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JEE Mains · Physics · STD 12 - 1. Electric charges and fields

A point particle of charge \(Q\) is located at \(P\) along the axis of an electric dipole 1 at a distance \(r\) as shown in the figure. The point P is also on the equatorial plane of a second electric dipole 2 at a distance r . The dipoles are made of opposite charge q separated by a distance \(2 a\). For the charge particle at P not to experience any net force, which of the following correctly describes the situation?

  1. A \(\frac{a}{r} \sim 10\)
  2. B \(\frac{a}{r} \sim 20\)
  3. C \(\frac{a}{r} \sim 0.5\)
  4. D \(\frac{a}{r} \sim 3\)
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Answer & Solution

Correct Answer

(D) \(\frac{a}{r} \sim 3\)

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Detailed explanation

\begin{aligned} & \Rightarrow \frac{1}{(r-a)^2}-\frac{1}{(r+a)^2}=\frac{2 a}{\left(a^2+r^2\right)^{3 / 2}} \\ & \frac{4 a r}{\left(r^2-a^2\right)^2}=\frac{2 a}{\left(a^2+r^2\right)^{3 / 2}} \\ & \left(r^2-a^2\right)^2=2 r\left(a^2+r^2\right)^{3 / 2} \\ &…

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