# Electric dipole moment, which charge is the $q$ for?

Electric dipole moment says $p = qd$. Which charge does the $q$ equal to?

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You book no doubt sets this up with a particular situation. Look back at how they defined it... – dmckee May 22 '13 at 20:54
$q$ is the magnitude of the charge of one of the two charges that integrate the dipole. – Ana May 22 '13 at 21:37
@Anuar that should be an answer – David Z May 23 '13 at 1:34

The dipole moment of a system of charges $q_i$ located at positions $\mathbf r_i$ is defined as the vector $$\mathbf d=\sum_i q_i\mathbf r_i.$$ If you have a single charge $q$ at $\mathbf r=d\hat{\mathbf e}$ then $\mathbf{d}$ has magnitude $qd$ and points along the unit vector $\hat{\mathbf e}$. Usually, however, this is introduced for two charges of equal but opposite charge $q$ and $-q$. In this case $\mathbf{ d}$ has magnitude $qd$, where $d$ is the charges' relative separation, and points from the $-$ charge to the $+$ one. If there are more charges you need to apply the general formula.