Skin effect occurs when a conductor carries current at _________ frequencies.

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Explanation: Skin effect is the tendency of an alternating electric current (AC) to become distributed within a conductor such that the current density is highest near the surface and decreases exponentially with depth. This effect is negligible at low frequencies but becomes significant as the frequency of the AC signal increases.

Working Principle: As AC flows through a conductor, it generates a time-varying magnetic field that induces eddy currents. These eddy currents oppose the main current flow at the center of the conductor and reinforce it near the surface, effectively forcing the current to the 'skin' of the wire. The penetration depth, or skin depth, is inversely proportional to the square root of the frequency.

Key Formulas: $\delta = \frac{1}{\sqrt{\pi f \mu \sigma}}$ — where $\delta$ is the skin depth, $f$ is frequency, $\mu$ is permeability, and $\sigma$ is conductivity.

Key Points: Skin effect causes an increase in the effective AC resistance of a conductor.; The depth of penetration ($\delta$) decreases as frequency ($f$) increases.; At high frequencies, the center of a solid conductor carries very little current, making stranded wires or hollow tubes more efficient.; Skin effect is dependent on the material properties (conductivity and permeability) of the conductor.

Advantages: Useful in induction heating and surface hardening processes.; Helps in reducing the eddy current losses in certain electromagnetic applications.

Disadvantages: Increases the effective resistance of power lines leading to higher ohmic losses.; Limits the current-carrying capacity of large-diameter conductors at high frequencies.

Applications: High-frequency transmission lines and waveguides.; RF shielding and microwave components.; Induction heating furnaces.

Additional Info: At power frequencies (e.g., 50 Hz), the skin effect is minimal in small conductors but becomes important in extremely large cross-section conductors.. Option A and B are incorrect because skin effect is essentially negligible for DC and very low-frequency currents where the current distribution remains uniform.

Answer: C is correct — Skin effect becomes significant only at high frequencies due to the increase in induced eddy currents within the conductor.

Exam Tip: In competitive exams, remember that skin depth ($\delta$) is inversely proportional to the square root of frequency, meaning the effect intensifies rapidly at radio frequencies (RF).

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