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

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 5.0xx 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 2.0s^(-1) . What is the moment of inertia of the coil about its axis of rotation ?

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

Soft iron is preferred as the core of transformers due to its :

If ac supply is used and an iron core is introduced in a coil connected in series with the bulb, then what happens to the intensity of the bulb?