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Measuring the EMF

  1. Mar 18, 2016 #1
    Guys, is it accurate to measure the battery's emf by putting voltmeter across the battery( it is not connected in circuit). I think this is accurate because there is no current flowing, this means that there is no voltage across the internal resistance.
     
  2. jcsd
  3. Mar 18, 2016 #2

    berkeman

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    Sure. There will be a small amount of current flowing through the voltmeter's detection circuit, but that is usually negligible. :smile:
     
  4. Mar 18, 2016 #3

    meBigGuy

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    Measuring the voltage across a battery is valid anytime you measure it. You just need to include the conditions as part of the measurement.

    For example you can measure at, no load, or 10 ohm load, or 100ma load and will get slightly different numbers. A 1.5V battery may measure 1.35V open circuit, or 1.2V under a 1 amp load (made up numbers).
    You just need to know what you are measuring.
     
  5. Mar 19, 2016 #4
    There is a slight difference between EMF and voltage. The two are usually used interchangeably, so unless there's a pedantic problem this can be ignored.

    Please let us know if you need the difference.
     
  6. Mar 19, 2016 #5
    Can you please tell me the difference
     
  7. Mar 19, 2016 #6
    Sure.

    Simply, EMF is a force. Volts (voltage) is the unit with which we measure the force.

    "But wait," I hear you cry. "Forces are measured in Newtons."

    EMF can also be measured in Newtons, but only if we know the charge of our object and its distance from the EMF generating object. We need to know our charge because more charge (on "our" side) means more force. We need to know the distance because more distance from the EMF generating object means less force.

    So Volts can be thought of as standard bundles of charge and distance. The standard charge is a Coulomb and the standard distance is a meter. So:

    V = Nm/C.

    This neatly packages the force allowing it to be applied in everyday situations.
     
  8. Mar 19, 2016 #7

    meBigGuy

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    That explanation simply confuses me by bringing up mechanical force, which is a different animal. Just because the word is the same doesn't mean they are closely related. They should call it "electromotive stuff"

    EMF is the voltage developed by any source of electrical energy.

    As wikipedia says: The word "force" in this case is not used to mean mechanical force, measured in newtons, but a potential, or energy per unit of charge, measured in volts.

    Maybe it would help if you illustrated with physical situation where emf and voltage cannot be used interchangeably.
     
  9. Mar 22, 2016 #8
    Here's an example where they point out the difference (which isn't really much of a difference.). Note the electric field can either be measured in Volts/meter or Coulombs/Newton. As a practical matter, in electrical engineering, we almost always use Volts. But it's not always the only way.
     
  10. Mar 22, 2016 #9

    sophiecentaur

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    The terms Electromotive Force crept in before anyone really understood the subject and when it was assumed that a Force was needed for anything to Happen. Imo, it was a shame it was ever born but we are stuck with it. It is not a force because its units are Volts. The bombproof way to define it is:
    emf = (limit as q→0) E/q
    Where E is the energy transferred and q is the charge transferred.
    Alternatively:
    emf = (limit as I→O) P/I
    P is power and I is current
     
  11. Mar 22, 2016 #10

    meBigGuy

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    Jeff, I still don't understand your difference between emf and voltage. An electric field is a different animal (not an emf) and is not measured in volts. (rather volts/meter). Where can I not use emf (electromotive force) and voltage interchangeably?
     
  12. Mar 22, 2016 #11
    The term "emf" was coined in a time where things were not as well understood as they are today. The "force" in "emf" is not a force at all. The term "emf" refers to a ratio of energy gained per unit charge. A battery rated at 12 volts provides 12 joules of energy per coulomb of charge. The actual force acting on a charge is Lorentz force. The electric force on a charge due to an electric field E is given by: F = qE. BR.

    Claude
     
  13. Mar 22, 2016 #12

    Merlin3189

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    I'm finding this very turbid and obfuscatory.
    I thought I understood emf and voltage before. It's a revelation to me that emf has anything to do with mechanical force - well any more than current does, say.
    My own favourite is PD. I call that voltage as well. I like PDs because I can see them (well, observe or measure.) Emfs I usually have to deduce from the pds they cause.
    In any case I measure them all in volts using a machine which tells me how far a spring is stretched by the current flowing through a coil in a magnetic field. (Aaargh! Hoist with my own petard! Voltage does have something to do with mechanical force.)

    You've really got me worried now. I shall have sleepless nights until I get myself a yellow plastic box that measures not just true RMS (is there any other sort?) but true emf as well.
     
  14. Mar 22, 2016 #13
    A needle type voltmeter measures current, not voltage. The voltage across the leads is connected across a resistor and the meter is calibrated so that the current in said resistor reads a specific voltage. Current is related to mechanical force at dc, but voltage is not related to force. In motor control, speed relates to voltage, torque relates to current. And vice-versa. Does this help? BR.

    Claude
     
  15. Mar 22, 2016 #14

    meBigGuy

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    It doesn't. At least not directly.

    Not sure yet what Jeff was getting on about with respect to EMF and Voltage not being used interchangeably in some situations. He has talked about electric fields, and mechanical forces, but still has me confused.
     
  16. Mar 22, 2016 #15

    jim hardy

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    Would not
    a nonconductive Ziploc sandwich bag
    containing one coulomb of charge
    when placed in an electric field of one volt per meter
    experience one Newton of force in direction of the field ?
     
  17. Mar 22, 2016 #16

    meBigGuy

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    Nobody is arguing that mechanical forces can't be created by electric fields acting on charges. Mechanical forces are off-topic and confusing the issue.

    At issue is the following statement:
    We have yet to hear of a situation where EMF (electromotive force) and Voltage are different. The confusion started at post #6.
     
  18. Mar 22, 2016 #17
    EMF is measured in Volts. They are different. It is a pedantic difference.

    EMF could also be measured in other units (not that I'm aware of any; pounds instead of newtons? feet instead of meters?). Voltage could not be measured in other units, because the name implies the the unit of measure.

    Volts are (among other things) Newton meters per Coulomb. That makes them related to force, but not the same as force.

    And yes, I'm picking nits, but that's what "pedantic" means.
     
  19. Mar 22, 2016 #18

    meBigGuy

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    Hmmm --- I don't get it even if you are pedantic. EMF and voltage are measures of the same thing so can be used interchangeably. Doesn't matter what you call them, or what the units are, they measure the same thing. Can you agree with that?

    BTW, to be pedantic, saying the word voltage implies measurement in volts is a big assumption. Voltage could easily be expressed in standard lemons.
     
  20. Mar 22, 2016 #19
    Here's an example of where they differ:

    What is the EMF of a 9V battery. Answer in long ton furlongs per mole charge.
     
  21. Mar 22, 2016 #20

    meBigGuy

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    And that will be exactly equal to 9 volts. They are identical.

    What is the Voltage of a 9V battery? Answer in long ton furlongs per mole charge.
     
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