Suppose you wish to make a solenoid whose self-inductance is 3.0 mH. The inductor is to have a cross-sectional area of 2.0*10^-3 m2 and a length of 0.052 m. How many turns of wire are needed?

Answer 1

Use the formula for inductance L that you will find here:
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In your case, you know L, l (the length), and area pi a^2, so solve for N