Using the expression for volume density of magnetic eneregy, demonstare that the amount of work contributed to magnetization of a unit volume of para or diamagentic, is equal to `A = -JB//2`.

Explanation

The total energy of the magnetic field is,
`(1)/(2) int (vec(B).vec(H)) dV = (1)/(2) int vec(B). ((vec(B))/(mu_(0)) - vec(j)) dV`
`= (1)/(2 mu_(0)) int vec(B).vec(B)dV - (1)/(2) int vec(J).vec(B) dV`
The secound tern can be interpreted as the energy fo magnetization, and has the density
`-(1)/(2) vec(J).vec(B)`.