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Homework Help: Theory behind real life emfs

  1. Jun 15, 2017 #1
    • Member advised to use the homework template for posts in the homework sections of PF.
    My question isn't exactly a problem,it's just a doubt which occured in my mind,while studying current electricity.The potential difference across the emf of an ideal battery is considered to be constant,whereas if the battery has an internal resistance (the case for all real life batteries),the potential difference across it's terminals changes as soon as a current flows through it.

    What is the exact reason for this??
  2. jcsd
  3. Jun 15, 2017 #2


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    The current flows through the internal resistance, and causes potential drop across it.
  4. Jun 15, 2017 #3
    Oo..that simple?
  5. Jun 16, 2017 #4


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    Yes that simple.

    Just for info... The internal resistance isn't always constant. Sometimes a used AA cell will read 1.2-1.4 V on a volt meter even though it's nearly empty. But if you try and draw any significant current from it the voltage falls much further than it would if the battery was new/full. The internal resistance has increased as the cell was used. The internal resistance of a cell can also vary with cell temperature. All this means that measuring the voltage of a cell isn't necessarily a good indication of how much energy is left in it.
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