how does magnetic field get affected when a core of any substance is placed inside a solenoid....

The magnetic field inside a solenoid is proportional to both the applied current and the number of turns per unit length. There was no dependence on the diameter of the solenoid or even on the fact that the wires were wrapped around a cylinder and not a rectangular shape. Most importantly, the result did not depend on the precise placement of the path inside the solenoid, indicating that the magnetic field is constant inside the solenoid.

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