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I Why does putting two batteries in series increase the voltage?

  1. Aug 17, 2016 #1
    So say we had 2 batteries, B1 and B2, and B2 is on top of B1.

    The + terminal of B1 connects to - terminal of B2, and the + terminal of B2 connects to - terminal of B1.

    Why does this double the voltage compared to the voltage of just B1?
  2. jcsd
  3. Aug 17, 2016 #2


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    It doesn't.

    V12 = V1 + V2 , ( not 2*V1 )

    Only if V1 = V2 , you can by substistution calculate:

    V1 + V2 = V1 + V1 = 2 * V1
  4. Aug 17, 2016 #3


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    Correction added in red.

    There is really no magic here. B1 adds V1 volts to the terminals of the circuit where it is connected, and B2 adds V2 volts to the terminals of the circuit where it is connected.

    In the series additive connection where the voltages add together, each battery acts independently and is unaware of the presence of the other, in any case.
  5. Aug 17, 2016 #4
    If you go back to the definition of voltage, it is a difference in electric potential. If the electric potential at the - end of B1 is A, and the electric potential at the + end of B1 (and - end of B2) is B, and the electric potential at the + end of B2 is C, then the voltage over both B1 and B2 is
    C-A, which is obviously the same as the voltage across B1 + the voltage across B2.
  6. Aug 17, 2016 #5
    Oh yeah, I was assuming the two batteries were of the same potentials.

    Yes, but why does voltage add? They do, but I don't understand why. Batteries are made of two half cells, right?

    So assuming we use the Copper/Zinc half reactions, we have Z1 connected to C1, C1 connected to Z2, Z2 connected to C2, C2 connected to a resistor, and from the resistor connected to Z1.


    So C1 pulls on the electrons of Z1 with some voltage V, and C2 pulls on the electrons of Z2 with the same voltage V. Why does having electrons going from Z1 to C2 lead to double the voltage?

    No water or gravitational potential energy analogy, please!


    Ok wait, if we have only 1 battery with voltage V, and we double the amounts of both half cell reactions, would that lead to double the voltage? Or just a battery with the same voltage that lasts longer?
  7. Aug 17, 2016 #6
    Oops, I deleted a reply because it was wrong. There's an electrolyte between Z1 and C1, and an electrolyte between Z2 and C2. Electrochemistry causes there to be a potential difference between Z1 and C1, and Z2 and C2. But C1 and Z2 are in direct contact, so there is no potential difference. The copper and zinc don't directly react with each other, but instead react with the water in the electrolyte. Look here: https://en.wikipedia.org/wiki/Voltaic_pile

    AFAIK, if you put an electrolyte between the two cells, then the cells would no longer add into a larger battery, since C1 would now react with Z2 (assuming the cells aren't sealed in some package of other metals).
    Last edited: Aug 17, 2016
  8. Aug 18, 2016 #7


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    The short answer is that Voltage is always relative. We don't have to limit this discussion to batteries...

    Pass a constant current through a resistor and you get a voltage across it. Put two resistors in series and the voltage across the combination is the sum of the individual voltages.
  9. Aug 19, 2016 #8


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    Kirchhoff's two laws are worth getting very familiar with. They are no more than re-statements of charge and energy conservation laws but they 'work'.They say nothing about 'why' but they demonstrate the mechanisms at work and will allow you to accept the things you are having doubts about. This is not a cop-out; all our personal understanding of Science is based on re-arranging ideas that are familiar with and producing a new one.
  10. Jul 4, 2017 #9
    The way I understand it best is like this:

    Electrolysis (movement of electrons/current within a battery) happens without the help of anything but the remaining circuitry; the wire connecting "+" and "-".

    A single battery in circuit will be performing electrolysis at the "normal" rate producing a set Voltage. Voltage is determined by magnetic forces that are produced by the difference in charge of a moving electron and a stationary one at each end of the battery in circuit (+/-).

    Placing two cells in series within this circuit will mean that whilst both batteries would be performing electrolysis/draining themselves at the normal rate on their own, one battery is increasing the number of electrons in circuit "arriving" at the cathode (+) (think of it as concentrations of particles if that's easier). This will mean that more electron exchanges will be able to happen at the cathode to begin and ending with the anode (-) and then back to the original cell with ever decreasing amounts of electrons going round the circuit.
  11. Jul 4, 2017 #10

    Vanadium 50

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    As suggested, go back to the definitions.

    Voltages add because potentials add, and potentials are defined with potential energy, and energies add - this is necessary for energy to be conserved.
  12. Jul 4, 2017 #11


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    An alternative way of thinking about why voltages simply add is to consider some of the absurdities that would arise if they did not ?
  13. Jul 4, 2017 #12


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    Old thread alert. Original question has been answered.
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