Discussion
Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) : A material for which \(X_{m} \gt 0\) and \(\mu_{r} \ge 1\) is said to be paramagnetic.
(Here \(X_{m}\) is magnetic susceptibility and \(\mu_{r}\) is relative permeability)
Reason (R) : A material for which \(X_{m} \gg 0,\ \mu_{r} \gg 1\) is said to be ferromagnetic.
In the light of the above statements, choose the most appropriate answer from the options given below.
Answer: C
(A) is false but (R) is true.
(R) is true. A ferromagnetic material has a very large positive susceptibility and a very large relative permeability, so \(X_{m} \gg 0\) and \(\mu_{r} \gg 1\) is the usual description of one.
(A) is false. A paramagnetic material has a small positive susceptibility, of the order of \(10^{-5}\) to \(10^{-3}\), so that \(\mu_{r} = 1 + X_{m}\) is only just above 1. The condition given in the Assertion, \(X_{m} \gt 0\) and \(\mu_{r} \ge 1\), has no upper limit and is met by ferromagnetic materials as well, so it does not define a paramagnetic one. A diamagnetic material, by contrast, has \(X_{m} \lt 0\) and \(\mu_{r} \lt 1\).
So option B (A true, R false) and options A and D (A true) fail on the Assertion, and the Board keyed option C. A student who reads (A) loosely, as the textbook line that paramagnets have positive susceptibility, is led to option A or D; the missing size limit is what makes the Assertion wrong.
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