The self-inductance L of a solenoid of length l and area of cross-section A, with a fixed number of turns N increases as

1. l and A increase
2. l decreases and A increases
3. l increases and A decreases
4. both l and A decrease

(b) The self-inductance of a long solenoid of cross-sectional area A and length l, having n turns per unit length, filled the inside of the solenoid with material of relative permeability (e.g., soft iron, which has a high value of relative permeability) is given by
                                    L=μrμ0n2Al
where,                       n=N/l