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 at the centre of a long solenoid is given by B = ₀ n I , where n is the number of turns per unit length and I is the current. The magnetic field intensity H is related to the magnetic field B in free space by the relation H = B ₀ . Substituting the value of B , we get H = ₀ n I ₀ = nI . Answer: nI