Field just outside a long solenoid

Simple lessons about static magnetic fields are often taught with the model of an “infinite” solenoid, outside of which the fields vanish. Just outside a very long but finite solenoid of length L, the field must be a decreasing function of L. We show that this external field is approximately uniform and decreases as L−2. Furthermore, we show that the study of this external field provides interesting and surprisingly simple illustrations of techniques for analyzing magnetic fields.