The magnetic field intensity H at the centre of a long solenoid having n turns per unit length and carrying a current I, when no material is kept in it is ( ₀= permeability of free space)

Step-by-step solution

The magnetic field B inside a long solenoid with n turns per unit length carrying current I in free space is B = ₀ n I , where ₀ is the permeability of free space. The magnetic field intensity H satisfies the relation B = H ; in free space, = ₀ . Substituting gives ₀ n I = ₀ H , and thus H = nI . Therefore, the magnetic field intensity at the center of the solenoid is nI . Correct option: C