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NCERT Class XII Chapter
Magnetism and Matter
Questions With Solutions

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Question : 23
Total: 25
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)
Solution:  
Dipole moment at complete saturation
M = 2 × 1024 × 1.5 × 10–23 = 30 J T–1
At 0.84 T and 4.2 K, 15% of sample is magnetised
M = C
B0
T0
0.15 × 30 = C
0.84
4.2
 ... (i)
At 0.98 T and 2.8 K
M = C
0.98
2.8
Dividing (i) by (ii), M = 7.875 J T–1
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