A slender ferromagnetic bar of length and negligible thickness is pivoted at its…
PHYSICS · Multi-part · CBSE · Physics · Moving Charges and Magnetism · Moving Charges and Magnetism
Problem
A slender ferromagnetic bar of length 8.0cm and negligible thickness is pivoted at its midpoint so that it can rotate freely in the horizontal plane. The bar possesses a permanent magnetic dipole moment μ that points from its geometric center toward its north–seeking end. The assembly is placed in a uniform horizontal magnetic field of magnitude B0=5.0×10−5T (comparable to Earth’s field) so that the bar makes an angle θ with the direction of the external field.
(a) Describe qualitatively how the magnitude and direction of the magnetic torque acting on the bar depend on the angle θ. In your description, indicate the angular positions at which the torque is zero and where its magnitude is maximum.
(b) Identify every orientation that corresponds to rotational equilibrium and classify each as stable or unstable. Justify your classification using the torque–angle relationship discussed in part (a).
(c) The external field is suddenly reversed in direction while the bar is initially aligned along the original field. Describe qualitatively the subsequent motion of the bar, commenting on energy transformations and on how the magnetic torque influences the motion.
(d) The ferromagnetic bar is replaced by an identically shaped paramagnetic aluminum bar that acquires the same magnitude of magnetic dipole moment while the field is on but loses that dipole moment when the field is switched off. Compare the behaviors of the ferromagnetic and paramagnetic bars immediately after the field is turned off, and explain the physical reason for any difference.