CHAP. 12: BASIC TERMS OF CHEMICAL PHYSICS [CONTENTS] 418
surrounding environment. Its value for vacuum isμ 0 = 4π× 10 −^7 J s^2 C−^2 m−^1. For other types
of environment the permeability is different and it is sometimes expressed by therelative
permeability
μr=μ
μ 0(12.11)
or more often by thesusceptibility(more precisely volume magnetic susceptibility)χ=μr− 1. (12.12)U Main units: permeability: J s^2 C−^2 m−^1 , relative permeability and susceptibility are dimension-
less numbers.
Since different substances can either weaken or strengthen the magnetic field, magnetic
susceptibility may be positive or negative. For most common substances it is very small (of
the order of 10−^3 and lower), and it can be determined by measuring the force acting on the
substance in the field of a strong magnet. Materials weakening the field (χ <0) are called
diamagnetic, materials strengthening the field (χ > 0) are called paramagnetic. Some
substances with freely moving electrons (iron, nickel,... ) strengthen the magnetic field many
times (χ≈ 102 up to 10^4 ). These substances are referred to asferromagnetic.
Note: The same symbolμis used in this chapter for two different physical properties—
permeability and molecule dipole moment (see12.1.3).12.1.7 Molar magnetic susceptibility
The quantity calledmolar magnetic susceptibilityis the magnetic-field analogy of the molar
polarization (see12.1.2),
χm=χVm. (12.13)12.1.8 Magnetizability and magnetic moment
For most substances, the temperature dependence of molar magnetic susceptibility can be
expressed by the relation
χM=NA(
αm+μ 0 m^2 m
3 kBT)
, (12.14)