Chapter Electrodynamics ple 7.6. The ring\" If a solenoidal Examl iron is there
ID: 1542956 • Letter: C
Question
Chapter Electrodynamics ple 7.6. The ring" If a solenoidal Examl iron is there to beef up the magnetic place coil around an iron core (the will jump several feet in the a metal ring on and plug it in, the ring 7.24). Why? Becau go up becaue FIGURE 7.24 Solution the flux through the ring was zero. Afterward a Before you turned on the current, in the ring led to a flux appeared in the diagram), and the enf generated a direction that (in the ring) which, according to Lenz's law was in such its field tended to cancel this new flux. This means that the current in the loop is to the cument in the solenoid. And opposite currents repel, so the rin flies off long solenoid, of radius (a is driven by an alternating current. that the field inside is Bo cos(at)ij A circular loop of wire, radius GE A. is placed inside the solenoid. and coaxial with it. Fins d resistanc induced in the loop as a function of time the current t roblem 7 square loop of wire, with sides of length a, lies in the first quadr of the ry plane, with one corner at the origin. In this region, there is a nonuniform me-dependent magnetic field B ky tz i (where k is a constan Find the mfind n the loopa Problem 7.14 As a lecture demonstration a short cylindrical bar magnet is dropped down a vertical aluminum pipe of slightly larger diameter, about 2 meters long. takes several seconds emerge at tbe bottom, whereas an otherwise identical piece to of unmagnetized makes the trip in a fraction of a second. Explain why the magnet falls more slowly 12 For further discussion of the jumping ring (and the related "floating ring", see c. s, Schneider and P Ertel, Am J Phys 66, 686 (998): P J H. Tjossem and E. C.Brost, Am. J Phys. 79, 353 2011 For a discussion of this amazing demonstration see K D. Hahn et al., Ann. J Phys. 66, 1066 19 nd Gr Donoso, c Etadera, and P Martin, Am. J Phys. 79, 193 (2011)Explanation / Answer
As the magnet comes down its magnetic field increaes ( As comes closer) so flux increases and hence as per faraday's law of electromagnetic induction there is some current in the aluminim pipe.
As per the lenz's law this induced current opposes the motion of magnet downward so there is repulsive force up in addition to its weight dowanward , So less net force ,less accelartion ,So takes longer to emerge at the bottom.
In case of unmagnetised iron there is no magnetic field so no change of flux hence no opposing force upwards so it goes down under the influence of its weight with the acceleration equal to 9.8 m/s^2 ,take less time.