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If the magnetic dipole moment of an atom of diamagnetic material, paramagnetic material and ferromagnetic material are denoted by $\mu_d$, $\mu_p$ and $\mu_f$ respectively, then
A
$\mu_p = 0$ and $\mu_f \neq 0$
B
$\mu_d \neq 0$ and $\mu_p = 0$
C
$\mu_d \neq 0$ and $\mu_f \neq 0$
D
$\mu_d = 0$ and $\mu_p \neq 0$
Detailed Solution
In a diamagnetic substance the magnetic moments of the electrons in each atom cancel out completely, so each atom has zero net magnetic moment: $\mu_d = 0$. A magnetic moment appears only by induction when an external field is applied, and it opposes the field.
In a paramagnetic substance each atom has a permanent (non-zero) magnetic dipole moment, because of unpaired electrons: $\mu_p \neq 0$. These moments are randomly oriented in the absence of a field.
In a ferromagnetic substance each atom also has a permanent magnetic moment: $\mu_f \neq 0$, and the moments are aligned within domains.
So $\mu_d = 0$, $\mu_p \neq 0$ and $\mu_f \neq 0$.
Among the given choices, the correct one is $\mu_d = 0$ and $\mu_p \neq 0$.
In a paramagnetic substance each atom has a permanent (non-zero) magnetic dipole moment, because of unpaired electrons: $\mu_p \neq 0$. These moments are randomly oriented in the absence of a field.
In a ferromagnetic substance each atom also has a permanent magnetic moment: $\mu_f \neq 0$, and the moments are aligned within domains.
So $\mu_d = 0$, $\mu_p \neq 0$ and $\mu_f \neq 0$.
Among the given choices, the correct one is $\mu_d = 0$ and $\mu_p \neq 0$.