Chapter-4|Moving Charges and Magnetism| NCERT 12th Physics:


A solenoid is a insulated long wire closely wound in the form of a helix. Its length is very large as compared to its diameter.

In television, we make use of solenoid to generate magnetic field needed for the deflection of electron in the picture tube.

Magnetic Field Due to a Solenoid:

In solenoid wire are turned like helical coil with cocentric rings very close to each other but insulated from each other.

When this setup is suspended freely, it rest in north- south direction.

One reversing the direction of current (by changing polarity of battery) the solenoid rotates 180⁰.

Inside the solenoid magnetic field is uniform and parallel to the solenoid Axis. Outside the solenoid magnetic field is assumed to be zero.

Derivation of Magnetic Field:let a solenoid having closely packed coil in which I is current, n is number of turns per unit length and B is magnetic field inside the solenoid.


Now, We consider ABCD as amperian loop, then by applying ampere’s circuital law in whole closed path ABCD.


From above it is clear that B is independent of the length and diameter of solenoid and it is uniform over the cross section of the solenoid.




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