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    can someone explain to me how this equation is derived. there was a question to prove this equation. tnx
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    (Original post by vp03)
    can someone explain to me how this equation is derived. there was a question to prove this equation. tnx
    I don't know the derivation but i E = I (R+r) is the same as V=IR.
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    It comes from the fact that the sum of voltages across components is equal to the supply voltage. Using V=IR for two resistances r and R gives the equation.
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    You just use Kirchoff's 2nd law (sum of EMFs around a closed loop = sum of p.ds around closed loop).
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    (Original post by vp03)
    can someone explain to me how this equation is derived. there was a question to prove this equation. tnx
    Kirchoff's 2nd law: sum of emf around a loop=sum of pds around loop

    Edit: oops got beaten by nylex and co
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    the revision thread might help. ive put kirchoffs' laws on it aswell

    http://www.uk-learning.net/t47596.html
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    THis seems like a long way rouind, but this is what I would write:
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    (Original post by vp03)
    can someone explain to me how this equation is derived. there was a question to prove this equation. tnx
    If you are talking about the internal resistance of a cell, then you assume that the cell is made up of a 'perfect cell' with no internal resistance, and a resistor with resistance r.

    Now imagine a complete circuit with a cell and resistor.

    Using Ohm's law,

    e.m.f. of cell = I(R + r)

    where R is the resistance of the external resistor and r is the internal resistance of the cell

    Hence,

    e.m.f. = IR + Ir
 
 
 
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