A coil is wound on iron core which carries current I. The self-induced voltage in the coil is not affected by

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Option B: Variation in voltage to the coil

Explanation & Concepts:
The self-induced voltage in a coil is determined by the rate of change of flux linkage, which depends on the coil's physical properties and the current flowing through it. While the supply voltage determines the current in the circuit (via Ohm's law and impedance), it does not directly define the self-induced voltage, which is purely a function of the magnetic circuit parameters.

Working Principle: According to Faraday's Law, the self-induced voltage is defined as $e = L \frac{di}{dt}$. Here, $L$ is determined by the number of turns $N$ and the reluctance of the magnetic path $\mathcal{R}$ as $L = \frac{N^2}{\mathcal{R}}$. The supply voltage affects $i$ and $\frac{di}{dt}$, but the self-induced voltage is specifically the internal back-EMF response to magnetic field changes, making it independent of the external voltage source's magnitude provided the current variation is consistent.

Key Formulas: $e = L \frac{di}{dt}$ — Faraday-Lenz Law of self-induction; $L = \frac{N^2}{\mathcal{R}}$ — Inductance in terms of turns and reluctance

Key Points: Self-inductance ($L$) is a physical property defined by geometry and core material.; The induced EMF is the reaction to the change in magnetic flux, not the source voltage itself.; Magnetic reluctance ($\mathcal{R}$) depends on core permeability, length, and cross-sectional area.

Advantages: Predictable back-EMF generation; Linear relationship between flux linkage and current for non-saturated iron cores

Disadvantages: Core saturation can lead to non-linear inductance; Eddy current losses in the iron core

Applications: Inductors in filter circuits; Transformer core design

Additional Info: Option A: Changing current changes $di/dt$, which directly changes $e$.. Option C: Changing $N$ changes $L$ because $L \propto N^2$, directly changing $e$.. Option D: Reluctance changes the magnetic flux path, affecting the inductance value $L$.

Answer: B is correct — The self-induced voltage is a consequence of magnetic flux change within the coil and is independent of the external potential difference applied to the circuit terminals.

Exam Tip: Always distinguish between the 'cause' (current variation) and the 'external source' (supply voltage). The self-induced EMF is a property of the coil's reaction to magnetic flux changes.

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Option A: Variation in coil current

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Option C: Change of number of turns of coil

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Option D: Resistance of magnetic path

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