When it comes to long coils and other such aspects, it is quite natural that there will be a magnetizing force that is acting on it.
The magnetizing force refers to a certain degree up to which a certain magnetic field can magnetize a particular material. In a solenoid, which is important for a number of experiments it is important to note the level of force that is applicable for a long solenoid.
When a straight and long coil is used for generating a magnetic field that is uniform in nature and is quite similar to a bar magnet, it is known as a solenoid. Having a number of practical applications, with an additional iron core this field is strengthened to a great extent.
It is based on this principle that sums are to be solved by students.