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 outsid e a very long but finite solenoid of length L, the field must be a decreasi ng 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 techni ques for analyzing magnetic fields. (C) 2001 American Association of Physic s Teachers.