Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why do voltages add up

  1. Mar 7, 2005 #1
    lets say i got two batteries; first has 200 electrons on plus terminal and 300 electrons on a negative terminal ( so there is 100 electrons difference in potential energy )
    and and second battery has 100 electrons on + terminal and 200 electrons on -.
    if connected in series +(200e) -(300e)_____ +(100e) -(200e)
    why would voltages add up ? wouldn't 100e from - terminal of the first battery be transferred
    to + terminal of the second battery making pic look like :
    +(200e) -(200e)_____ +(200e) -(200e) making potential difference between + terminal of the first battery and - terminal of the second battery zero ?
  2. jcsd
  3. Mar 8, 2005 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Transfer of electrons is current not voltage. To maintain a constant voltage, the difference in "electrons" must remain contant. Voltage is not a transfer of electrons, you can (and commonly do have) a voltage, or potential difference, with no movement of electrons.
  4. Mar 8, 2005 #3


    User Avatar
    Science Advisor

    Recall that batteries work off of chemical energy -- they typically create a steady EMF generated from that chemical stuff. That's what keeps the electrons doing the right thing, and allows batteries in series to do the right thing. You will find this discussed in more-or-less detail in most any freshman physics text.
    Reilly Atkinson
  5. Mar 9, 2005 #4
    Thank you for your help. I am still at lost though. I thought if you connect two
    objects with a diffrent number of free electrons by a wire -- there will be current
    going from a higher to a lower potential point. So will there be a current going
    between the two connected terminals of batteries in series if they differ in potential ?
    I do undestand that chemical energy in a battery is what creates and keeps
    the voltage at a constant level.
  6. Mar 9, 2005 #5


    User Avatar
    Staff Emeritus
    Science Advisor

    If you hook two battereies up in parallel, then move one of them to a series connection, the only current flow will be to charge up stray capacitances.

    The capacitances across the batteries won't contribute to the curreent flow - if you draw the circut out, you should see that any stray capacitance from either of the second batteries's terminals to ground will have to be charged or discharged.

    Voltage is energy / charge. The reason that voltages add in a loop is that energy is conserved.
  7. Mar 9, 2005 #6
    a battery is divided up into two half-cells of ions (positive and negatively charged atoms) in solution, connected by a salt bridge. Both are electrically neutral, so they have the same amount of positive and negative charge, but the half-cell that's the negative terminal has ions that are better at attracting electrons than the half-cell that makes up the positive terminal. Now, both half-cells have to stay neutral, so any charge that leaves the positive terminal is replaced by ions from the salt bridge dissolving into the solution, and a similar situation occurs for the negative terminal.

    So I don't know if it helps to talk about the chemistry of a battery, but maybe it's useful or interesting.
  8. Mar 17, 2005 #7

    You need to study the Capacitor, Q=CV.

    You don't know the voltage if all you know is that you have 200 electrons.

    You need to know the capacitance, and THEN this doesn't apply to a battery.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook