1. The problem statement, all variables and given/known data A student makes an electromagnet by winding 320 turns of wire around a wooden cylinder of diameter 4,80 cm. The coil is connected to a battery producing a current of 4.20 A in the wire. At what axial distance z>>d will the magnetic field of this dipole be 5.0 μT? 2. Relevant equations Biot-Savard Law: (in vector form) B=(μ/4π) i(L x u)/r^2 Maybe Ampère's Law: ∫Bds=μi 3. The attempt at a solution Unfortunately I can't even imagine how to integrate... I tried by using Ampère's Law on a circular path around the solenoid. I mean: imagine the solenoid and an imaginary circumference that lays on the same plane of the coils, then inside this circumference there's the current i. I actually treated it as a straight wire because the result is the same and it's wrong... mhh, help?