something that has had me confused: How can I calculate the time varying magnetic field produced by a solenoid, outside the solenoid along the axis of the core. I know that: inside the solenoid : apply amperes law For DC (magnetostatic) case outside the solenoid : apply Biot-savart law but what about a AC (time varying field) outside the solenoid? is the attenuation calculated like in EM waves attenuation and skin-depth? I also read something about replacing biot-savart with Jefimenko's equations if magnetostatics does not apply. Or is this not possible to calculate analytically. What is going on?