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Use of EMF v Voltage

  1. Sep 14, 2010 #1
    First of all this isn't really Homework problem, but since it is based on homework problem, I'll post on this section of the forum. Feel free to move if I was supposed to post on different part of the forum.
    Either way, I'll use homework as an example since that is what brought this up..

    Regardless of how long this post looks, it's really a simple question =p

    1. The problem statement, all variables and given/known data
    This question involves circuit system using...
    Power, Voltage, Current, and Resistance (internal and load resistance), and EMF
    This is probably really basic question for most of you

    Question is when measuring Current and Power of any part of the circuit, do I use EMF? or Voltage? (So it's two questions. 1. when measuring current, 2. when measuring power)
    P=V^2 / R or P=E^2 / R ?
    and I= V / R or I = E / R ?
    and when using other Power formula such as P=I*V, P=I^2 * R
    do I use the current that is derived from using EMF ? or Voltage?
    assuming I use I = E/ Rt

    2. Relevant equations
    V=E-Ir (or V=E+Ir)

    Rl-Load Resistance
    Ri- Internal resistance
    Rt-total resistance
    E-EMF (Electro Motive Force ithink...)

    3. The attempt at a solution
    This is a conceptual question

    Don't read this part, unless you feel it's neccessary...It's not really quotation, I just wanted to box it somehow ...
    Is there something that I don't know and should know? (maybe about V vs V(terminal)?)
  2. jcsd
  3. Sep 14, 2010 #2


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    In 2010, I think the term "electromotive force" is at risk for becoming an archaic term. Sort of like "kilocycles" or "kc" for kiloHertz. I have led a happy life without being able to distinguish an EMF from a Volt.

    Think of EMF as the "concept" equivalent to voltage measured in Volts.
    Last edited: Sep 14, 2010
  4. Sep 14, 2010 #3
    I believe i am aware of what EMF is. correct me if im wrong.
    EMF is a type of V that's conceptually pure volt from (say a battery) unaffected by any resistance
    whereas V(terminal) is type of V that is actually used. (more calculated)

    it's not understanding EMF/ V(terminal) that i'm having trouble with. I'm having trouble with actually applying them in an equation.
  5. Sep 14, 2010 #4


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    EMF is voltage. You would do well to forget you ever heard of EMF. If you see the term "EMF" in a book, white-it-out and replace with ink the word "voltage". I'm not kidding.

    Just use these formulas and you will be fine for all things ohmic:

  6. Sep 15, 2010 #5


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    I should spend more time actually reading original posts. Sorry. In this case you have a battery modeled as an ideal voltage source of 15V in series with an internal resistance.
    In this case, they are simply telling you that the teminal voltage, Vt, is what is appearing across the load resistor and so P= Vt^2/R applies.
  7. Sep 15, 2010 #6
    thank you for reply
    so I use V(terminal) for calculating power and EMF for calculating total current?
  8. Sep 15, 2010 #7


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    For power dissipated by the load resistor, from the point of view of the load resistor-- Vt is applied across it so that's what you use. For current going through the resistor, why not use I=V/R with V = Vt and R = Rload?

    Vemf can be used if you want to calculate the total power dissipation of the circuit (power dissipated by Rload and Rint), because Vemf is the voltage across the series combination of the two resistors)
  9. Sep 15, 2010 #8
    Ah I see it now. Thank you very much
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