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Series battery

  1. Sep 2, 2011 #1
    Hello, why when I connect 2 or more battery in series the voltage at the extremes of batterys will be the sum of single batteries voltages? what happen phisically?
    how the electrons of second battery go to the positive pole if they must pass from the first battery?

    Thanks!
     
  2. jcsd
  3. Sep 28, 2011 #2
    To answer your question lets make a simple picture
    A---------(-V1+)------------(-V2+)----------B,
    o-->
    where V1 and V2 are your batteries. When an electron travels from point A and passes through point V1 there is a rise in the electric potential energy of the electron, similar to water flowing if you imagine the water going uphill at V1 and V2. Each rise will increase the potential energy, both by V1 and V2. The key thing to realize is that each battery is just a voltage gain. The "positive pole" you are referring to is kind of a misnomer. The plus sign just indicates which end of the battery is at a higher relative electric potential.

    For the details behind what physically happens to batteries I would recommend quickly going over this site: http://en.wikipedia.org/wiki/Battery_(electricity)#Principle_of_operation
     
  4. Sep 28, 2011 #3

    sophiecentaur

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    Talking in terms of electrons flowing round really does make life harder. In a low power circuit, an electron has an average speed of much less than 1mm/second. So much so that, with a longish piece of wire in the circuit, a watch battery will be flat before an electron gets from one terminal, round the circuit and back to the other terminal. Despite the slightly more abstract model, it really does make more sense to talk in terms of charge flow (which is the net effect of zillions of electrons in extremely fast, random motion and with an extremely low mean velocity). The Electrical Potential Differences across the two cells gives the charge two lots energy which is dissipated in the elements of the circuit (resistor / lamp /motor etc). The higher the total Potential difference, the more energy each Coulomb of charge dissipates.
    imho, the electron flow model is such an oversimplification that it actually detracts from understanding what is going on and damages the possibility of a good understanding.
    One Volt of potential gives one Coulomb of Charge one Joule of energy. The volts just add up.
     
  5. Sep 28, 2011 #4

    jim hardy

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    there's a pretty down-to-earth explanation of the difference between current and electron motion here:


    http://amasci.com/miscon/speed.html

    it's written by a hobbyist wth a knack for explaining things.
    would be interesting to delve into his references.

    old jim
     
  6. Sep 29, 2011 #5

    sophiecentaur

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    I think he gives a good description. I'm not sure he's really prepared to give enough of an opinion as to how useful it may be to use one approach or another. Perhaps he reckons it's not his job. Fair enough.
    He definitely pours cold water on the model that is implied in the way school kids are taught. This is good; all Science teachers should be required to read it.
     
    Last edited: Sep 29, 2011
  7. Feb 21, 2012 #6
    If I measure an electrical potential difference of 32 V it means every electron has the power of 32 Joule?
    Is the speed of electricity related to its potential difference or current?

    Thanks!
     
  8. Feb 21, 2012 #7

    NascentOxygen

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    A volt is defined as a Joule per coulomb.

    Greater current means more electrons are flowing.
     
  9. Feb 21, 2012 #8

    sophiecentaur

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    "Speed of electricity" is not a meaningful quantity because "electricity" is not a defined quantity (try to look it up). As stated earlier, the mean drift speed of electrons is around 1mm/s but the speed that the effect of turning on the supply will propagate at not far short of the speed of light.
     
  10. Feb 21, 2012 #9
    but a 32 V current flow is moving a bit faster than a 12 V current flow if they both have the same amperage and in the same material like Cu, right?
     
  11. Feb 21, 2012 #10

    sophiecentaur

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    Go and do the sums. The formula for drift velocity is available all over the place. You need to get this in proportion. Find out the numbers involved before you say "surely".
     
  12. Feb 21, 2012 #11
    I've seen the drift velocity formula and it's proportional to the current instead of voltage but why?
    Doesn't more energy mean more speed available?
     
  13. Feb 21, 2012 #12

    sophiecentaur

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    Ideas about drift velocity tend to be based on intuitive 'mechanical' thinking - water wheels and the like. If you think in terms of a wide velocity distribution of conduction electrons and that a potential difference is just going to change the probability of a direction of motion by a minuscule amount, would that make more sense?
     
  14. Feb 22, 2012 #13
    The last time I discussed Drift Velocity was in an EE class ( in the 80s) - I have never seen any practical application of the topic - it is more of a material science issue, current density of material etc. Really has nothing to do with the circuit, power etc - and nothing to do with the battery case of the OP.
     
  15. Feb 22, 2012 #14

    sophiecentaur

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    Absolutely. But people insist on a flowing water / KE explanation and drift velocity shows it's not like that.
     
  16. Feb 27, 2012 #15
    The way I think is the more voltage is applied the more electrons' speed increase because higher voltage = more energy = more speed
    I think an higher energy object can be more speed than a low energy one but in the drift velocity formula only appears the current not the voltage so what are the errors of my reasoning?

    Thank you!
     
  17. Feb 27, 2012 #16

    sophiecentaur

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    But it ISN'T like Kinetic Energy. The KE of the electrons is absolutely microscopic. You need to remember that or you will draw some very wrong conclusions. Like I said before, it isn't the Kinetic Energy in your (very low mass) bicycle chain that takes the power to the wheels - and it's not even the speed at all, because you can be in a range of gears for the same power transfer.
     
  18. Feb 27, 2012 #17
    if not more speed, how can be imagined an electrons flow at low voltage and at high voltage?
    for "change the probability of a direction of motion" you mean the higher the voltage is the more electrons will be physically "involved" in the current flow?
     
  19. Feb 27, 2012 #18

    sophiecentaur

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    I am saying that the Kinetic Energy is IRRELEVANT and is of no significance in the transfer of energy.
    To say otherwise is to give the wrong impression.
     
  20. Feb 28, 2012 #19

    NascentOxygen

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    That applies to the electron beam in a cathode ray tube (CRT).

    If you apply more voltage to the sea of swirling electrons in a wire, then it's a case of more electrons getting pushed along, rather than the original number being pushed faster.
     
  21. Feb 28, 2012 #20

    jim hardy

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    it is more similar to the pv term in fluid flow, potential energy from pressure,
    than it is to mv2 as in kinetic energy from velocity.
     
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