Introduction:
A solenoid is a coil of wire wound in the shape of a cylinder. When an electric current flows through the solenoid, it produces a magnetic field. The strength of the magnetic field depends on various factors, including the number of turns in the coil, the current flowing through it, and the length of the solenoid.

Explanation:
When the length of the solenoid increases, the magnetic field produced by the solenoid decreases. This can be explained using the following points:

1. Magnetic field produced by a solenoid:
The magnetic field inside a solenoid is uniform and parallel to the axis of the solenoid. It can be approximated as a series of closely spaced circular loops, each carrying a current in the same direction. The magnetic field inside the solenoid is given by the equation:

B = μ₀ * n * I

Where B is the magnetic field strength, μ₀ is the permeability of free space, n is the number of turns per unit length, and I is the current flowing through the solenoid.

2. Effect of length on magnetic field:
When the length of the solenoid increases, the number of turns per unit length decreases. This means that the value of 'n' in the equation decreases. As a result, the magnetic field strength 'B' decreases. This is because the magnetic field produced by each turn of the solenoid contributes to the overall magnetic field strength, and when the number of turns decreases, the overall magnetic field strength decreases.

3. Relationship between length and magnetic field:
The relationship between the length of the solenoid and the magnetic field strength can be understood by considering the equation for the magnetic field. As the length of the solenoid increases, the number of turns per unit length decreases, resulting in a decrease in the magnetic field strength. Conversely, if the length of the solenoid decreases, the number of turns per unit length increases, leading to an increase in the magnetic field strength.

Conclusion:
In summary, when the length of a solenoid increases, the magnetic field strength decreases. This is because the number of turns per unit length decreases, resulting in a decrease in the overall magnetic field produced by the solenoid.

Magnetic field outside the solenoid is cancelled by its own flux, in a solenoid magnetic flux exists only inside the solenoid