In what scenario is the density of eddy currents at its maximum?

The density of eddy currents is at its maximum when the rod is at its surface level in the coil. This situation occurs because eddy currents are induced in the conductive material (the rod) by the changing magnetic field produced by the coil.

When the rod is positioned at the surface level of the coil, it experiences the strongest magnetic field interaction due to proximity to the coil's conductive path, leading to the highest rate of change in magnetic flux. This increase in magnetic flux results in a larger number of eddy currents being generated and circulating within the rod. The induced eddy currents are critical for non-destructive testing techniques, such as eddy current testing, where detecting flaws or variations in the material's conductivity is essential.

In contrast, being removed from the part or further inside the coil would reduce the magnetic field interaction, thereby reducing eddy current intensity. High frequency can lead to skin effect, which might not necessarily correlate to maximum eddy currents depending on the position of the material relative to the coil.

Thus, the scenario where the rod is at the surface of the coil maximizes the eddy current density due to the strongest electromagnetic interaction.