1. The problem statement, all variables and given/known data Hey everyone, I got a question wrong on my homework that I'm trying to figure out. I'm given this: "a 500uH solenoid(inductor) with an air core is 10cm long has an inside diameter of 1cm. How many turns of wire does it have? 2. Relevant equations L = N2*u*A / l N is the number of turns u is the permeability of core material A is the Area of the coil in square meters l is the average length of the coil in meters. 3. The attempt at a solution So first of all I rewrote the equation to be N = sqrt([L*l]/[u*A] L = 0.5H, l = 0.1m, u is u0 * ur, or 1 * (1.26x10-6, and my area is going to ∏*(0.005)2 So, N = √(0.5*0.1)/(1 * ([1.26x10-6]*[∏*(0.005)2] When I calculated this however, I'm getting an answer of 22477.9 turns, which I know is wrong. The answer actually is 712. I'm thinking I either have my permeability of wrong (which I double checked, an air core is permeability 1 and the permeability of free space is 1.26x10-6, so the permeability is 1.26x10-6. Do I possibly have my area wrong? The diameter is 1cm, so we know that the area of a circle is pi * r squared, so pi * (0.01/2), which I got 0.000079m2. Am I missing something obvious here? When I solve for area, I get (0.5*0.1)/(7122*(1.26x10-6)) which gives me 0.078278, which I'm guessing that's the area I SHOULD have gotten, but how is that?