1. The problem statement, all variables and given/known data I'm having problems figuring out the difference between a coil and a solenoid. My book provides two equations for magnetic field, and they are similar, but one depends on the radius (coil) while the other depends on the length (solenoid). 2. Relevant equations Bcoil = μNI/2a (field at the center of N circular loops) Bsolenoid = μNI/L (field in a solenoid) 3. The attempt at a solution As you can see, the Bcoil depends on the a (radius) while the solenoid depends on L (length).