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A level physics help!!!! Electric fields

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    Hi
    Could someone please help me with part b of this problem?

    Calculate
    a) the force between 2 charges of +1.4nC and +1.6nC on point conductors 40cm apart in air.
    b)What size of charge on a third point conductor placed midway between the first two conductors would result in doubling of the magnitude of the force on the 1.4nC charge?

    My answer for the first part was 1.2586x10^-7

    Thanks in advance!!!
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    (Original post by Skipper12345)
    Hi
    Could someone please help me with part b of this problem?

    Calculate
    a) the force between 2 charges of +1.4nC and +1.6nC on point conductors 40cm apart in air.
    b)What size of charge on a third point conductor placed midway between the first two conductors would result in doubling of the magnitude of the force on the 1.4nC charge?

    My answer for the first part was 1.2586x10^-7

    Thanks in advance!!!
    Hi,
    you know the force in part a) is  \vec{F_1} = \frac{-q_1q_2}{4\pi\epsilon_o r^2}\vec{x}
    So you want this force to be doubled when we introduce a charge q3 between q1 and q2

     2\vec{F} = -2\frac{q_1q_2}{4\pi\epsilon_o r^2}\vec{x}= -\frac{q_1q_2}{4\pi\epsilon_o r^2}\vec{x}-\frac{q_1q_3}{4\pi\epsilon_o (r/2)^2}\vec{x}

    now you can solve this for q_3
 
 
 
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