[Physics] Meaning of the phrase “dipole moment of the combination”

definitiondipoleelectrostatics

Here is a question I came across in a book:

Three point charges $-q$,$-q$ and $2q$ are placed on the vertices of an equilateral triangle of side length $d$ units.What is the dipole moment of the combination?

I had learnt that the dipole moment is defined for 2 point charges only with equal magnitudes but opposite character.I am in a bit of a fix regarding what the author means by that phrase in bold.Thank you in advance!

Edit:I just noticed theoretical physics has been merged here;I sincerely apologise if the question is unbearably stupid.

Best Answer

Dipole moment is a vector and can be calculated using formula

$$\vec{p} = \sum_i q_i \vec{r}_i.$$

It can be shown easily using the formula above that in case of two charges separated by distance $d$

$$\vec{p} = q \vec{d},$$

where vector $\vec{d}$ goes starts at negative ends at positive charge.

http://en.wikipedia.org/wiki/Electric_dipole_moment#Dipole_moment_density_and_polarization_density

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