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# 3: Dipole and Quadrupole Moments

• 3.1: Introduction
The maximum torque that the dipole experiences when placed in an external electric field is its dipole moment. This is a vector quantity, and the torque is a maximum when the dipole moment is at right angles to the electric field.
• 3.2: Mathematical Definition of Dipole Moment
Previously, we gave a physical definition of dipole moment. I am now about to give a mathematical definition.
• 3.3: Oscillation of a Dipole in an Electric Field
• 3.4: Potential Energy of a Dipole in an Electric Field
The amount of work you would have to do to increase the angle between a dipole on a external field is the potential energy of the dipole.
• 3.5: Force on a Dipole in an Inhomogeneous Electric Field
While a dipole in a homogeneous field experiences no net force, but we can see that it does experience a net force in an inhomogeneous field.
• 3.6: Induced Dipoles and Polarizability
A charged rod will attract an uncharged pith ball since it induces a dipole moment in the uncharged pith ball, which now has a dipole moment, is attracted in the inhomogeneous field surrounding the charged rod. How may a dipole moment be induced in an uncharged body? Well, if the uncharged body is metallic, then once placed in an electric field, the free electrons are attracted to one end of the metal, leaving an excess of positive charge at the other end; this induces a dipole moment.
• 3.7: The Simple Dipole
As you may expect from the title of this section, this will be the most difficult and complicated section of this chapter so far. Our aim will be to calculate the field and potential surrounding a simple dipole.