A sample of paramagnetic salt contains 2.0 × 1024 atomic dipoles each of dipole moment 1.5 × 10–23 J T–1. The sample is placed under a homogeneous magnetic field of 0.64 T, and cooled to a temperature of 4.2 K. The degree of magnetic saturation achieved is equal to 15%. What is the total dipole moment of the sample for a magnetic field of 0.98 T and a temperature of 2.8 K? (Assume Curie’s law)

Number of atomic dipoles (n) = 2 × 1024

Initially, Dipole moment of each atomic dipole (M) = 1.5 × 10-23JT-1


Magnetic field (B1) = 0.64 T and Temperature (θ1) = 4.2K


Degree of magnetic saturation = 15%


Effective Dipole moment (M1) = 15% of Total dipole moment


M1 = × (no. of atomic dipole × dipole moment of each)


M1 = × 2 × 1024 × 1.5 × 10-23JT-1 = 4.5 JT-1


When, Magnetic field (B2) = 0.98 T and Temperature (θ2) = 2.8 K


Let the dipole moment be M2.


According to the Curie’s Law, magnetization of a paramagnetic material is directly proportional to applied magnetic field and inversely proportional to temperature.


Ratio of magnetic dipole moments (i.e. M1:M2) = (B1θ2/B2θ1)


M2 = M1 × M2 = 4.5 JT-1 × = 10.336 JT-1


Thus, total dipole moment of the sample for a magnetic field of 0.98 T and a temperature of 2.8 K = 10.336 JT-1


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