with conductors. It cannot occur with insulators because charge is not free to move

within such substances.

Hollow conductor

Shields contents from external fields

23.12 - Electric dipoles

Electric dipole: Two point

charges with equal strength

but opposite signs separated

by a fixed distance.

A configuration of two charges that are equal in

strength but opposite in sign is important enough to

merit its own name: the electric dipole. The two

charges are at a fixed distance from each other.

The diagram in Concept 1 shows the basics of an

electric dipole. The equal and opposite charges have magnitude q and are separated

by the distance d. An electric dipole can be described by its electric dipole moment

p = qd, where d is the displacement vector from the negative charge to the positive

charge. The dipole moment vector points in the same direction as the displacement

vector.

When an electric dipole is placed in an external electric field, the positive end

experiences a force in the direction of the external field, and the negative end

experiences a force in the opposite direction. As the diagram in Concept 2 shows, if the

dipole moment is not aligned with the field, the dipole will experience a torque that

rotates it into the direction of alignment with the external electric field.

One reason why electric dipoles are important is that the individual molecules in what is

arguably the most vital substance for human existence í water í behave as electric

dipoles. The oxygen side of a water molecule tends to be negatively charged, while the

hydrogen side tends to have an equal but opposite positive charge. The result is a

dipole. The fact that water molecules are dipoles accounts for many of water’s crucial

properties. The illustration below shows a conceptual model of a water molecule

together with its electric dipole moment.

A convenient device, the microwave oven, takes advantage of the dipole nature of

water molecules. The oven uses an oscillating electric field to cause water molecules to

rotate back and forth, increasing their energy and the thermal energy of the food that

contains them.

The world's oceans contain about 7.8x10^46 electric dipoles: water molecules.

Electric dipole

Two point charges

Dipole moment vector p

·Separated by fixed distance

·Equal strength, opposite sign

·points from “í” to “+” charge

Electric dipole in an external

field

Moment tends to align with external field

(^428) Copyright 2007 Kinetic Books Co. Chapter 23