Question

Pick out the WRONG statements about magnetic substances (\(\chi\) = magnetic susceptibility) (\(\mu_r\) = relative permeability).
I. Substances with \(-1 \le \chi<0\) are diamagnetic
II. Substances with \(\chi \gg 1\) are paramagnetic
III. Substances with \(\chi \ll 1\) are ferromagnetic
IV. Substances with \(\mu_r \gg 1\) are ferromagnetic

• A. I and II
• B. III and IV
• C. II and III
• D. II and IV

Hint:

Remember the broad trends for \(\chi\):
- Diamagnetic: Small and Negative.
- Paramagnetic: Small and Positive.
- Ferromagnetic: Large and Positive.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks to identify the incorrect statements among the four provided, which describe the magnetic properties of different classes of substances based on their susceptibility (\(\chi\)) and relative permeability (\(\mu_r\)).
Step 3: Detailed Explanation:
Let us examine each statement individually:
Statement I: Diamagnetic substances exhibit a small, negative magnetic susceptibility. For a perfect diamagnet (like a superconductor), \(\chi = -1\). For typical diamagnetic materials, \(-1 \le \chi<0\). This statement is correct.
Statement II: Paramagnetic substances have a small, positive magnetic susceptibility (typically \(10^{-3}\) to \(10^{-5}\)). It is ferromagnetic substances that have \(\chi \gg 1\). Therefore, this statement is wrong.
Statement III: Ferromagnetic substances are characterized by very high magnetic susceptibility, where \(\chi \gg 1\). The condition \(\chi \ll 1\) (small positive) describes paramagnetic materials. Therefore, this statement is wrong.
Statement IV: The relationship between relative permeability and susceptibility is given by \(\mu_r = 1 + \chi\). Since ferromagnetic substances have very large values of \(\chi\), their relative permeability is also very large (\(\mu_r \gg 1\)). This statement is correct.
Step 4: Final Answer:
The wrong statements are II and III. This corresponds to option (3).