A current I flows in a long solenoid with n closely wound coil-turns per unit length. The...

A current I flows in a long solenoid with n closely wound coil-turns per unit length. The cross-sectional area of its iron core, which has permeability µ. is S. Determine the force acting on the core if it is withdrawn to the position shown in Figure.

Figure: A long solenoid with iron core partially withdrawn