1. The problem statement, all variables and given/known data Can I derive an equation for magnetic field inside a solenoid using the formula for magnetic field on the axis of a current carrying coil? 2. Relevant equations B = μI/2r ( Magnetic field at the centre of a current carrying coil) 3. The attempt at a solution B = μI/2r Let the number of turns per unit length of the solenoid be 'n' and its length be 'a' So, B = μnaI/2r Which is definitely not equal to μnI (Calculated using Ampere's Circuital Law) What's wrong?