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I don't understand Electric Potential in battery

  1. Apr 18, 2013 #1
    I understand potential as follows:

    V(r) = Q/(4*pi*e0*r)


    If current flows from high potential to low potential in a conductor, I am keen to know where is the charge Q located that cause this potential difference across the two ends of the conductor. I would also like to know its polarity.

    Thanks in advance.

    Regards,
    wirefree
     
  2. jcsd
  3. Apr 18, 2013 #2
    The equation you have quoted gives the potential at a distance r from a point charge Q.
    A battery is something completely different
     
  4. Apr 18, 2013 #3
    Please allow me to rephrase:

    If current flows across a conductor from A to B, then how can I understand the potential difference between the two end points?

    NOTE: I am not given any information about batteries in this case.

    Best,
    wirefree
     
  5. Apr 19, 2013 #4
    The potential difference is because of the presence of charges at both ends of the conductor.
    It's really not important just how big these charges are.
    What is of interest is the potential difference wich could depend on the amount of energy a chemical reaction in a battery produces per unit charge, or the strength of the magnetic field and the speed that a coil rotates in it.
     
  6. Apr 19, 2013 #5

    rcgldr

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    Homework Helper

    There has to be some resitance in the circuit, either in the conductor or within the battery. Assuming the conductor has some amount of resistance, then a battery will maintain a charge difference between it's terminals that corresponds to the battery's rated voltage, depending on the amount of current being consumed. The voltage from most batteries will decrease somewhat as current increases, but for an idealized case, you can assume a battery's voltage is constant until the battery is depleted.
     
  7. Apr 21, 2013 #6
    I have distilled my thoughts. Thanks for your inputs. Here's the question:

    Lets slap two oppositely charge circular discs onto two ends of a cylindrical conductor. Clearly current flows. There is also a Potential Difference between the two ends.

    It is this Potential Difference that I care to know more about in terms of the elementary definition, which is: work done on a charge by external force against an electric field.

    Question: Since current (conventional) flows from higher potential to lower potential, I care to know how is it that the potential is calculated at either ends in order to determine which end is higher. (See first response above which discourages use of the point-charge formula.)

    Regards,
    wirefree
     
  8. Apr 21, 2013 #7

    CWatters

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    Science Advisor
    Homework Helper

    Ok so those circular discs have capacitance. Just before you short out the plates the voltage will be given by Vi = Q/C.

    If you short out that charged capacitor with an ideal conductor the voltage will fall instantly to zero and the current will be infinite.

    Real conductors have resistance. The voltage will fall according to...

    V = Vi * e-(t/RC).

    http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capdis.html#c2
     
  9. May 8, 2013 #8
    Thank you, CWatters.
     
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