1. The problem statement, all variables and given/known data Prove that the number of turns per length on a solenoid is equal to 1/(diameter). 2. Relevant equations A= 2(pi)(R) 3. The attempt at a solution number of turns= length/(2pi*radius) number of coils per length=(length/(2pi*radius))/length number of coils per length=1/(2pi*radius) I'm not sure why I have an extra pi. Did I do something wrong? Thanks for any help.