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

EMF and charging of a battery

  1. Aug 13, 2009 #1
    My doubt is that when we charge a battery we say that energy is transferred into it , but does the emf of a battery change in this process ? Is the emf same before and after charging , if yes then what actually happens during charging a battery ??

    pls help !!!!!!!!!!!
  2. jcsd
  3. Aug 13, 2009 #2
    When you charge a battery, you apply a voltage greater than that of battery's in order to reverse the internal chemical reaction. The battery's emf is overcome by the voltage you apply to the leads.

    The net result is a potential gradient large enough to reverse the chemical flow by pumping energy into the reactions. This is where the energy is stored.
  4. Aug 13, 2009 #3
    so this means that the emf does not change , the extra potential applied just gets converted to chemical energy , did i get it right ??
  5. Aug 13, 2009 #4
    The emf is the electromotive force. This is what pushes the electrons through the circuit and back to ground. Charging a battery sort of provides a bigger emf in the other direction, driving the current backwards. So while the battery is charging the total emf is the other direction but the emf provided by the battery remains the same.
  6. Aug 13, 2009 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    A battery's emf does actually change as it is charged or discharged.
  7. Aug 13, 2009 #6
    now i am a bit confused , even i think that the emf changes , but i am unable to figure out the reason , could someone just explane it . pls....
  8. Aug 13, 2009 #7


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Hmm, it's been a while since I studied electrochemistry. I know it has to do with the change in Gibbs Free Energy (ΔG) for the chemical reaction involved, and ΔG depends on the concentration of ions in the battery solutions, which in turn depends on the charge state of the battery.

    This site goes into it in detail, but assumes you are familiar with Gibbs Free Energy:

    If nobody else can provide a better answer, I'll recommend you look at an introductory chemistry book (perhaps high school honors level, or college freshman level) that has a chapter devoted to electrochemistry. You'll need to be familiar with concepts like enthalpy and free energy, which would also be covered in such a book.

    In particular, I can recommend the book Chemistry by Steven and Susan Zumdahl.
    Last edited by a moderator: Apr 24, 2017
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook