What does the separation of turns of a solenoid affect in the experiment of the “world’s simplest electric train" made by a solenoid and a battery with spherical magnets at its poles?

A solenoid with 4 x 107 turns/m has an iron core placed in it whose magnetic susceptibility is 4.0 × 103 . (a) If a current of 2.0 A flows through the solenoid, what is the magnetic field in the iron core? (b) What is the effective surface current formed by the aligned atomic current loops in the iron core? (c) What is the self-inductance of the filled solenoid?

or the future Electron Ion Collider (EIC) a proposed magnet for the detector is a solenoid with the following properties: B = 3 Tesla (assume uniform inside, 0 outside) Length L = 4.0 m Inner Radius R = 1.5 m Use the long thin solenoid approximation of uniform field, even though this is a short squat solenoid A). Assuming the coil has radius R = 1.5 m, what is the current density (Amp per unit length) B) Calculate the total stored energy in the magnetic field, in Joules. Given that 1 gallon of gasoline contains about 10 8 Joules of chemical energy is this a large, medium, or small amount of stored energy? C) The coil would be superconducting.  If the coil were to suddenly lose it's superconductivity and the magnetic field ramped down extremely rapidly, where would the stored energy go?

A copper ring and a gold ring of the same dimensions are placed in magnetic fields so that there is the same change in magnetic flux through them. Compare the induced electric fields and currents in the rings.