But when I was doing another problem which deals with calculating the self inductance of a toroid with a small gap, whose permeability is μ for the iron part and μ0 for the gap, the answer uses μ0 to calculate the B field in the gap (used "imaginary wire loop")...And the answer is definitely...
Original Problem:
"A sphere of radius a is made of a nonconducting material that has a uniform volume charge density [PLAIN]http://jkwiens.com/2007/10/24/answer-electric-field-of-a-nonconducting-sphere-with-a-spherical-cavity/d2606be4e0cd2c9a6179c8f2e3547a85_2.gif. A spherical cavity of...