A small strong magnet of mass is dropped down a long vertical copper pipe with its…
PHYSICS · Multi-part · Fermi University Physics · Electricity, Magnetism & Electromagnetism · Electromagnetic induction and alternating-current circuits · Electromagnetic induction and alternating-current circuits
Electricity, Magnetism and Electromagnetism, Ch Faraday's law, motional emf and Lenz's law
Problem
A small strong magnet of mass 35g is dropped down a long vertical copper pipe with its north pole downwards. After a short distance it settles to a steady measured speed of 8.0cms−1 and keeps that speed for the rest of the pipe. Air resistance and any rubbing on the pipe wall are negligible, and the magnet does not touch the wall. Take g=9.81ms−2. A student argues: "The magnet moves at constant speed, so its kinetic energy is not changing and no work is being done; therefore no energy is being dissipated in the pipe."
((a)) Find the magnitude of the retarding magnetic force on the magnet while it falls at its steady speed.
((b)) Find the rate at which energy is dissipated as heat in the copper, and say where that energy comes from, thereby answering the quoted argument.
((c)) State the sense of the induced current in the pipe wall just below the falling magnet, as seen by an observer looking down the pipe from above.