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Fix my intuition!

  1. Oct 30, 2009 #1
    Excuse the newby question, but this is bothering me...

    Could someone explain to me why two identical batteries when connected in series do not form internally a circuit between the touching positive end of one battery and the touching negative end of the other and proceed to short the ends in contact out?

    Obviously this doesn't happen since many electronics use batteries in series all the time, but what is wrong with this explanation? Why is there no potential difference between opposite ends of different batteries touching, but there is a potential (battery is shorted) between opposite ends of the same battery when connected. If the batteries are manufactured identically why should there be a difference between its own negative terminal and someone else's negative terminal?

    Thank you!
     
  2. jcsd
  3. Oct 30, 2009 #2

    vk6kro

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    Science Advisor

    You might have picked up some wrong concepts.

    A battery only has potential between its opposite ends and current can only flow out of a battery if it can return to the opposite terminal of the battery.

    So you can touch batteries to each other and nothing will happen unless you arrange a path between opposite ends of the two batteries.

    So, if you connect the positive terminal of a battery to the negative terminal of another one, nothing will happen until you arrange a path between the two unconnected ends of the batteries.

    If you measured the voltage between those ends, you would measure the sum of the two battery voltages.
     
  4. Nov 2, 2009 #3
    Could you explain at a lower-level why a potential would not exist with an opposite end of another battery. As I see it, both its opposite and another opposite end have exactly the same opposite charge, why and how (could say the positive end) differentiate between the its own and another opposite end when in contact?
     
  5. Nov 2, 2009 #4

    es1

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    ElJustica, do you have familiarity with other circuit components? Say perhaps capacitors? Do you understand why two precharged capacitors stacked in series, like in the given battery example, do not conduct?
     
  6. Nov 2, 2009 #5
    Yes, I am familiar with basic circuit elements, like capacitors, though now that you mention it, I would have the same conceptual gap for a capacitor in series. Most of my electronics knowledge comes more through tinkering than through classes (CS by training), so I am not very good at the low-level EM theory or circuit analysis. (I did make a robot in college, search R2-D3 in youtube, but that was mostly all Digital logic).
     
  7. Nov 2, 2009 #6
    You need to take a step back and think about it.
    Power supplies incuding batteries supply a voltage but it is also called a potential difference.
    For example - a 12V battery only has voltage between its own positive terminal and its own negative terminal. The potential to supply power is only if you connect a circuit between the two terminals.
    Therefore if you connect two 12V batteries in series you have 24V potential between the positive on battery1 and the negative on battery 2.
    They do not complete the circuit that you mention because the potential is only between each batteries positive and negative terminals.
    It seems confusing for some people at first.
    Each of the 2 batteries when they are disconnected have absolutely no interaction between each other. Connecting the second battery only ties the positive potential of the one battery to the negative potential of the other. There is no potential difference or voltage between these two points.
    Don't get overly worried that you are finding this difficult. You will learn more and things will become a lot clearer with time.

    http://www.calibrepower.co.uk" [Broken]
     
    Last edited by a moderator: May 4, 2017
  8. Nov 2, 2009 #7

    es1

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    I was going to type something up but happily I decided to check wikipedia first and they have a good page describing a deniell cell, which is the standard classroom demonstration battery.

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

    Think about the salt bridge and what it is for. Then once you understand that draw two Daniell Cells in series and I think you'll have your intuition.
     
  9. Nov 2, 2009 #8

    es1

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    Oh ya, some hints. Think about where all the charges are and keep in mind, on average, current can only flow in one direction inside a wire (or salt bridge).
     
  10. Nov 2, 2009 #9

    russ_watters

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    Staff: Mentor

    I'd try thinking about the chemical reaction going on inside the battery. Electrons travel out through the negative terminal and back in the other in order to complete the chemical reaction. If electrons aren't coming back in the positive terminal, the chemical reaction can't proceed.
     
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