emf and voltage?

Discussion in 'Homework Help' started by Praveenponniah, Aug 20, 2015.

  1. Praveenponniah

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    Aug 20, 2015
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    both emf and voltage are same? if its not so explain the concept clearly..
     
  2. WBahn

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    For most purposes, the terms are used interchangeably. I think a strict interpretation is that EMF is specifically the voltage produced by a source, whereas voltage is applicable to ANY potential difference.
     
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  3. nsaspook

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  4. WBahn

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    How does that show a difference between the terms "electromotive force" and "voltage"? Even Lewin used the terms interchangeably. His entire demo could have been done using one or the other term exclusively (flip a coin to pick which). His demo delves into the difference between conservative and non-conservative electric fields.
     
  5. nsaspook

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    I think he describes motional EMF as voltages which are generated by changing fields vs voltage drops from energy loss in resistors.

    Edit:
     
  6. crutschow

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    I use the definition that a volt is the unit of EMF (Electro Motive Force), just as the newton/dyne is the unit of physical force.
     
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  7. crutschow

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    I use the definition that a volt is the unit of EMF (Electro Motive Force), just as the newton/dyne is the unit of physical force.
     
  8. alfacliff

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    the e in ohms law is electromotive force, or voltage. there is no difference between emf and voltage, if there were, then ohms law would have to have different formulas. by the way, the I in ohms law is current in amps, just an older term for current.
     
  9. nsaspook

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    They have the same units but emf IMO is typically used when there is a conversion of electrical energy like in a coil ("induced emf"), battery, motor or generator as the generated potential source. A real battery emf [​IMG] (voltage at zero current) has internal resistance so the Ohms law voltage would be exactly the same as emf only with negligibly small currents from the battery. So as we increase current from the battery the emf will remain the same (assuming the chemical reaction can sustain the current draw and generate the same [​IMG]) but the voltage from the cell will decrease.
     
    Last edited: Aug 20, 2015
  10. WBahn

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    But he also used the term EMF for the battery, saying explicitly, "The battery has an EMF of 1V." (25 seconds into the Part 1 video).

    I don't recall him using the term "motional EMF", but I very possibly missed it. At 4:40 he talks about the magnetic flux change being 1V and moments later says, "E-induced at that moment in time is 1V".

    In general, the terms, "induced EMF" and "induced voltage" are widely used and I'm not aware of any difference in meaning, but I don't know that that is strictly the case.
     
  11. WBahn

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    Ohm's Law is more commonly written as V=IR, but E=IR is still widely used. Just as P=IV and P=IE are both used.

    The "I" for current comes from "intensity", an older term for current. Voltage was also known as "tension", which we still hear occasionally in the form of "high tension lines".
     
  12. nsaspook

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    Yes he would as that's the proper term for historic reasons to use.
    https://en.wikipedia.org/wiki/Electromotive_force
     
  13. WBahn

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    So how is that different from what I posted in my first response, namely, "For most purposes, the terms are used interchangeably. I think a strict interpretation is that EMF is specifically the voltage produced by a source, whereas voltage is applicable to ANY potential difference."

    I'm still not seeing any fundamental distinction. It is completely reasonable to talk about the output voltage of a source. While I would recommend against it, I would not say that it is wrong to talk about the EMF at the output of a voltage divider.
     
  14. nsaspook

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    It's more a matter of the origin of that voltage not semantics and why when we measure circuits with a voltmeter we can get results that seem counter to the expected results if we think electricity is just a matter of current flow in a wire.

    I wouldn't call it wrong either, just imprecise. An understanding of why we use the physical source emf (a term we rarely use out of context) instead of voltage at times leads directly to the subject of losses and efficiency in motor/generator, transformer or solar energy (emf generated proportional to photon flux) circuits.
     
    Last edited: Aug 20, 2015
  15. Lool

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    Personally, I view voltage as a general term that can apply to potential voltage, or emf voltage. I think of EMF as anything that is not potential. I view potential energy as the conservative force potential energy (per unit charge).

    No doubt this is still confusing to many people.

    Another view is to think of potential as applying to charges that actually have that potential energy implied by the potential voltage, while EMF could be thought of as the potential that charges can and will have, when needed, or when available.

    Let's take the case of an EMF generated by a generator. If the generator is not hooked up to anything, then there are very few charges that are actually at a potential voltage corresponding to the EMF voltage. However, close the circuit, and the charges in the current are given the potential energy to do work (create kinetic energy in the charge for example, or heat up a resistor, as the charge loses energy)

    Similarly, a battery has this ability to provide potential energy to any charges needed to do work, but very few actual charges have this potential energy if the circuit is open.

    Now, let's contrast this with potential voltage from a capacitor, under the assumption that we are trying to use the capacitor like a battery, to power a circuit. In this case, all the charges available to do work are actually at that potential energy, as implied by the voltage.Hence, I would call this a potential, although I have heard people refer to this as emf too. So, I don't claim this is cut and dried. Now, even though all charges are at the potential energy, once the chrges start to get used, the potential decreases, and charges used later have less potential. The only way to keep the charges at the same voltage is to use an emf.

    This has always been a confusing question, and it has never been easy to explain to others or fully comprehend in my own brain, but this is the best I can do.

    It is not unreasonable to not worry about the definition and just get familiar with what we call the various voltages. You dont always need a clear definition to know what something is. Biologists struggle to define the word "life", yet we all know what life is. I once read an essay where a scientist made the point of how hard it can be to define what a "wave" is, yet we usually know what phenomenon are wave-like, and which are not
     
    Last edited: Aug 20, 2015
  16. JoeJester

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    My view is:

    Add an adjective to voltage and you have a whole lot of terms including bias, de-ionizing, ionizing, etc.

    Why shouldn't we. If we want effective communications where the receiver understands the message, adherence to the definitions is a must. Otherwise, it's time to play 20 questions with the sender.
     
  17. nsaspook

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    Good points but I would be careful on how you see kinetic energy moving into charge and lost energy from charge in a circuit.
     
  18. JoeJester

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    I disagree. Assuming the generator is turning at the proper RPM, you can measure the potential difference between two points ... with a VOLTMETER. A battery not connected to anything, you can measure the potential difference between two points ... with a VOLTMETER. The voltmeter "completes" the circuit causing some current to flow, and given todays DVMs, we are talking little current.
     
  19. Lool

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    I dont mean that we should not try to define things as best we can. My point was that sometimes perfect definitions are not possible and we have working definition and general knowledge about which things fit into which classifications. I'm not sure EMF/Potential necessarily fits into this class, but there seems to be so much confusion about this question, even among knowledgeable people, that perhaps it is a factor in this case.
     
  20. Lool

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    Lol, you took my quote where I say "if the generator is not hooked up to anything" and then you hooked it up to a voltmeter.

    That seems a little bit not fair. :)
     
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