A perfectly diamagnetic sphere has a small spherical cavity at its centre, which is filled with a paramagnetic substance. The whole system is placed in a uniform magnetic field B. Then the field inside the paramagnetic substance is: (a) B (b) zero (c) much large than |B| and parallel to B (d) much large than |B| but opposite to B

Answer: (b). A perfect diamagnet has χ = -1, so μr = 0 and B = μ₀ μr H = 0 inside it. It expels the field completely, as a superconductor does. No field lines enter the diamagnetic shell, so none reach the cavity at its centre. With no field applied to it, the paramagnetic substance has no magnetisation, so the field there is zero. So the answer is (b) zero.

A perfectly diamagnetic sphere has a small spherical cavity at its… | JEE Main 2020 (3 Sep, Shift 2)

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Magnetic Materials & Curie Law

A perfectly diamagnetic sphere has a small spherical cavity at its centre, which is filled with a paramagnetic substance. The whole system is placed in a uniform magnetic field $\vec{B}$ . Then the field inside the paramagnetic substance is:

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Answer: (b)

A perfect diamagnet has $\chi = -1$ , so $\mu_r = 0$ and $B = \mu_0 \mu_r H = 0$ inside it. It expels the field completely, as a superconductor does.
No field lines enter the diamagnetic shell, so none reach the cavity at its centre.
With no field applied to it, the paramagnetic substance has no magnetisation, so the field there is zero.
So the answer is (b) zero.

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