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Why battery voltages add in series?

  1. May 26, 2015 #1
    If I connected 2 batteries together in series Positive-Negative-Positive-Negative, I've read many times the voltage (joules/coulomb) adds. I cannot understand what is going on to cause this at the ATOMIC level.

    Electrons in a battery are produced by a redox reaction and are attracted to the positive (cathode) part of the battery. The only way to do this is if one connected a wire from Positive and Negative side of the battery. However, if 2 batteries were connected in a P1-N1-P2-N2 with a wire from P1 to N2, when the electrons from N2 make it to P1, aren't they attracted into the cathode electrode of the battery? What keeps the electrons from moving on past P1?

    Electrons get the voltage, or potential energy, from the chemical reaction, so how does it get the voltage of 2 chemical reactions, i.e. why voltage adds in this case. Can you explain at the small scale of what forces are making the electrons gain energy and move the way they do?

    Thanks for any help to help me understand.
  2. jcsd
  3. May 27, 2015 #2

    Simon Bridge

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    You have to connect the batteries using a conductor of some kind.
    The redox reaction basically shmovesifts charges from one end of the battery to the other - doing work. The voltage is the work that was done by the reaction.

    The next battery shifts the charges again, doing the same work again.
    It's the same as adding more cells to an existing battery.

    It's the same as when you put a chair on top of a table - the height of the stack is the sum of the heights of the chair and the table.
    The mechanism for moving a ball to the top of the table and then to the top of the chair is analagous to the batteries shifting charges.
  4. May 27, 2015 #3
    I thought thatan electronwould still experience the same amount of work if there were 1 or 2 batteries because it still is only moving from a negative to a positive terminal. Whether itsN1 to P1or N1 toP2, isnt the same amountof work done?

    Side question: when you connect N2 to P1 wwith a conductor, dont electrons flow extremely fast to balance charge? why doesnt this short circut
  5. May 27, 2015 #4
    A battery does not "add" electrons to a circuit, it adds "force" to the electrons in the circuit.
  6. May 27, 2015 #5


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    A battery is a device that pushes charge around until the positive terminal is at a higher potential than the negative terminal.

    If there is no wire between the two terminals, the battery just needs to move a tiny amount of charge to get the positive terminal to a higher potential, and then the charge just stays there because there's nowhere for it to go and the current stops flowing. But if there's a wire between the two terminals to form a circuit, then the higher potential at the positive terminal causes a current flow through the wire to the negative terminal, and the battery has to push more charge into the positive terminal to make up for the charge that flows out through the wire, and then that also flows away through the wire... and we have a short circuit.

    An analogy would be a pumping water into a reservoir at the top of a hill. If there are no leaks, the pump just runs until the reservoir reaches its fill level and then it shuts off, and there's no more flow. But if I run a pipe from the reservoir back down to the pump inlet, then I have a circuit and the pump will run continuously as water flows out of the reservoir back downhill as quickly as the pump can push it uphill.

    Now, suppose I had two pumps. They're both strong enough to push water ten meters uphill, but no higher... And unfortunately my hill with the reservoir on top is twenty meters high. How do I fill the reservoir? That's easy - I just put a bucket at the ten-meter level, use the first pump to pump water from ground level into the bucket, then use the second pump to lift the water another ten meters from the bucket to the reservoir. That's analogous to two 10V batteries connected in series, positive of one to the negative of the other, to get twenty volts. And as before, if there's no return path or leaks from the reservoir the system just runs long enough to move enough water to fill the reservoir... but if I run a pipe from the reservoir back to to the inlet of the first pipe, there will be a continuous flow.
  7. May 27, 2015 #6
    isnt there a second redox reaction inside the added battery.
    exactly how is force added
  8. May 27, 2015 #7
    Yes, when electrons are flowing through a battery below its physical limits, the reactions add energy to the flow of electrons allowing them to overcome a greater resistance or do more work.
    "Exactly" doesn't really occur in physics, neither in description nor representation. How complex and/or accurate would you like to get?
  9. May 28, 2015 #8

    Simon Bridge

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    Your thinking on this matter is in error - perhaps you have been given an oversimplified picture of what is going on?

    Consider: work is change in energy.
    If the batteries are the same, then the electron experiences the same amount of work from each battery ... so the first battery does work W which makes it gain energy E, then the second battery does work W making it gain energy E again. A gain in energy E and then another gain in energy E is a total gain in energy of 2E.

    The work, and thus the energy gains, comes from the redox reactions you keep mentioning.
    Once you see that the redox reactions add energy, you should see that the fact of energies adding together does not actually depend on the chemistry.
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