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Math Help - need help in transposition

  1. #1
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    need help in transposition

    Hi,
    i need help in transposing this fomula

    u=e-(i x r)
    where,
    u=terminal voltage
    e=E.M.F
    i=current
    r=internal resistance

    i want to caculate r (internal resistance). i cant transpose this formula to the extend as it confuses me.
    i have ended up doing in this way and it may be right or may be wrong but please explain me.

    (u-e) divided by i = r

    therefore

    r = u-e divided by i

    please explain me.
    thankyou.
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  2. #2
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    I'm assuming that you want to express r in terms of the other variables. Is this correct?

    If you indeed want this (I'm changing the variables to those most commonly used to the equation I think it is.

    V = \varepsilon - Ri

    Now you sum Ri - V in BOTH sides of the equations:

    <br />
V + Ri - V = \varepsilon - Ri + Ri - V<br />

    EDIT: it was wrong before:

    Ri = \varepsilon - V

    and finally,

    R = \frac{\varepsilon - V}{i}

    So your calculations are, indeed, correct. Is this clear why it works?
    Last edited by Rafael Almeida; October 26th 2008 at 04:34 PM. Reason: Error correction
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  3. #3
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    Negative problem

    The problem here is that the product has a minus in front. Easiest way to solve this is to X by -1 it get -u=-e + ir, which can then be rearranged to give e-u = ir, so e-u/i = r.
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  4. #4
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    Ri = \varepsilon - (-V)

    There is no need for both - here, the first disappears when you cancel the [tex]Ri
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  5. #5
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    Quote Originally Posted by Here to Help View Post
    Ri = \varepsilon - (-V)

    There is no need for both - here, the first disappears when you cancel the [tex]Ri
    Surely not, I just left this to increase detail level.
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  6. #6
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    e (- Ri + Ri) - V simplifies to e - V not e + V, the correct transposition it e-V/i.
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  7. #7
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    Quote Originally Posted by Here to Help View Post
    e (- Ri + Ri) - V simplifies to e - V not e + V, the correct transposition it e-V/i.
    True. Edited my previous post to make it right, thanks for pointing it out. I should have noticed this before, specially because the electromotive force \varepsilon should be greater than tension V
    Last edited by Rafael Almeida; October 26th 2008 at 06:11 PM.
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