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Homework Help: Cicuits and emf

  1. Jun 8, 2006 #1

    Iam stuck on a topic (have been for a while !) about the electro motive force around a circuit (emf). Iam also confused about how internal resistance plays a role in this.
    I do vaguely remember a teacher telling me that max current is at max internal resistance (or something around those lines).
    I dont do advanced electronics and circuits so I dont think you'll hav to launch into long,advanced explanations :).
    Sorry if this is a bit vague. If you're nto sure ill try and reword my problems :).thanks in advance
  2. jcsd
  3. Jun 8, 2006 #2


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    Hi there Lil ze and welcome to PF,

    As your inquiry is a little vague I don't really know how deep to go but I'll start with the basics.

    Electromotive Force

    The electromotive force (emf) of a source is defined as the potential difference across a source when no current flows. This means that the load resistance (not the internal resistance) is infinitly high (or at least very very high for a good approximation). It is also said the the emf is work done per unit charge.

    Internal Resistance (r)

    Inside a chemical cell (battery), the chemicals present a resistance to the flow of current, this is known as the internal resistance.

    There are usually three things to remember when dealing with an internal resistance problem;

    • When no current flows in a circuit then emf = pd across source
    • When a current flows in a circuit there is a p.d. across the load resistor(R) ([itex]V_{R} = IR[/itex]) and a p.d. across the internal resistance(r) ([itex]V_{r} = Ir[/itex]).
    • From Kichhoff's second law - the current in any part of the circuit is the same, thus emf = sum of the p.d.'s - [itex]emf = IR + Ir[/itex]
    Last edited: Jun 8, 2006
  4. Jun 8, 2006 #3
    thanks a lot ! that seems to hav cleared up some of my problems, ill do some exam Qs and see how it goes :D

    thanks hootenanny
  5. Jun 8, 2006 #4


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    My pleasure :smile: Don't hesitate to come back if you get stuck.
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