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Diferences b/w emf & voltage,Resistance & resistivity

  1. Dec 20, 2009 #1
    hi im new member of this forum..Plz tell me how to use this forum & how to ask Question from other members...Plz tell me differences b/w emf & voltage,Resistance & resistivity..
     
  2. jcsd
  3. Dec 20, 2009 #2

    tiny-tim

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    Welcome to PF!

    Hi hafiz16! Welcome to PF! :smile:

    emf ("electromotive force") means different things in different books.

    usually, it means the same as voltage

    See http://en.wikipedia.org/wiki/Electromotive_force#Terminology"
    … etc etc etc

    Resistivity of a material is resistance times cross-section area per length … see http://en.wikipedia.org/wiki/Resistivity#Definitions" :wink:
     
    Last edited by a moderator: Apr 24, 2017
  4. Dec 20, 2009 #3
    I believe that "emf" usually refers to a potential due to an energy sourcing device, like a battery, or a generator. This emf is indeed a voltage, as well as being a potential. But, if an energy dissipating device, i.e. passive element, incurs a voltage drop when carrying current, this is a drop in potential, or voltage, but the term "emf" is not used here.

    Thus, potential, and voltage are general terms. The term emf is included, as emf is a voltage and potential. But emf is used specifically to describe a voltage/potential generated by an energy sourcing device. The term "drop" denotes dissipation. The emf and the drop are both measured in volts. One indicates energy being sourced, the other indicates dissipation.

    Clear?

    Claude
     
  5. Dec 20, 2009 #4
    The emf of a device measures the energy gained by unit charge passing through a cell or dynamo etc. The PD between two points in a circuit measures the energy lost per unit charge as it passes through those points. Both are measured in Volts (joule per coulomb)
    One is energy gained. The other is energy lost.
    In a closed circuit, energy gained equals energy lost. This is often expressed as Kirchhoff's Rule.
     
  6. Dec 20, 2009 #5
    The way I like to remember it, emf is the negative line integral of the electric field between two points, whereas a potential difference is the negative difference between the potential function of the electric field. When the electric field actually has a potential function, these would be the same thing (but we don't call it emf when the field is the gradient of some function). However, some electric fields don't have potential functions, for example a circular electric field can't be written as the gradient of a potential. But it will still have an emf.

    The important thing here is that the voltage across a closed loop is always zero, but you can have a nonzero emf by integrating around a closed look. As you know,

    [tex]\oint_C\vec{E}\cdot d\vec{l} = -\dfrac{d}{dt}\iint\vec{B}\cdot d\vec{A}[/tex]

    So Faraday's Law itself gives an example of how emf can be different from a voltage. If the electric field had a potential function, then the left side would always be zero.

    You could also think of the emf as the "work" done by an electric field.
     
  7. Dec 21, 2009 #6
    thxk for all..but i still not clear & i am confuse..What is emf & voltage..
     
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