Discussion
The ratio of skin depth in copper at 1 kHz to that at 100 MHz is approximately :
Answer: C
The ratio is about 300.
The skin depth \( \delta = \sqrt{\frac{2}{\omega\mu\sigma}} \) varies as \( 1/\sqrt{f} \), so
\[ \begin{aligned} \frac{\delta_{1\,\text{kHz}}}{\delta_{100\,\text{MHz}}} &= \sqrt{\frac{10^{8}}{10^{3}}} \\ &= \sqrt{10^{5}} \approx 316 \end{aligned} \]
In copper that is about 2 mm at 1 kHz against 6.6 μm at 100 MHz. 30 and 3000 would follow from frequency ratios of \( 10^{3} \) or \( 10^{7} \), not \( 10^{5} \), and 3 is far too small.
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