= Electric quadrupole moment
{title2=$Q_{ij}=\sum_a q_a(3r_{a,i}r_{a,j}-|r_a|^2\delta_{ij})$}
The trace-free <electric quadrupole moment> is a symmetric tensor measuring the second-order term in an <electric multipole expansion>. For a localized charge distribution its contribution to the far-field <electrostatic potential> is $Q_{ij}\widehat r_i\widehat r_j/(8\pi\epsilon_0r^3)$, with summation over indices. The moment may be nonzero even when the total <electric charge> or <electric dipole moment> is nonzero; it should not be confused with an <electric quadrupole> whose lower moments vanish. Changing the origin generally changes the tensor when lower moments are nonzero.
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