Which one of the following statements regarding a current-carrying solenoid is not correct?
The magnetic field inside the solenoid is uniform.
The current-carrying solenoid behaves like a bar magnet.
The magnetic field inside the solenoid increases with increase in current.
If a soft iron bar is inserted inside the solenoid, the magnetic field remains the same.
Answer is Wrong!
Answer is Right!
This question was previously asked in
UPSC NDA-2 – 2022
A) The magnetic field inside a *long* solenoid is approximately uniform and directed along the axis of the solenoid, except near the ends. This statement is correct for an ideal or long solenoid.
B) A current-carrying solenoid creates a magnetic field pattern similar to that of a bar magnet, with magnetic poles at its ends. This statement is correct.
C) The magnitude of the magnetic field inside a solenoid is given by $B = \mu n I$, where $\mu$ is the permeability of the core material, $n$ is the number of turns per unit length, and $I$ is the current. The field is directly proportional to the current ($I$). So, increasing the current increases the magnetic field. This statement is correct.
D) If a soft iron bar (a ferromagnetic material with high permeability) is inserted inside the solenoid, the magnetic field inside increases significantly. This happens because the soft iron gets strongly magnetized in the direction of the solenoid’s field, and its own magnetic field adds to the field produced by the current. The permeability of soft iron is much greater than the permeability of air or vacuum ($\mu >> \mu_0$). The magnetic field does *not* remain the same; it increases. This statement is incorrect.