Dropping strong magnets down metal tubes. (A qualitative question. No computation!)
Using straight metal tubes of aluminum (aluminium) and copper of the same length, about a meter (yard) in length, the same internal diameter and wall thickness and a small, very strong, cylindrical magnet (I actually do this with two identical magnets) that fits easily with plenty of room for air to flow around the magnet, drop the magnet down the inside of the tubes and note how long it takes to travel the length. Compare time to dropping the magnet the same distance in the open air.
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1 solution
The moving magnet generates a current which travels around the wall of the tube in which it is dropped. That current in turn creats a magnetic field that opposes the downward motion of the magnet. No significant current is not created in the open air because air is not significantly conductive. As copper is more conductive than aluminum, more of that current survives to produce the counter-magnetic field and less goes into resistive heat loss. Therefore, the magnet drops slower in copper than in aluminum. In the limit case of a superconducting tube, the magnet would not even enter the tube unless the superconductivity were destroyed by the magnetic field.
Magnet levitating above a superconducting plate