question_answer 33)

  Consider a metallic pipe with an inner radius of 1 cm. If a cylindrical bar magnet of radiuscm is dropped through the pipe, it takes more time to come down than it takes for a similar unmagnetised cylindrical iron bar dropped through the metallic pipe. Explain.

Answer:

                  When a cylindrical bar magnet is dropped through the metallic pipe, amount of magnetic flux linked with the pipe changes. Eddy currents are induced in the pipe, which oppose the free fall of bar magnet. The acceleration of bar magnet will be less than 'g', acceleration due to gravity. Therefore, it takes more time to come down the pipe. When a similar unmagnetised cylindrical iron bar is dropped through the metallic pipe, there are no eddy currents as magnetic flux linked with the pipe does not change. Hence, there is no opposition to free fall of the iron bar under gravity, i.e., its downward acceleration = g. Hence, the iron bar takes lesser time to come down the pipe.