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What is the definition of EMF?

  1. Mar 18, 2012 #1
    Actually my sir asked me the definition of EMF so I just tell him that "Suppose a resistance(R)
    is connected across the terminals of a battery.A potential difference is developed across its ends.Current(or positive charge) flows from higher potential to lower potential across the resistance by itself.But inside the battery work has to be done to bring the positive charge from lower potential to higher potential.The influence that makes current flow from lower to higher potential(inside the battery) is called EMF.If w is the work done by battery in taking a charge q from negative terminal to positive terminal,then work done by battery per unit charge emf(E) of the battery.
    thus, E=W/q"
    then he asked from someone else then that boy just tell that
    "Emf is equal to voltage at zero current i.e
    V=E-IR or E= V+IR

    and when I=0,

    so my sir said me that this is the exact definition??
    so my question is who is more correct??
  2. jcsd
  3. Mar 18, 2012 #2
    Your definition is better.
    EMF is the energy supplied to electric charge. It is measured in Joules per Coulomb and 1 Volt means 1Joule per Coulomb
    POTENTIAL DIFFERENCE is energy dissipated (given up) by charge passing through resistance. This also measured in Joules per Coulomb or Volts.
  4. Mar 18, 2012 #3
    EMF refers to voltage generated by a battery or by the magnetic force according to Faraday's Law, which states that a time varying magnetic field will induce an electric current.
    Electromotive "force" is not a force (measured in newtons) but a potential, or energy per unit of charge, measured in volts. Formally, emf is the external work expended per unit of charge to produce an electric potential difference across two open-circuited terminals. The electric potential difference produced is created by separating positive and negative charges, thereby generating an electric field. The created electrical potential difference drives current flow if a circuit is attached to the source of emf. When current flows, however, the voltage across the terminals of the source of emf is no longer the open-circuit value, due to voltage drops inside the device due to its internal resistance.

  5. Mar 18, 2012 #4

    jim hardy

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    Science Advisor
    Gold Member

    I've got to where i try to understand why things got the name they were given.

    To that end it's sometimes helpful to parse acronyms backward like we do for RMS.

    Term EMF is acronym for Electro-Motive-Force , that is a Force that Moves Electric Charge. (in some circles, that'd be electrons)

    It is not implied by the name of that term whether the origin of the Force is elecrostatic, magnetic , thermoelectric , or electrochemical.

    And that's how i answer that question.

    GM's "Electromotive Division" makes those beautiful railway locomotives.
    Every time i see one my thoughts leap to the huge generators and traction motors inside, and QV cross B. Now THERE"S some torque !
    Wouldn't it be fun to be see the charge getting shoved sideways inside those conductors?
  6. Mar 18, 2012 #5


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    Staff: Mentor

    Hi saurabhtiwari. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

    Discussion of EMF is usually careful to point out that its measurement requires that the generator (or battery) be on open-circuit or no-load condition, (I=0), so that the voltage you measure at the terminals is equal to the EMF. If there is current flowing, then internal resistance of the generator or motor or battery will cause the voltage you measure at the terminals to differ from the true EMF.

    I hope that clears up your concern. Please visit again with more questions. :smile:
    Last edited by a moderator: May 5, 2017
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