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Electromotive force

  1. Sep 6, 2011 #1
    electromotive force....

    why is v called electromotive force if it is not a force??
    is there a reason for this??
     
  2. jcsd
  3. Sep 7, 2011 #2
    Re: electromotive force....

    The term electromotive force was coined in the days when some sort of

    "driving force" was attributed to lots of physical phenomenon eg life itself, evolution, thermodynamics and so on.

    So it is not suprising that a "driving force" was attributed to electric phenomenon.

    Electromotive force actually refers to the external sources of energy to a system.

    go well
     
  4. Sep 7, 2011 #3
    Re: electromotive force....

    Well, conceptually it is like a force.
     
  5. Sep 7, 2011 #4

    sophiecentaur

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    Re: electromotive force....

    Best not to conceive it that way without a number of caveats. Electricity is a pretty subtle and abstract thing and the quantities used are, in my opinion, best kept within their own realm. You are OK treating it as the same sort of force as, for instance 'force of personality' or 'I was forced to do it', which don't actually involve anyone pushing anyone necessarily.
    You can end up thinking of a battery pushing electrons around a circuit and letting that picture govern your appreciation of what happens in a transformer or a radio antenna will give you a big disappointment.
     
  6. Sep 7, 2011 #5
    Re: electromotive force....

    Actually, no it is not. Coulomb's law relates the actual force between 2 charges as

    F = kq1q2 / r2

    The "F" above is literally force. Similarly, we can write equations for the force between 2 wires carrying current, or plates of a charged cap, etc. Force has units of mass times length divided by time squared.

    The quantity "emf" is work per charge, which is mass times length squared divided by (time squared times charge). The 2 quantities are different & should not be compared as similar.

    Claude
     
  7. Sep 7, 2011 #6

    jim hardy

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    Re: electromotive force....

    i dont know what you're studying tina

    my background is more practical than theoretical. I have to think in physical pictures and try to figure out why the formulas apply.

    for analyzing how electronic and electrical machinery works
    i use the simple notion that EMF is, as the name implies, a "motive force" that motivates electrons to move.
    In an electrical field the electron is pushed parallel to the direction of the field
    and in a moving magnetic field it's pushed mutually perpendicular to the field and its motion relative to the field - remember Q*V cross B ?

    it has unit of joule per coulomb not newtons or dynes

    As Cabrahaom points out voltage is really work done moving a charge from one place to another. Hence potential difference.

    I guess maybe if you multiplied his kq1q2/r^2 by dr and integrated from r1 to r2 you'd get the volts? I notice k has newtons in it so you'd get F X D ?
    http://en.wikipedia.org/wiki/Coulomb's_law

    But back to wrapping your brain around why things work -
    in figuring out why an electric motor makes torque, i think of the electrons inside the wire being physically pushed sideways against the insulation by the magnetic field through which they are moving.
    Hall effect is another manifestation of physical force on electrons. It squeezes them to one side of a sensor.

    So if you use my oversimplified thought picture technique to get your mind around how things works it will get you started . But keep on questioning.
    And develop a habit of checking your mental pictures against the formulas and make the numbers work out. It'll keep your thinking straight.

    "
    When you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge of it is of a meager and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced it to the stage of science. ""

    Sir William Thompson, Lord Kelvin (1824-1907)

    Perhaps you are one of those fortunate people who can think in formulas. I envy them.



    old jim
     
  8. Sep 8, 2011 #7
    Re: electromotive force....

    okh...i seem to hav got the point..
    thank you for sharing your opinion and coming up wid such analytical answers..
    i m really grateful to ol dose who bothered to reply!!!1
     
    Last edited: Sep 8, 2011
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