Final answer:
The assertion that the self-inductance L of a solenoid increases with insertion of a ferromagnetic substance is correct as it enhances the magnetic field. However, the reason claiming that increasing current increases L is incorrect; self-inductance is a constant for a given system and geometry. The correct answer to the question is: C. Assertion is correct but the reason is incorrect.
Step-by-step explanation:
The question asks about increases in the coefficient of self-induction (L) of a solenoid when a ferromagnetic substance is placed inside it, and whether changing the current in the coil affects L. The assertion that inserting a ferromagnetic material into the solenoid increases L is correct. A ferromagnetic substance greatly enhances the magnetic field within the solenoid due to its high magnetic permeability compared to air or vacuum. Consequently, this increase in the magnetic field results in a higher magnetic flux for a given current, leading to an increased self-inductance of the coil.
The reason cited, that increasing the current increases L, is incorrect. While it is true that increasing the current will increase the magnetic field and magnetic flux, the coefficient of self-induction, L, depends on the physical properties and geometry of the coil and the magnetic permeability of the core material, which are constants for a given system. Therefore, L remains constant for a solenoid with fixed properties regardless of the current flowing through it. Changing the current will change the induced emf but not the coefficient of self-induction itself.
The correct answer to the question is: C. Assertion is correct but the reason is incorrect.