What is a dipole moment?

Asked by  | 29th Mar, 2009, 06:59: AM

Expert Answer:

the electric dipole moment (or electric dipole for short) is a measure of the polarity of a system of electric charges called dipole.

In the simple case of two point charges, one with charge + q and one with charge q, the electric dipole moment is:

where r is the displacement vector pointing from the negative charge to the positive charge. This implies that the electric dipole moment vector points from the negative charge to the positive charge. Note that the electric field lines run in the opposite direction, i.e. away from the positive charge and toward the negative charge. There is no inconsistency here, because the electric dipole moment has to do with the positions of the charges, not the field lines.

On the other hand Moment of force (often just moment) is a synonym for torque, an important basic concept in physics, civil engineering, and mechanical engineering. In the context of mechanical engineering, the terms are not necessarily interchangeable, but one or the other may be preferred in a specific context. For example, "torque" is usually used to describe a rotational force down a shaft, for example a turning screw-driver, whereas "moment" is more often used to describe a bending force on a beam.

More generally, for a system with an arbitrary number of point charges, the electric dipole moment is

where each is a vector from some reference point to the charge qi.

Answered by  | 29th Mar, 2009, 04:29: PM

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