If toroid uses bismuth for its core, will the field in the core be (slightly) greater or (slightly) less than when the core is empty ?

When the resistance connected in series with a cell is halved, the current is equal to or slightly less ro slighty greater than double. Why?

A toroid has a core (non-ferromagnetic) of inner radius 25 cm and outer radius 26 cm, around which 3500 turns of a wire are wound. If the current in the wire is 11 A, what is the magnetic field (a) outside the toroid, (b) inside the core of the toroid, and (c) in the empty space surrounded by the toroid.

Given an A.P whose terms are all positive integers. The sum of its nine terms is greater than 200 and less than 220. If the second term in it is 12, then its 4th term is

A circular coil of 16 turns and radius 10 cm carrying a current of 0.75 A rests with its plane normal to an external field of magnitude 8.0 xx 10^(-2) T . The coil is free to turn about an axis in its plane perpendicular to the field direction. When the coil is turned slightly and released, it oscillates about its stable equilibrium with a frequency of 2.0 s^(-1) . What is the moment of intertia of the coil about its axis of rotation.