Two dipoles `p_1 and p_2` are placed along the same axis at a distance x apart as shown in (Fig. 3.119). What is the magnitude of the force of interaction ? What is the nature of force, attractive or repulsive ?
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Answer

`(3)/(2 pi epsilon_0) (p_1 p_2)/(x^4)`

Explanation

Potential energy of system is
`U = - vec p _2 . vec E_(21) = - p_2 (1)/(4 pi epsilon_0) (2 p_1)/(x^3) cos 0 = -(1)/(4 pi epsilon_0) (2 p_1 p_2)/(x^3)`

`F = - (del U)/(del x) = - (6)/(4 pi epsilon_0) (p_1 p_2)/(x^4) = -(3)/(2 pi epsilon_0) (p_1 p_2)/(x^4)`
F comes out to be negative, so it is an attractive force.