A cylindrical shell made of a high-permeability alloy (often called a magnetic…
PHYSICS · Multi-part · A Level · Physics · Fields and their consequences · Fields and their consequences
Problem
A cylindrical shell made of a high-permeability alloy (often called a magnetic shield) has an inner radius of 2 cm, a wall thickness of 1 mm, and is much longer than it is wide. The cylinder’s axis is horizontal. A bar magnet 8 cm long, with its North pole facing the open end of the cylinder, is positioned 3 cm from that end and aligned so that its dipole axis is collinear with the cylinder’s axis.
(a) Draw a qualitative sketch of several representative magnetic field lines that originate near the North pole of the magnet and interact with the cylindrical shell. Show how the lines propagate both outside and inside the shell.
(b) Explain how the presence of the high-permeability shell modifies the magnetic field magnitude inside the hollow cavity of the cylinder compared with the field that would exist there if the shell were absent.
(c) A compass placed at the center of the hollow cavity deflects by about 5∘ from its original orientation when the shell is slipped over the magnet. State the direction of this deflection and explain why both the direction and magnitude change.
(d) When the shell is removed, the compass needle returns to its original orientation, yet no noticeable heating or cooling of the magnet or shell is observed. Discuss the energy transfers that occur during the insertion and removal of the shell, identifying the systems that gain or lose energy.