A dipole of moment p is placed on the axis of a fixed point charge Q at distance r, with p parallel to the electric field. Assuming r is much larger than the dipole size, the magnitude of the net force on the dipole is:
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Question

A dipole of moment p is placed on the axis of a fixed point charge Q at distance r, with p parallel to the electric field. Assuming r is much larger than the dipole size, the magnitude of the net force on the dipole is:

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G1 Physics · Accepted Answer

For a dipole aligned with a non-uniform field, F=p(dE/dr) in magnitude. E=Q/(4πε₀r²), so |dE/dr|=2|Q|/(4πε₀r³). Hence |F|=2|Qp|/(4πε₀r³).