Question

Inductance of a coil is measured as $10\,\text{mH}$ using an LCR meter at $10\,\text{kHz}$, when no objects are near the coil. If a pure copper sheet is brought near the coil, the same LCR meter will read _________.

• A. less than $10\,\text{mH}$
• B. $10\,\text{mH}$
• C. more than $10\,\text{mH}$
• D. less than $10\,\text{mH}$ initially and then stabilizes to more than $10\,\text{mH}$

Hint:

A conductive object near a coil acts like a shorted secondary: stronger eddy currents $\Rightarrow$ opposing flux $\Rightarrow$ \textbf{lower} effective inductance (and higher losses) at AC.

Answer 1

The Correct Option is A

A nearby copper sheet forms an eddy-current (shorted turn) shield. The changing magnetic field from the coil induces currents in the sheet whose magnetic field opposes the coil’s flux (Lenz’s law). This reduces the {net} flux linkage of the coil, so the measured inductance decreases: $L_{\text{measured}}<10\,\text{mH}$. Therefore \fbox{(A)} is correct.