1. The problem statement, all variables and given/known data Determine the magnetic flux density at a point on the axis of a solenoid with radius b and length L, and with a current I in its N turns of closely wound coil. 2. Relevant equations Biot-Savart law: myI/(4pi)integral(dlcross er/R^2) there er is the unit vector in r-direction of cylindrical coordinates, and R =[(x-x')^2+(y-y')^2+(z-z')^2] 3. The attempt at a solution I think that the magnatic flux can be calculated for one circle and then multiply with N. I'm not sure what dl is in this case so I can't calculate the magnetic flux density.