Electric Dipole Moment

The ideal dipole

An electric dipole consists of two point charges $+q$ and $-q$ separated by a small distance $d$. The system has zero net charge, yet produces a field that does not vanish — because the two charges are spatially separated.

Dipole moment vector

The dipole moment is defined as:

where $\vec{d}$ is the displacement vector from $-q$ to $+q$.

• Magnitude: $p = qd$, SI unit: C·m
• Direction: from the negative charge to the positive charge

Why this quantity matters

For distances $r \gg d$, the field of a dipole falls as $1/r^3$ (faster than a point charge's $1/r^2$), but its spatial pattern depends entirely on $\vec{p}$ — not on $q$ and $d$ separately. The dipole moment is the natural single quantity characterising the dipole at large distances.

Atomic dipoles

In atoms and molecules, asymmetric electron distributions create permanent or induced dipole moments. These are measured in Debye: $1\text{ D} = 3.336 \times 10^{-30}$ C·m.