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## Main Question or Discussion Point

In a book about stat. mech., I read the following relaiton for magnetic system

[tex]A = G + hM[/tex]

where A is the Helmholtz energy, G is the Gibbs energy and h is the external magnetic field, M is the magetization. I know from thermodynamic, we have

[tex]A = U - TS[/tex]

or

[tex]A = G - PV[/tex]

so, [tex]hM = -PV[/tex] ?

I don't understand what is [tex]hM[/tex]. If I know magnetization, external field and Gibbs energy, how to get Helmholtz energy?

[tex]A = G + hM[/tex]

where A is the Helmholtz energy, G is the Gibbs energy and h is the external magnetic field, M is the magetization. I know from thermodynamic, we have

[tex]A = U - TS[/tex]

or

[tex]A = G - PV[/tex]

so, [tex]hM = -PV[/tex] ?

I don't understand what is [tex]hM[/tex]. If I know magnetization, external field and Gibbs energy, how to get Helmholtz energy?