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

Is electromotive force always constant?

  1. Feb 20, 2010 #1
    is electromotive force always constant??

    i've been cross-referencing different sources as to the exact definition of the electromotive force, and i keep getting the following:

    emf = terminal voltage

    but i know terminal voltage changes due to the voltage drop that occurs when an electrical circuit is closed. what i just need to absolutely confirm is that emf is practically constant, given the equation:

    terminal V = emf - IR

    where the ONLY time emf = terminal V = open-circuit voltage is when the electric circuit is open. And that open-circuit voltage is ALWAYS equal to emf and that open-circuit voltage = terminal voltage ONLY when the circuit is open.

    moreover, what exactly is the role of emf once the circuit is closed? I know it's responsible for driving the current, but if this is the case, what's the role of the terminal V then if not the same thing?


    1) during discharge: terminal V < emf, due to the voltage drop from internal resistance
    2) during recharge: terminal V > emf, due to restoration of potential difference across the terminals of the battery (i.e. voltage source)
  2. jcsd
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Can you offer guidance or do you also need help?
Draft saved Draft deleted