Curie law and Curie temperature 

(a)          Curie law:  (i) The magnetic susceptibility of paramagnetic substances in inversely proportional to its absolute temperature i.e.. 

 ii.)Where C = Curie constant, T = absolute temperature

(iii)    On increasing temperature, the magnetic susceptibility of paramagnetic materials decreases and vice versa.
(iv)    The magnetic susceptibility of ferromagnetic substances does not change according to curie law.
(b)    Curie temperature (T_C): (i) The temperature above which a ferromagnetic material behaves like a paramagnetic material is defined as curie temperature (T_C).
(ii)    The minimum temperature at which a ferromagnetic substance is converted into paramagnetic substance is defined as curie temperature.
(iii)    For various ferromagnetic materials its values are different. e.g. for Ni

(iv)    At this temperature the ferromagnetism of the substances suddenly vanishes.

Curie-Weiss law
χ–T Cure

Other important features and formulae
(A)    Angle of declination and Geographical meridian:

(i)    The horizontal component of earth's magnetic field is from S to N.
(ii)    If at any place the angle of dip is θ and magnetic latitude is λ then,
        tan θ = 2 tan λ
    The total intensity of earth's magnetic field

Here M and R are the magnetic moment of bar magnet of earth and radius of earth respectively.
(iii)    At magnetic equator of earth λ = 0° and at poles λ = 90°.

(iv)    At the poles and equator of earth, the values of total intensity are 0.66 and 0.33 oersted respectively.

(B)    (i)    In vacuum :– B₀ = μ₀H
    (ii)    In medium :– B =  μH
    (iii)    Resultant magnetic field due to I and H :–
        (a)    B = B₁ + BH
                = μ₀I + μ₀H
            B = μ₀ (I + H)
            B = μ₀H (1 + I/H)
             or    B = μ₀ (1 + χ)H + = μH
    (iv)    (a)    μ = μ₀ [1 +  χ]
        (b)    μ = μ₀μ _r

(vii)    The magnetic potential due to a small magnet at a point distant r is given by :–

(viii)    The mutual interaction force between two small magnets of moments M₁ and  M₂ is given by

(C)    Magnetic torque τ

(D)    Magnetic potential energy (U_B)