PHYSICS · Single correct · AP · Physics C: Electricity and Magnetism · Magnetic Fields and Electromagnetism · Magnetic Fields and Electromagnetism
Problem
A small bar magnet is centered inside a long, hollow cylindrical shell made of soft iron (high magnetic permeability). The shell is open at both ends and its inner radius is much larger than the size of the magnet. Point Q lies on the central axis just outside one open end of the shell. Compared with an identical bar magnet in free space (with no surrounding shell), which statement about the magnitude of the magnetic field at point Q is most accurate?
(a) It is greater because the shell provides a path of low magnetic reluctance that draws additional field lines through Q. (Distractor)
(b) It is smaller because many field lines are redirected through the high-permeability shell instead of passing through Q. (Correct)
(c) It is greater because the shell’s induced south pole near Q adds to the field of the magnet. (Distractor)
(d) It is unchanged because magnetic shielding is effective only for time-varying fields, so a steady bar magnet is unaffected. (Distractor)