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B Confusion about Electro-motive force (EMF)

  1. Sep 1, 2016 #1
    The definitions of electromotive force of a cell that i have read include:

    1.When no current is drawn from a cell,i.e., when the cell is in open circuit, then potential difference between the terminals of the cell is its electromotive force.

    2.The electromotive force of a cell is defined as the energy spent or the work done per unit charge in taking a positive charge around the complete circuit of the cell i.e., in the circuit outside the cell as well as in the electrolyte inside the cell.

    I do not understand what definition 1 means. I think it is probably refers to the work done in moving the charge inside the cell since in the definition it is said that the circuit is open but then its meaning does not match with definition 2.
    My question is, what is the actual meaning of definition 1?
     
  2. jcsd
  3. Sep 1, 2016 #2

    sophiecentaur

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    It means that the work done on an infinitesimally small charge q will be Vq, where V is the PD across the terminals. Same definition (Joules per Coulomb) but for a vanishingly small Charge. The Energy dissipated due to any internal resistance will be q(squared) r. This gets smaller faster than qV so it is less and less significant.
     
  4. Sep 1, 2016 #3
    Here does pd across the terminal refer to the work done in moving the charge inside the cell or outside the cell.
     
  5. Sep 1, 2016 #4

    sophiecentaur

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    However it's done will involve the same work done 'on' the charge. The PD can be the result of action between two chemicals or the result of the cell being 'charged' from an external source - or the cell could be a capacitor. You have to be careful to remember this is the work done on the charge and not the actual work done to get it there.
     
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