# Difference between revisions of "Dipole moment"

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The electric dipole moment is a measure of polarity. It is the second term in a multipole expansion of a field (the first one being the monopole, or Coulombic, term.) Its definition, for a system of point charges, is $\mathbf{p} = \sum_{i=1}^N \, q_i \, \mathbf{r}_i.$

For molecular systems, it is customarily given in units of Debyes, after the physicist Peter J. W. Debye. In SI units, $1$D equals approximately $3.33564\times 10^{-30}$ Coulomb-meter (exactly $10^{-21}$C m$^2$/s divided by $c$, the speed of light in vacuum). Conversely, 1 C m = $2.9979 10^{29}$D. This is unit is convenient for molecular systems since 1Å$\times 1 e=4.803$D.