MAGNETIC FIELDS IN MATTER — Edward Purcell & David Morin
Physics — E&M · induced-dipole-force
Problem
Between any ion and any neutral atom there is a force that arises as follows. The electric field of the ion polarizes the atom; the field of that induced dipole reacts on the ion. Show that this force is always attractive, and that it varies with the inverse fifth power of the distance of separation r. Derive an expression for the associated potential energy, with zero energy corresponding to infinite separation. For what distance r does this potential energy have the same magnitude as kT at room temperature (which is 4⋅10−21 joule) if the ion is singly charged and the atom is a sodium atom? (See Table 10.2.)