(Figure 3.118) shows two dipole moments parallel to each other and placed at a distance x apart. What is the magnitude of force of interaction ? What is the nature of force, attractive or repulsive ?
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Answer

`(3)/(4 pi epsilon_0) (p_1 p_2)/(x^4)`

Explanation

Potential energy of dipole system
`U = - vec p_2 . vec E_(21) = - p_2 (1)/(4 pi epsilon_0) (p_1)/(x^3) cos pi`
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`E_(21)` is the field due to `p_1` at `p_2` as shown.
`U = (1)/(4 pi epsilon_0) (p_1 p_2)/(x^3)`
`F = -(del U)/(del x) = (3)/(4 pi epsilon_0) (p_1 p_2)/(x^4)`
F comes out to be positive, so it is a repulsive force.