from the -pole and electrons need to be added to the +pole. If you only remove electrons from the -pole without adding any to + then the battery as a whole becomes positively charged and that causes the current to stop. The potential difference between + and - still stays at 1.5V though.
  13. Nov 12, 2010 #12
    I think the thing inside is :

    let's see a simple battery : a plastic can of HCL = H+ and CL- solution.2 strips one Zinc, one Copper.What will happen ?

    fermi energy of Zinc > fermi energy of Copper .

    So that :

    + At Zinc strip , CL- and OH- try to attack the Zinc nuclei,a Zinc nuclei easy to left some electron on Zinc strip and then migrate away into solution .now we got a Zn nuclei is positive(Zn+) and Zinc strip is negative .Zinc strip should attract Zn+ (by electric force ) back.But we also still have OH- and CL- they will surround Zn+ very quitly and prevent Zn+ back to Zinc strip .Now we really have Zinc strip in negative so it push (electric force)CL- and OH- far way and didn't let they take any more Zinc nuclei. it something like chemical balance status .

    + At Copper strip : Same thing happened .

    Both Zinc strip and Copper strip now is have more electrons and become negative charge .But Zinc more negative => Vc - Vz=Vb is the potential of battery .

    When we have 2 of battery as above : A and B .

    When we connect A+ (copper ) to B-(Zinc) => electron from B- now move to A+ how to make exactly Vb on both A+ and B+ .But when B-(Zinc) lose electron it will be attacked by OH- and CL- again and copper at B+ will be attacked at same time until something like chemical balance status happen .Same thing will happened at A .And result is Zinc strip of A (-pole on A) is more negative also Copper strip of B(+pole on B) is more negative .But cause total electron more just same as Vc - Vz=Vb => we got (Vb+) - (Va-) = 2Vb .

    When we connect B+ and A- electron will flow and chemical balance status is broken again and it's loop again until HCL is used up!

    so i think there's current when you connect 2 difference pole on 2 different batteries .But it just very short time .
  14. Nov 12, 2010 #13


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    That's the case. I must admit I have troubles understanding your description of what you think is happening, but the final conclusion is correct - when you connect batteries some (very small) charge is transferred till new equilibrium is reached.

    In a way it is similar to what is happening when you put just a piece of copper into water. Copper is - on the reactivity scale - well above hydrogen, so in general it doesn't react directly with water (as opposed to zinc or sodium). However, that's not entirely true. There are two redox systems present, both described by Nernst equations:

    [tex]E_{Cu} = E_{0Cu/Cu^{2+}} + \frac {RT} {2F} \ln [Cu^{2+}][/tex]

    [tex]E_H = E_{0H_2/H^+} + \frac {RT} {F} \ln \frac {[H^{+}]}{p_{H_2}}}[/tex]

    For system to be in equilibrium both potentials must be identical, or ECu = EH. For that some H+ has to be reduced (so that pH2 is not zero) and some copper has to be oxidized (so that [Cu2+] is not zero). That in turn means some charge has to flow from metal to the solution. I am not sure about numbers now, but from what I remember we are talking about less than single electron per mole of copper (it can be even several orders of magnitude less than that, it is not difficult to calculate but I am in a hurry now).
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