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

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

• You book no doubt sets this up with a particular situation. Look back at how they defined it... – dmckee --- ex-moderator kitten May 22 '13 at 20:54

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.