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# A level physics help!!!! Electric fields

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1. Hi

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

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Just quoting in Fox Corner so she can move the thread if needed
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(Original post by Fox Corner)
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3. (Original post by Skipper12345)
Hi

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

Hi,
you know the force in part a) is
So you want this force to be doubled when we introduce a charge q3 between q1 and q2

now you can solve this for

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