What is solenoid ? Give information of magnetic field by qualitative discussion.

Explanation

1. A solenoid means an insulated copper wire wound closely in the form of a helix.
2. By a long solenoid, we mean that the length of the solenoid is very large as compared to its diameter.

3. If its length is shorter than radius then it is called short solenoid.
4. It consist of a long wire wound in the form of a helix where the neighboring turns are closely spaced. So each turn can be regarded as a circular loop.
5. Enamelled wires are used for winding so that turns are insulated from each other.
6. The net magnetic field is the vector sum of the fields due to all the turns.
7. Figure (a) shows a section of this solenoid in an enlarged manner.
8. Figure (b) shows the entire finite solenoid with its magnetic field.
9. In figure (a), it is clear from the circular loops that the field between two neighboring turns vanishes.
10. In figure (b), we see that the field at the interior mid point P is uniform, strong and along the axis of the solenoid.
11. The field at the exterior mid point Q is weak and moreover is along the axis of the solenoid with no perpendicular or normal component.

12. Its the solenoid is made longer it appears like a long cylindrical metal sheet.
13. Figure represent this idealised picture.
The field outside the solenoid approaches zero. We shall assume that the field outside is zero. The field inside becomes everywhere parallel to the axis.