Resistivity Variations

Dropping a strong magnet through a \(20 \textrm{ cm}\) copper tube took about \(3 \textrm{ s}\) when the pipe was at room temperature and \(4 \textrm{ s}\) when the pipe had been cooled in liquid nitrogen for a short period of time.

Part 1

The magnet fell more slowly through the cooled pipe because:

Answer Section

• The resistivity of the copper pipe increased at lower temperatures, resulting in decreased induced currents, and the induced magnetic fields opposed the magnet’s falling.

• The resistivity of the copper pipe decreased at lower temperatures, resulting in increased induced currents, and the induced magnetic fields opposed the magnet’s falling.

• The rate of change in the magnetic flux in the pipe caused by the falling magnet was greater in the colder pipe, resulting in a larger induced emf.

• Copper became magnetic at the lower temperature, so the magnet stuck to its side.

• Copper became superconducting at the lower temperature so it expelled the magnet.

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Attribution

Problem is licensed under the CC-BY-NC-SA 4.0 license.