The magnetic field inside a solenoid can be found exactly using ampère's law only if the solenoid is infinitely long. otherwise, the biot-savart law must be used to find an exact answer. in practice, the field can be determined with very little error by using ampère's law, as long as certain conditions hold that make the field similar to that in an infinitely long solenoid.

which of the following conditions must hold to allow you to use ampère's law to find a good approximation?

(a) consider only locations where the distance from the ends is many times d.
(b) consider any location inside the solenoid, as long as l is much larger than d for the solenoid.
(c) consider only locations along the axis of the solenoid.